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DEPARTMENT   OF   THE    INTERIOR 


MONOGRAPHS 


United  States  Geological  Survey 


VOLUME   Y 


WASHINGTOjST 

government   printing-  office 

1883 


UNITED  STATES  GEOLOaiCAL  SURVEY 

CLARENCE  KING  DIRECTOR 


THE 


COPPER-BEAEING  ROOKS 


LAKE    SUPERIOR 


Bv   ROLAND    IDXJER   IRVIN'a 


WASHINGTOls^ 

GOVERNMENT     PRINTING     OFFICE 

1883 


Digitized  by  the  Internet  Archive 

in  2009  with  funding  from 

Boston  Library  Consortium  IVIember  Libraries 


http://www.archive.org/details/copperbearingrocOOirvi 


Newport,  R  I.,  October  23,  1881. 
Sir:  I  have  the  honor  to  transmit  herewith  the  manuscript  of  the  text 
and  ilhistrations  of  Prof  Roland  D.  Irving's  memoir  on  the  Copper-Bearing 
Rocks  of  Lake  Superior. 

Very  respectfully,  your  obedient  servant, 

RAPHAEL  PUMPELLY, 
In  charge  of  Division  of  Mining  Geology. 
Hon.  Clarence  King, 

Director  of  the  United  States  Geological  Survey. 


University  op  Wisconsin, 
Department  of  Geology  and  Mineralogy, 

Madison,  October  15,  1881. 
Sir:  I  send  you  herewith  the  manuscript  of  my  memoir  on  the  Copper- 
Bearing  or  Keweenawan  Rocks  of  Lake  Superior. 

I  am,  sir,  with  great  respect,  your  very  obedient  servant, 

ROLAND  D.  IRVma. 
Raphael  Pumpelly, 

In  charge  Division  Mining  Geology, 

United  States  Geological  Survey,  Newport,  B.  J. 


CONTE]SrTS, 


Page. 

Chapter  I. — Introductory 1 

LiTEKATURF, 14 

Chapter  II. — Extent  and  General  Nature  of  the  Keweenaw  Series 24 

Chapter  III. — Lithology  of  the  Keweenaw  Series 34 

Section  I. — Basic  Original  Rocks 35 

Section  II. — Acid  Original  Rocks 91 

Section  III. — Summary  View  of  the  Original  Rocks  of  the  Keweenaw  Series 126 

Section  IV. — Detrital  Rocks '. 127 

Chapter  IV. — Structural  Features  of  the  Three  Classes  of  Rocks  of  the  Keweenaw 

Series 134 

Chapter  V. — General  Stratigraphy  of  the  Keweenaw  Series 152 

Chapter  VI.— The  Keweenaw  an  Rocks  op  the  South  Shore  of  Lake  Superior 161 

Introductory 161 

Section  I.— Keweenaw  Point 163 

Section  II.— The  Region  between  Portage  Lake  and  the  Ontonagon  River 19S 

Section  III. — The  South  Range £01 

Section  IV. — The  Region  between  the  Ontonagon  River  and  Numakagon  Lake  of 

Wisconsin,  including  the  Porcupine  Mountains 206 

Section  V. — NoRTH^VESTERN  Wisconsin  and  the  adjoining  part  of  Minnesota 234 

Chapter  VII.— The  Keweenaw  an  Rocks  of  the  North  and  East  Shores  of  Lake  Supe- 
rior   260 

Introductory 260 

Section  I. — The  Minnesota  Coast 262 

Section  II. — Isle  Royale  to  Nipigon  Bay 329 

Section  III.— Michipicoten  Island  and  the  East  Coast  of  Lake  Superior 341 

Chapter  VIII.— Relations  of  the  Keweenaw  Series  to  the  Associated  Formations...  350 

Section  I. — To  the  Newer  Formations 351 

Section  II. — To  the  Older  Formations 367 

Chapter  IX. — Structure  of  the  Lake  Superior  Basin 410 

Chapter  X. — The  Copper  Deposits 419 

Notes 431 

IX 


ILLXJSTR^TIOIN^S. 


Pag«i 

Plate      I. — General  geological  map  of  the  Lake  Superior  Basin.    Scale,  Tirahm 24 

II. — Thin  sections  of  olivine-free  gabbros 35 

Fig.  1. — From  half  a  mile  southwest  of  south  point  of  Beaver  Bay,  Minn. 
N.  W.  i,  Sec.  3,  T.  55,  E.  8  W.  Anorthite;  non-diallagic 
augite.     Scale,  21  diameters. 

Fig.    2. — The  same  in  polarized  light.    Scale,  21  diameters. 

Fig.  3.— From  the  North  Shore,  near  Duluth,  Minn.  N.  W.  i,  Sec.  24,  T.  50, 
K.  14  W.  Anorthite;  diallagic  augite;  titaniferous  mag- 
netite ;  altered  olivine.    Scale,  30  diameters. 

Fig.    4. — ^The  same  in  polarized  light.     Scale,  30  diameters. 
in. — Thin  sections  of  olivine-gabbros 38 

Fig.  1.— From  bed  of  French  Eiver,Miun.  N.  line,  N.  E.  J,  Sec.  7.  T.  51,  E. 
12  W.     Anorthite;  diallage;  olivine.     Scale, 25 diameters. 

Fig.    2. — The  same  in  polarized  light.     Scale,  25  diameters. 

Fig.  3. — From  the  north  shore  of  Lake  Superior,  near  the  east  point  of 
Sucker  Eiver  Bay,  Minn.  S.  E.  i.  Sec.  2,  T.  51,  E.  12  W. 
Anorthite ;  diallage ;  titaniferous  magnetite ;  olivine.  Scale, 
25  diameters. 

Fig.    4. — Large  olivines  from  the  same  rock  as  represented  in  Figs.  1  and  2. 

Scale,  21  diameters. 

IV. — Thin  section  of  coarse  olivine-gabbro  from  Bladder  Lake,  Ashland  County, 

Wis.    Labradorito;  augite;  diallage;  olivine,  mostly  fresh, 

but  partly  altered  to  biotite,  viridite,  talc  and  iron  oxide; 

titaniferous  magnetite.     Scale,  10  diameters 40 

V. — Thin  sections  of  orthoclase-gabbro 50 

Fig.  1.— From  near  Lester  Eiver,  Minn.  N.  E.  i,  Sec.  29,  T.  51,  E.  13  W. 
Labradorite;  orthoclase;  augite  in  Iwinned  blades;  dial- 
lage ;  titaniferous  magnetite ;  brown  alteration-product  of 
a'lgite ;  secondaiy  quartz.     Scale,  20  diameters. 

Fig.  2. — Another  portion  of  the  same  section  in  polarized  light,  much 
enlarged.  Orthoclase;  labradorite;  twinned  augite;  net- 
work of  secondary  quartz.     Scale,  37  diameters. 

Fig.  3. — From  Bohemian  Mountain,  near  Lac  La  Belle,  Keweenaw  Point. 
N.  E.  i,  Sec.  32,  T.  58,  E.  29  \V.  Eed-stained  oligoclase 
and  orthoclase ;  diallage  altered  to  uraUte ;  titaniferous 
magnetite  and  gray  substance  from  ita  alteration;  large 
apatites.     Scale,  20  diameters. 

Fig.    4.— From  Brunschweiler's  Eiver,  Wis.    S.  E.  J,  See.  16,  T.  45,  E.  4  W. 
Labradorite ;  orthoclase ;   titaniferous  magnetite ;  augite ; 
secondary  quartz ;  apatite.    In  polarized  light.    Scale,  25 
diameters. 

XI 


-JSai  ILLUSTEATIONS. 


Page. 


Plate    VI. — Thin   section  of  very  coarse  ortboclase-gabbro,  from  near  Duluth,   Minn. 

N.  W.  i,  Sec.  28,  T.  50,  E.  14  W.  (1800  N.  2000  W.).  Oligo- 
claae ;  some  orthoclase  ;  diallage,  largely  altered  to  uralite, 
whicli  on  cross-section  shows  the  hornblende  cleavage; 
titauiferous  magnetite  mostly  surrounded  by  a  border  of 
brown  oxide  of  iron,  iu  which  are  developed  biotite  blades; 
very  large  apatites ;  a  little  chlorite ;  rare  pyrite  particles 
surrounded  by  an  ocherous  alteration.    Scale,  10  diameters.  53 

VII. — Thin  sections  of  hornblende-gabbro  and  of  anorthite  rock 56 

Fig.  1.— From  Ashland  County,  Wis.  N.  W.  cor.  Sec.  35,  T.  45,  E.  4  W. 
Labradorite ;  orthoclase ;  augite  partly  altered  to  viridite 
and  uralite;  brown  hornblende  in  and  away  from  the 
augito;  titaniferons  magnetite;  apatite.  Scale,  36  diame- 
ters. ,  . 
Fig.  2  —From  Ashland  County,  Wis.  N.  lino  of  N.  W.  i,  Sec.  17,  T.  44, 
E.  8  \V.  Oligoclase;  orthoclase;  brown  hornblende;  dial- 
lage  altered  to  green  uralite;  titaniferons  magnetite ;  rare 
quartz ;  very  abundant  and  large  apatites.  Scale,  26  diame- 
ters. 

Fig.  3.— From  English  Lake,  at  outlet,  S.  i,  Sec.  5,  T.  44,  E.  3  W.,  Wis. 
Brown  hornblende  grading  into  augite ;  labradorite  ;  mag- 
netite.   Scale,  a'.i  diameters. 

Fig    4. — Anorthite-rock  from  north  shore  of  Lake  Superior.     N.  i,  S.  W. 
i,  Sec.  5,  T.  54,  E.  8  W.    Scale,  17  diameters. 
VIII. — Thin  sections  of  pseudamygdaloidal  diabase  aud  diabase CI 

Fig.  1.— From  Union  Mine  "vein,"  N.  W.  i,  Sec.  27,  T.  51,  E.  42  W.  (1950 
N.  1750  W.),  Porcui)ine  Mountains,  Mich.  Oligoclase  or 
labradorite;  augite  wholly  altered  to  green  and  brown 
pseudomorphs ;  calciie;  chlorite  pseudamyjidules.  Scale, 
26  diameters. 

Fig.  2. — From  the  Fond  du  Lac  Mine,  Douglas  County,  Wis.  N.  E.  J, 
Sec.  8,  T.  47,  E.  14  W.  Oligoclase;  augite  altered  to  red- 
dish brown  pseudomorph  ;  magnetite;  pseudamygdules  of 
chlorite  and  epidote.    Scale,  28  diameters. 

Fig.  3.— From  near  the  Gogogashugun  Eiver,  Wis.  Sec.  16,  T.  46,  E.  2 
E.  Oligoclase,  largely  altered  to  chlorite;  magnetite  in 
rod-like  forms;  augite;  chlorite  pseudamygdules.  Scale, 
36  diameters. 

FlQ.  4. — Diabase  from  north  shore  of  Lake  Superior  below  mouth  of 
Split  Eook  Eiver.  S.  W.  i,  Sec.  5,  T.  54,  R.  8  W.  Labrador- 
ite, iu  small  tabular  crystals  arranged  in  a  common  direc- 
tion ;  augite,  including  many  labradorites  within  each  crys- 
tal ;  magnetite,  very  abundant  in  the  interspaces  of  the 
iingites.  Scale,  42  diameters. 
IX. — Thin  section  of  fine-grained  oliviue-diabase  or  melaphyr  from  the  north  shore 
of  Lake  Superior,  Minn.  S.  E.  i,  Sec.  9,  T.  51,  E.  12  W. 
Augite  in  large  areas,  including  many  plagioclaaes;  anor- 
thite; olivine  and  magnetite,  chiefly  in  the  interspaces  of 
theaugites;  chlorite  pseudamygdules.     Scale,  20  diameters  -  68 

X. — Thin  sections  of  diabase-porphyrite  and  ashbed-diabase 77 

Fig.  1. — Diabase-ijorphyrite  from  the  southeast  shore  of  Michipicoten 
Island.  Base  of  red  and  brown  stained  unindividnalized 
material,  minute  plagioclases  and  black  magnetite.  Por- 
phyritic  labradorite  and  altered  augite.  Scale,  7  diameters. 
Fig.  2. — Part  of  the  same  section  enlarged  and  as  seen  between  the 
crossed  nicols.     Scale,  21  diameters. 


ILLUSTRATIONS.  xiit 

Plate  X— Continued. 

Page. 
Fig.    3. — Diabase-porpbyrite  from  Duliith,  Minn.,  bed  of  Brewery  Creek. 

S.  E.  i,  Sec.  22,  T.  50,  R.  14  W.  Porphyritic  orthoclase  and 
oligoolase,  in  a  base  consisting  of  plagioclase,  magnetite, 
augite,  and  unindividnalized  snbstance.  Scale,  14|  diam. 
Fig.  4. — Ashbed-diabase  from  Frog  Creek,  Douglas  County,  Wis.  S.  W. 
i,  Sec.  iiS,  T.  42,  E.  II  W.  P]agioclase;  augite  in  grains; 
magnetite.  Scale,  CO  diameters. 
XI.— Thin  section  of  amygdaloid  from  Ike  bay  above  tke  Great  Palisades  on  tke 
Minneeeta  coast.  S.  W.  i,  Sec.  22,  T.  56,  E.  7  W.  Unindi- 
vidnalized ferritic  base  witbplagioclases;  gas  cavities  filled 
^vitb  fragniental  material  from  above,  and  containing  cal- 
cite  deposited  aftertkefragmental material.  Scale,  24 diam.  87 

XII. — Tkiu  sections  of  quartzii'erons  porphyry 95 • 

Fig.  1. — From  Torek  Lake  Railroad,  Keweenaw  Point.  Sec.  36,  T.  56, 
R.  33  W.  Matrix  witk  particles  skewing  flowage;  ortko- 
clase;  ronnded  qnartz  crystals.     Scale,  15  diameters. 

Fig.  2. — From  a  pebble  of  tke  Calumet  conglomerate,  Calumet  Mine,  Ke- 
weenaw Point,  Mick.  Matrix ;  porpkyritic  crystals  of 
orthoclase  and  quartz.     Scale,  12  diameters. 

Fig.  3. — From  the  Great  Palisades  of  tke  nortk  shore  of  Lake  Superior, 
N.  E.  i.  Sec.  22,  T.  56,  E.  7  W.,  Minn.  Matrix  showing  flow- 
age  ;  porphyritic  orthoclase  an<l  quartz.  Scale,  9  diame- 
ters. 

Fig.    4. — Part  of   tke  same  section   enlarged,   skewing   flowage  lines. 
Scale,  32  diameters. 
XIII. — Tkin  sections  of  quartziferous  porpkyry : 10I> 

Figs.  1, 2,  3,  4,  and  7  all  represent  quartzes  in  the  quartziferous  porphyry 
of  Bead  Island,  at  the  mouth  of  Nipigon  Straits  on  the 
north  shore  of  Lake  Superior,  Ontario,  Canada.  Scale,  26 
diameters. 

Fig.  5. — Quartz  in  quartziferous  porphyry  Iroiu  east  shore  of  Michipiooten 
Island.      Scale,  26  diameters. 

Fig.  6. — Quartz  in  quartziferous  jiorphyry  from  Baptism  River  Point, 
north  shore  of  Lake  Superior,  Minn.  S.  E.  i,  Sec.  11,  T.  56, 
R.  7  W.      Scale,  26  diameters. 

Fig.  8. — Glass  inclusions  in  the  quartz  of  Fig.  6  greatly  enlarged,  show- 
ing hair-like  devitriUcation  and  black  hexagonal  crystals. 
Scale,  400  diameters. 

Fig.    9. — Banded  quartz-porphyry  from  south  side  of  Great  Palisades. 

S.  W.  i,  Sec.  22,  T.  56,  R.  7  W.     Drawn  irom  the  polished 

surface  of  ii  hand  specimen.     The  quartiies  appear  as  black 

spots.    Natural  scale. 

Fig.  10. — Thin  section  of  porpkyry  skown  in  Fig.  9,  skewing  flowage 

banding.     Scale,  11  diameters. 
Fig.  11. — Portion  of  tke  same  section  further  enlarged.     Scale,  33  diam.. 
Fig.  12. — Portion  of  the  same  section  enlarged.     Scale,  33  diameters. 
Fig.  13. — Purple  felsitie  porpkyry  from  the  bed  of  Carp  River,  Porcupine 
Mountains,  Mick.     N.  W.J,  See.  20,  T.  51,  R.  42  W.     Base 
of  unindividnalized  ferrite-bearing  substance  charged  with, 
secondary  quartz  ;  porpkyritic  ortkoclase.     Scale,  22  diam. 
Fig.  14. — The  same  section  in  polarized  light.     Scale,  22  diameters. 
Fig.  15. — Pink  felsite  from  north  shore  of  Lake  Superior,  S.  W.  J,  Sec. 
28,  T.  56,  E.  7  W.,  Minn.     Cloudy  base  completely  saturated 
with  unusually  coarse  networked  secondary  quartz;  ferrito 
particles  and  strings  of  i)articles.     Scale,  2  diameters. 
Fig.  16. — Part  of  the  same  section  further  enlarged.     Scale,  20  diameters.- 


.31T       .  ILLUSTEATIOKS.  

Page. 
>  Plate        XIV. — Thin  sections  of  granitic  porphyry  or  granitell 112 

Fig.  1.— From  Eagle  Mountain,  T.  63,  R.  2  W,,  Minn.  Orthoclase  crys- 
tals saturated  with  corrosion  quartz.     Scale,  35  diameters. 

Pig.  2. — The  same  section  in  polarized  light,  showing  how  numbers  of 
adjacent  quartz  particles  polarize  together.  Scale,  35 
diameters. 

Fig.  3. — From  vein  in  gabbro  of  Eice  Point  Quarry,  Duluth,  Minn. 
Orthoclase  decomposed,  reddened  by  iron  oxide,  and  filled 
with  secondary  quartz,  which  is  largely  in  regularly  out- 
lined forms  Uke  the  quartz  of  "graphic  granite."  Scale, 
25  diameters. 

Fig.    4. — From  area  of  red  rock  in  the  Duluth  gabbro.     Drawn  with  the 
polarized  light  to  show  the  way  in  which  the  larger  quartz 
areas  and  smaller  adjoining  ones  iiolarize  together.    Scale, 
25  diameters. 
XV. — Thin  sections  of  augite-syenite,  and  granitell  or  granitic  porphyry 115 

Fig.  1. — Uralitic  augite-syenite,  from  area  of  red  rock  within  the  coarse 
gabbro  of  Duluth,  N.  W.  J,  Sec.  27,  T.  50,  R.  14  W.,  Minn. 
Orthoclase  and  oligoclase  saturated  with  secondary  quartz, 
which  forms  some  large  areas;  greenish  uralite;  chlorite 
as  an  alteration  of  the  feldspars;  magnetite.  Scale,  20 
diameters. 

Fig.  2. — Angitic  granitell  from  the  north  shoie  of  Lake  Superior,  N.  E.  J, 
Sec.  32,  T.  56,  R.  7  W.,  Minn.  Oligoclase  and  orthoclase 
in  large  altered  crystals ;  quartz  in  large  areas,  possibly 
primary;  augite,  mostly  altered  to  a  greenish  substance, 
occurring  in  clusters  between  the  large  feldspars  along  with 
smaller  feldspar  particles. 

Fig.  3. — Red  granitic  porphyry  from  the  old  Ironton  trail,  Ashland 
County,  Wis.  Sec.  34,  T.  46,  R.  1  W.  Reddened  feldspar 
crystals  saturated  with  graphically  arranged  secondary 
quartz  ;  magnetite  ;  greenish  chloritio  alteration-product. 
Scale,  27  diameters. 

Fig.  4. — Augite-syenite  pebble  from  conglomerate  at  south  foot  of  Mount 
Bohemia,  north  side  Lac  La  Belle,  Keweenaw  Point,  N.  E.  J, 
Sec.  32,  T.  58,  R.  29  W.,  Mich.  Orthoclase  (twinned)  and 
labradorite  saturated  with  secondary  quartz,  both  graph- 
ically arranged,  and  in  exceedingly  fine  lines  along  the 
cleavage  directions;  augite  altered  to  ferrite.  Scale,  27 
diameters. 
XVI. — Thin  sections  of  sandstones 127 

Fig.  1.— The  "Nonesuch"  copper-bearing  sandstone,  from  the  Nonesuch 
mine,  S.  E.  },  Sec.  1,  T.  51,  R.  43  W.,  Mich.  Diabasic  detri- 
tus; porphyry  detritus;  abundant  quartz  fragments  evi- 
dently derived  from  the  quartzes  of  quartz-porphyry; 
magnetite;  native  copper  indicated  by  the  deep  red  color; 
one  large  pebble  of  a  quartziferous  porphyry  with  a  char- 
acteristic quartz-saturated  matrix.     Scale,  16  diameters. 

Pig.  2. — Sandstone  from  the  bed  of  Montreal  River,  N.  E.  J,  Sec.  20,  T. 
47,  R.  1  E.,  Wis.  Diabasic  detritus;  porphyry  detritus; 
rare  quartzes;  calcite  matrix.     Scale,  27  diameters. 

Pig.  3. — Sandstone  from  near  the  Copjier  Falls  Mine,  Keweenaw  Point, 
Mich.  Fragments  from  the  matrix  and  porphyritic  feld- 
spars of  a  quartziferous  porphyry  and  from  an  augite- 
syenite,  with  rarer  quartz  fragments,  magnetite  particles 
and  fragments  of  amygdaloid  matrix;  calcite  cement. 
Scale,  37  diameters. 


ILLUSTRATIONS.  XV 

Pajie. 
Plate    XVI. — Continued. 

Fig.  4. — Sandstone  from  the  Calumet  Conglomerate,  Calumet  and  Hecla 
Mine,  Keweenaw  Point.  Detritus  of  quartziferous  por- 
phyry, consisting  of  feldspar  and  quartz  fragments  and 
fragments  of  matrices;  magnetite  particles;  rather  largo 
particles  of  basic  rocks;  native  copper.  Scale,  28  diame- 
ters. 

XVII. — Geological  map  of  Keweenaw  Point,  Mich 1G2 

XVIII. — Geological  sections  of  Keweenaw  Point,  Mich 166 

XIX.^Geological  map  of  the  Porcupine  Mountains,  Mich 208 

XX. — Geological  section  illustrating  the  structure  of  the  Porcupine  Mountains, 

Mich 210 

XXI. — Geological  sections  illustrating  the  structure  of  the  Porcupine  Mountains, 

Mich 214 

XXII. — Geological  map  of  the  region  between  the  Ontonagon  Eiver,  Mich.,  and  Nu- 

makagon  Lake,  Wis 224 

XXIII. — Generalized  geological  sections  of  the  region  between  Portage  Lake,  Mich., 

and  the  Saint  Croix  Eiver,  Wis 234 

XXIV. — Map  showing  the  positions  of  the  exposures  of  Keweenawan  rocks  and  Pots- 
dam sandstone  along  the  lower  portions  of  Snake  and  Kettle 

Kivers,  Minn 241 

XXV. — Map  showing  rock  exposures  in  tbe  vicinity  of  the  Upper  Saint  Croix  Eiver, 

Wis 246 

XXVI.— Geological  map  of  the  northwest  coast  of  Lake  Superior 262 

XXVII. — Geological  map  of  Isle  Eoy ale  and  the  neighboring  mainland 329 

XXVIII.— Map  of  the  Lake  Superior  Basin,  designed  to  show  the  structure  and  extent 

of  the  Keweenawan  synclinal 410 

XXIX. — Generalized  geological  sections  of  the  Lake  Superior  Basin 416 

Fig.    1. — Outlines  of  coast  hills  for  20  miles  above  Grand  Marais 142 

Fig.    2. — Surface  contour  and  arrangement  of  beds  at  Eagle  Harbor.     Eepresents  a  distance  of 

about  200  feet 178 

Pig.    3. — Map  of  exposures  in  the  vicinity  of  west  branch  of  Ontonagon  River,  T.  46,  E.  41  W., 

Mich 204 

Fig.    4. — Map  of  exposures  on  upper  Carp  Eiver,  Porcupine  Mountains.     One  inch  =  90  paces.  215 

Fig.    5. — Cross-section  on  line  C  D  of  Fig.  4 216 

Fig.    6.— Section  online  A  B  of  Fig.  4 216 

Fig.  7. — Showing  unconformable  contact  between  Keweenawan  diabase-porphyry  and  Pots- 
dam sandstone  at  Taylor's  Falls,  Minn. ;  after  Chamberlin 237 

Fig.    8. — Map  of  exposures  on  Kettle  Eiver,  Sec.  22,  T.  41,  E.20  W.,  Minn.     Scale,  4  inches  to 

themile 245 

Fig.    9. — Section  on  line  AB  of  Fig.  8.     Scale  horizontal,  8  inches  to  the  mile;  vertical  300 

feet  to  the  inch 245 

T"iG.  10. — Section  on  the  gorge  of  Black  Eiver,  Douglas  County,  Wis 253 

T'lG.  11. — Section  in  the  gorge  of  Cojiper  Creek,  Douglas  County,  Wis 255 

Fig.  12. — Sketch  of  a  clift'-side  near  Agate  Bay,  Minn.,  illustrating  the  warped  bedding  of 

layers.     The  dip  is  toward  the  observer 290 

Fig.  13.— Sandstone  ''veins,"  Minnesota  coast — plan 292 

Fig.  14. — Sketch  of  cliff  on  Minnesota  coast,  showing  penetration  of  fissures  of  amygdaloid  by 

sandstone , 293 

Pig.  15. — Dike,  two  miles  below  Lester  Eiver,  Minnesota  coast 294 

Fig.  16. — Generalized  section  of  Portage  Bay  Island,  Minnesota  coast 297 

Fig.  17. — Sketch-map  of  rock  exposures  near  Split  Eock  Eiver,  Minnesota  coast,  T.  54,  E.  8  W.  300 
Fig.  18. — Sketch-map  of  rock  exposures  in  the  vicinity  of  Beaver  Bay,  Minnesota  coasi,  T.  55, 

E.  8W 305 


xvi  ILLUSTRATIONS. 

Page 

Fig.  19. — Section  on  Beayer  River,  Minn 308> 

Fig.  20. — Sketcli  map  of  exposures,  Minnesota  coast.  Sec.  32,  T.  56,  E.  7  W 310' 

Fig.  21.— Section  of  wall  DE  of  Fig.  20 311 

Fig.  22. — Flowage  structure  in  felsite 313. 

Fig.  23. — Sketch-map  of  rock  exposures  in  the  vicinity  of  Great  Palisades  and  Baptism  River, 

Minnesota  coast ---  315 

Fig.  24. — Section  on  south  cliff  of  the  Great  Palisades,  Minnesota  coast 316 

Fig.  25. — Dike  in  quartz-porphyry,  Red  Rock  Bay 323. 

Fig.  26. — Profile  of  island  at  mouth  of  Two  Islands  River 325 

Fig.  27. — Section  on  Temperance  River 325. 

Fig.  28. — Section  on  Minnesota  coast,  near  Temperance  River 326 

Fig.  29. — Section  on  Minnesota  coast,  near  Baptism  River - 326 

Fig.  30.— Showing  relation  of  the  Eastern  Sandstone  and  Keweenawan  melaphyr,  Bete  Grise 

Bay.     Length  of  section,  about  150  feet 353. 

Fig.  31. — Section  on  the  Hungarian  River,  Keweenaw  Point 355 

Fig.  32. — Section  showing  relation  of  Eastern  Sandstone  to  Keweenawan  diabase,  T.  50,  R.  29 

W.,Mich 359 

Fig.  33. — Bed  of  intrusive  diabase  in  Animikie  slates.  Pigeon  Bay,  Canada 373 

Fig.  34.— Generalized  section,  showing  hypothetical  relation  of  the  Animikie  Group  to  the 

Vermillion  Lake  schists 399 

Fig.  35.— Outline  geological  map  of  the  Nipigon  Lake  region,  after  a  manuscript  by  R.  Bell ..  408. 

Fig.  3d. — Hypothetical  section  of  Lake  Superior  Basin 417 

Fig.  37.— Cross-sections  of  gorge  of  Black  River,  Douglas  County,  Wis 442. 


THE    COPPER-BEARING    ROOKS    OF    LAKE 

SUPERIOR. 


BY  R.  D.  IRTING. 


CHAPTER      I. 

INTRODUCTORY. 

Aim  of  tbiB  memoir.— Obstacles  to  the  earlier  accomplishmeut  of  this  aim.— Investigations  necessary  to 
it.— Time  allotted  to  these  investigations.— Plan  of  field  wort.— Special  areas  studied  by  the  sev- 
eral parties.— Acknowledgments  of  outside  assistance.- Explanation  of  the  irregularities  in  the 
amount  of  detail  used  in  this  memoir.- Previous  examinations  and  accounts  of  the  copper-bearing 
rocks  of  Lake  Superior.— List  of  works  from  which  facts  have  been  drawn  for  this  memoir.— 
General  nature  of  the  information  drawn  from  each.— Account  of  different  theoretical  views  as 
to  the  origin  and  geologicalrelationsof  the  copper-bearing  rocks.— Different  views  as  to  the  origin 
of  the  conglomerates;  of  the  "traps"  and  amygdaloids;  of  the  felsitic  rocks.- Different  views 
as  to  the  geological  relations  of  the  copper-bearing  rocks.— Use  of  the  term  "Keweenawan."— These 
conflicting  views  have  arisen  in  large  measure  from  the  small  scope  of  the  examinations  of  any 
one  geologist. — Literature  of  the  subject. 

This  memoir  aims  at  a  general  exposition  of  tlie  nature,  structure, 
and  extent  of  the  series  of  rocks  in  which  occurs  the  well-known  native 
copper  of  Lake  Superior. 

This  is  a  work  which  has  never  been  attempted  before,  nor  could  it 
have  been  accomplished  sooner.  A  number  of  geologists  have  written  on 
different  portions  of  the  Lake  Superior  basin  during  the  past  fifty  years, 
but  until  very  recently  any  attempt  to  compile  a  general  account  of  the 
copper-bearing  rocks  would  have  met  with  some  insuperable  obstacles.  Not 
to  speak  of  the  difficulties  to  be  encountered  in  trying  to  reconcile  the  con- 
flicting statements  of  different  authors,  any  one  making  the  attempt  would 
have  met  the  two  serious  obstacles  of  nearly  complete  ignorance  as  to  the 
nature  of  the  crystalline  rocks  which  form  so  large  a  part  of  the  series,  and 
complete  ignorance  as  to  the  distribution  and  structural  relations  of  the 

(1) 


2         COPPEE-BEARING  EOCKS  OF  LAKE  SUPEEIOE. 

formations  of  the  western  end  of  the  Lake  Superior  Basin,  both  on  the  Minne- 
sota and  Wisconsin  sides.  The  first  of  these  obstacles  was  in  part  removed 
by  the  microscopic  studies  of  Professor  Pumpelly,  whose  conclusions  as  to 
the  nature  of  the  Keweenaw  Point  rocks  were  published  in  1878.'  The 
second  was  also  in  part  removed  by  the  investigations  of  the  Wisconsin 
Geological  Survey,  whose  results  as  to  that  portion  of  Wisconsin  which  bor- 
ders on  Lake  Superior  were  first  published  in  1880.^  There  still  remained 
unknown,  however,  a  wide  extent  of  the  series  in  Minnesota,  a  most  import- 
ant omission,  since  in  this  region  only  is  it  possible  to  connect  the  rocks  of 
the  South  and  North  Shores.  The  rocks  of  the  southern  range  of  Keweenaw 
Point  were  also  still  very  vaguely  known,  and  the  structure  of  the  Porcu- 
pine Mountains  had  never  yet  been  made  out. 

It  thus  became  necessary  for  any  one  attempting  to  present  anything  like 
a  general  account  of  the  series  to  cover  these  gaps,  and  extend  the  micro- 
scopic investigations  begun  by  Professor  Pumpelly,  so  far  as  possible,  over 
the  whole  extent  of  the  series.  Tt  was  also  necessary  that  he  sliould  famil- 
iarize himself  as  much  as  possible  with  those  districts  which  had  already 
been  more  or  less  thoroughly  worked  up,  by  studying  the  descriptions  of 
others  on  the  ground.  This  work,  including  also  the  preparation  of  this 
memoir,  I  have  undertaken  to  accomplish  during  the  months  between  Jidy, 
1880,  and  March,  1882 — short  enough  a  time,  especially  when  a  large  share 
of  it  has  been  given  to  other  work.  I  was,  howevei-,  already  quite  familiar 
with  the  series  as  developed  in  Wisconsin,  with  the  microscopic  characters 
of  its  rocks,  so  far  as  previously  known,  and  with  the  literature  of  the  sub- 
ject; so  that,  with  the  aid  of  several  assistants,  I  have  been  enabled  to 
accomplish  the  main  objects  aimed  at.  My  chief  regret  in  connection  with 
the  field  work,  which  was  of  necessity  confined  to  the  season  of  1880,  has 
been  my  inability,  on  account  of  lack  of  time,  to  visit  Isle  Royale,  the  region 
about  Lake  Nipigon,  and  Michipicoten  Island.  The  full  descriptions  of 
these  places,  given  respectively  by  Foster  and  Whitney,  Bell  and  Macfarlane, 
together  with  a  type  suite  of  specimens  collected  by  the  latter  gentleman  at 

1  "Metasomatic  Development  of  tbe  Copper-Bearing  Eocks  of  Lake  Superior."  Proo.  Am.  Acad., 
1878,  XIII,  253-309. 

^E.  D.  Irving,  E.  Pumpelly,  E.  T.  Sweet,  T.  C.  Chamberlin,  .and  M.  Strong,  in  the  Geology  of 
WiBconsiu,  Vol.  III. 


PLAN  OF  FIELD  WORK. 


Michipicoten,  and  kindly  sent  me  by  the  Director  of  the  Geological  Survey 
of  Canada,  help  to  supply  this  deficiency  in  some  measure. 

My  plan  of  field  work,  as  carried  out,  included:  (1)  a  study  of  the 
north  shore  of  the  lake  from  Duluth  to  Nipigon  Bay,  including  excur- 
sions up  the  principal  streams  for  distances  of  from  five  to  twenty  miles ; 
(2)  a  trip  from  Grand  Marais,  on  the  North  Shore,  northward  and  westward 
to  the  headwaters  of  Cascade  and  BruM  Rivers,  including  an  examination 
of  the  country  about  BruM  Lake  and  Eagle  Mountain,  a  region  never  be- 
fore visited  by  any  geologist ;  (3)  a  very  rapid  examination  of  the  east 
coast  from  the  Sault  to  Mamainse ;  (4)  a  study  of  the  well-known  mining 
region  of  Keweenaw  Point,  including  the  little-known  district  in  the 
vicinity  of  Mount  Houghton ;  (5)  a  comparatively  thorough  examination 
of  the  Porcupine  Mountains,  a  knowledge  of  whose  hitherto  unknown 
structure  was  necessary  to  the  understanding  of  that  of  the  entire  lake 
basin;  {6)  an  examination  of  Silver  Mountain,  southwest  from  L'Anse; 
(7)  a  rapid  exploration  of  the  "South  Copper  Range,"  eastward  from  Lake 
Agogebic;  (8)  an  examination  of  the  valleys  of  the  Kettle  and  Snake 
rivers  in  Minnesota,  with  the  view  of  determining  the  extent  of  the  copper- 
bearing  rocks  in  that  direction,  and  the  manner  in  which  they  terminate 
westward;  (9)  an  examination  of  the  valley  of  the  Cloquet  River  to  its 
junction  with  the  Saint  Louis,  and  of  the  latter  stream  from  this  junction 
to  its  mouth. 

Such  extended  investigations  could  not  be  all  carried  out  in  person 
in  one  season.  My  own  immediate  attention  was  given  to  the  north 
shore  of  the  lake  from  Duluth  to  Nipigon  Bay,  to  the  region  of  Portage 
Lake  and  Mount  Houghton,  and  to  the  region  south  of  Ontonagon.  In 
the  other  work  I  was  aided  by  Messrs.  W.  M.  Chauvenet,  A.  C.  Camp- 
bell, R.  McKinlay,  L.  G.  Emerson,  and  B.  N.  White.  Messrs.  McKinlay 
and  Campbell  made  the  river  trips  between  Duluth  and  Grand  Marais; 
Messrs.  Chauvenet  and  McKinlay  together  examined  the  country  from 
Grand  Marais  to  Brul^  Lake,  and  the  valleys  of  the  Cloquet  and  Saint 
Louis  Rivers;  Mr.  McKinlay  alone  examined  the  valleys  of  the  Snake  and 
Kettle  Rivers;  Mr.  A.  C.  Campbell  made  the  trip  from  the  Sault  to  Ma- 
mainse; Messrs.  Chauvenet  and  White  together  studied  the  eastern  half  of 


4  COPPEE-BEAEING  EOOKS  OF  LAKE  SUPBEIOE. 

the  Porcupine  Mountains,  while  Messrs.  Campbell  and  McKinlay  took  the 
western  part. 

Besides  the  aid  received  from  those  directly  connected  with  the  work, 
I  have  to  thank  for  assistance  also  a  number  of  others,  and  especially  the 
following  gentlemen:  Professor  T.  C  Chamberlin,  for  notes  of  observations 
on  Snake  River,  Minnesota;  Professor  A.  H.Chester,  of  Hamilton  College, 
Clinton,  N.  Y.,  for  a  series  of  notes,  accompanied  by  specimens,  on  the  rocks 
of  the  Vermillion  Lake  region,  and  of  the  north  shore  of  Nipigon  Bay; 
A.  R.  C.  Selwyn,  Director  of  the  Geological  Survey  of  Canada,  for  informa- 
tion and  for  a  suite  of  specimens  illustrating  Macfarlane's  reports  on  Michipi- 
coten  Island;  Professor  N.  H.  Winchell,  State  Greologist  of  Minnesota,  for 
loan  of  township  plats  of  the  Minnesota  coast,  and  for  information  with 
regard  to  the  back  country  between  Pigeon  River  and  the  lake  shore;  Frank 
Klapetko,  of  the  Delaware  Mine,  Keweenaw  Point,  for  measurements  made 
in  the  vicinity  of  Lac  La  Belle;  Messrs.  John  Chassells  and  J.  R.  Devereux, 
of  Houghton,  for  sundry  favors ;  B.  C.  Chynoweth,  agent  of  the  Mass  mine, 
Ontonagon,  for  assistance  in  that  region;  and  B.  N.  White,  of  Ontonagon, 
for  information  as  to  the  course  of  the  slate  belt  of  the  Porcupine  Moun- 
tains to  the  eastward.  Mr.  White  had  traced  out  this  belt  to  some  distance 
east  of  the  Ontonagon,  and  kindly  gave  to  me  all  the  results  of  his  labor. 

The  combination  of  the  results  of  previous  work  and  of  new  original 
observations,  by  which  this  memoir  must  be  made  up,  have  compelled  great 
irregularity  in  the  amount  of  detail  given  in  describing  the  different  dis- 
tricts. So  far  as  the  statements  are  based  upon  older  work,  detail  has  been 
generally  omitted,  but  whenever  I  have  felt  it  necessary  to  differ  from  pre- 
vious writei's,  and  in  all  new  descriptions,  I  have  used  detail  freely. 

The  list  of  geologists  who  have  during  the  last  fifty  years,  from  time  to 
time,  written  on  the  group  of  rocks  which  forms  the  subject  of  this  memoir 
is  a  long  one.  Omitting  the  names  of  those  whose  writings  have  not  been 
based  on  personal  examinations,  or  have  been  preceded  only  by  very 
slight  examination,  or  are  obviously  unworthy  of  notice  from  lack  of  geo- 
logical knowledge  on  the  part  of  the  authors,  I  may  mention  the  fol- 


EAELIER  WRITERS. 


lowing  geologists  as  having  written  more  or  less  copiously  upon  this 
subject,  the  dates  of  the  first  and  last  publications  being  added  in  each 
case:  Bigsby,  1824-1852;  Bayfield,  1829;  Houghton,  1831-1844;  Jackson, 
1845-1869;  Hubbard,  1846-1850;  Logan,  1846-1866;  D.  D.  Owen, 
1847-1852;  Foster  and  Whitney,  1849-1861;  Louis  Agassiz,  1850-1859; 
Marcou,  1850-1859;  Norwood,  1852;  Whittlesey,  1852-1877;  Rivot, 
1855-1856;  Hunt,  1861-1878;  Macfarlane,  1866-1889;  Alexander  Agas- 
siz, 1867;  Bell,  1869-1875;  Pumpelly,  1872-1880;  Marvine,  1873;  Brooks, 
1873-1876;  Rominger,  1873-1876;  Irving,  1874-1880;  Sweet,  1875-1880; 
Chamberlin  and  Strong,  1880;  N.  H.  Winchell,  1879-1881;  and  Wads- 
worth,  1880.  The  preparation  of  anything  like  a  satisfactory  account  of 
the  explorations  upon  which  the  numerous  writings  of  these  geologists  have 
been  based,  or  of  the  writings  themselves,  would  have  taken  away  more 
time  than  I  could  aff'ord  to  lose  from  the  original  studies  necessary  to  my 
own  work,  and  I  therefore  have  not  attempted  it. 

I  have,  of  course,  famiharized  myself  with  these  writings,  biit  the  pro- 
portion of  them  from  which  I  have  drawn  facts  for  incorporation  into  this 
memoir  is  a  small  one.  I  name  them  here  in  order  of  time  of  pubHcation: 
J.  W.  Foster's  and  J.  D.  Whitney's  joint  "Report  on  the  Geology  and 
Topography  of  a  Portion  of  the  Lake  Superior  Land  District  in  the  State 
of  Michigan,  Part  I,  Copper  Lands,"  made  to  the  Commissioner  of  the  Gen- 
eral Land  Office,  1850;  Sir  W.  E.  Logan's  "Geology  of  Canada,"  1863; 
Thomas  Macfarlane's,  "Report  on  Lake  Superior,"  in  the  Report  of  Prog- 
ress of  the  Geological  Survey  of  Canada  for  18fi3-'66;  the  same  gentle- 
man's paper  "On  the  Geology  and  Silver  Ore  of  Wood's  Location,  Thunder 
Cape,  Lake  Superior,"  pubhshed  in  the  Canadian  Naturahst  for  1869; 
Robert  Bell's  reports  on  the  regions  north  and  east  of  Lake  Superior, 
published  in  the  Report  of  Progress  of  the  Geological  Survey  of  Canada 
for  the  years  1866-'69,  1870-'71,  and  1872-'73;  the  report  of  R.  Pumpelly 
and  A.  R.  Marvine  on  the  copper-bearing  rocks  of  Keweenaw  Point,  in 
the  Geological  Survey  of  Michigan,  Vol.  I,  1869-'73,  Part  II;  Jl.  Pum- 
pelly's  paper  on  the  "Metasomatic  Development  of  the  Copper-Bearing 
Rocks  of  Lake  Superior,"  published  in  the  Proceedings  of  the  American 
Academy  of  Arts  and  Sciences,  Vol.  XIII,  p.  268,  1878,  and  "Lithology 


6         COPPEE-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 

of  the  Keweenawan  System,"  published  in  Vol.  Ill  of  the  Geology  of  Wis- 
consin, 1880;  my  own  reports  on  the  "  Greological  Structure  of  Northern 
Wisconsin"  and  "Geology  of  the  Eastern  Lake  Superior  District"  of  Wis- 
consin, both  in  Vol.  Ill,  Geology  of  Wisconsin,  1880;  E.  T.  Sweet's 
report  on  the  "Geology  of  the  Western  Lake  Superior  District,"  in  the 
same  volume;  T.  C.  Chamberlin's  and  M.  Strong's  joint  report  on  the 
"Geology  of  the  Upper  St.  Croix  District,"  also  in  the  same  volume;  and  N. 
H.  Winchell's  "Ninth  Annual  Report  of  the  Geological  and  Natural  History 
Survey  of  Minnesota,"  1880. 

In  every  case  where  I  have  taken  information  from  these  works  I  have 
indicated  it  by  direct  reference.  It  will,  perhaps,  be  well  to  give  a  gen- 
eral statement  here  as  to  the  extent  of  my  indebtedness  to  each  of  them. 
Foster  and  Whitney's  work  applies  solely  to  that  portion  of  the  copper- 
bearing  series  which  is  included  within  the  state  of  Michigan,  i.  e.,  to  the 
region  lying  between  the  eastern  extremity  of  Keweenaw  Point  and  the 
Montreal  Eiver,  and  to  Isle  Royale.  For  my  statements  as  to  Isle  Royale  I 
have  had  to  depend  almost  exclusively  upon  this  work,  read  in  the  light 
of  my  own  experience.  The  former  district  I  have  myself  examined,  and 
have  only  used  Foster  and  Whitney's  statements,  chiefly  to  aid  in  mapping, 
as  supplementary  to  my  own  observations.  Logan,  in  his  geology  of 
Canada,  describes  briefly  all  of  the  I'ocks  of  the  north  and  east  shores  of 
Lake  Superior  from  the  Minnesota  boundary  to  the  Sault.  I  have  only 
drawn  from  him,  however,  for  the  east  coast,  Michipicoten,  and  the  islands 
south  of  Nipigon  Bay.  Macfarlane's  "  Report  on  Lake  Superior"  includes 
a  further  description  of  Michipicoten  Island  and  the  east  coast,  having  been 
based  on  more  detailed  observation  than  that  of  Logan.  I  have  drawn 
somewhat  from  this  report,  but  especially  have  used  it  for  locating  the  series 
of  Michipicoten  rocks  sent  me  by  Mr.  A.  R.  C.  Selwyn,  and  which  I  de- 
scribe microscopicall}^  in  Chapters  III  and  VII.  The  same  writer's  account 
of  the  Geology  of  Wood's  Location  has  aided  me  in  reading  rapidly  the 
structure  of  the  peninsula  between  Black  and  Thunder  bays,  though  I 
differ  from'  him  in  some  of  his  most  important  conclusions.  Dr.  Bell's  sev- 
eral reports  on  the  geology  of  that  portion  of  Canada  north  and  east  of 
Lake  Superior,  including  a  manuscript  map  based  on  them,  have  been 


FOEMER  VIEWS  AS  TO  ORIGIN  OF  CONGLOMERATES.  7 

drawn  upon  liberally  for  accounts  of  those  regions  I  have  not  myself  seen, 
more  especially  of  the  Nipigon  Lake  basin,  and  for  data  for  mapping 
away  from  the  lake  shore,  although  I  have  here  again  departed  from  the  au- 
thor in  some  important  points.  The  descriptions  and  maps  of  Messrs. 
Pumpelly  and  Marvine,  in  the  first  volume  of  the  Geological  Survey  of 
Michigan,  have  furnished  a  large  part  of  the  material  from  which  I  have 
made  up  my  account  of  the  structure  of  Keweenaw  Point.  Pumpelly 's 
microscopic  descriptions  of  the  "  trap "  and  amygdaloid  of  Keweenaw 
Point  have  formed  the  basis  for  my  own  microscopic  studies,  whose 
results  I  give  especially  in  ChajDter  III,  in  which  I  have  occasion  to  quote 
freely  from  his  memoirs  above  mentioned.  The  general  reports  on  the 
geology  of  Northern  Wisconsin  included  in  the  third  volume  of  the  Geology 
of  Wisconsin,  by  Messrs.  Chamberlin,  Strong,  Sweet,  and  myself,  have  sup- 
plied all  of  the  material  for  the  description  of  that  region  which  I  shall 
give  in  Chapter  VI.  Upon  N.  H.  Winchell's  reports  I  have  depended  for  a 
few  locations  of  rocks  in  the  country  back  of  the  Minnesota  coast  and 
along  the  national  boundary  line. 

It  is  desirable  that  I  should  give  here  a  brief  account  of  the  different 
views  that  have  been  held  as  to  the  origin  and  relations  of  the  copper-bear- 
ing rocks,  although  this  will  involve  reference  to  a  number  of  points  whose 
explanation  must  be  deferred  to  subsequent  pages.  This  series  of  rocks 
is  described  by  all  writers  as  made  up  of  reddish  sandstones,  conglomerates, 
"traps,"  and  amygdaloids,  while  a  few  of  the  authorities  recognize  also  the 
local  development  of  reddish  felsitic  or  "jasper ''-like  rocks.  As  to  the  ori- 
gin of  these  several  kinds  of  rocks,  however,  as  to  their  mutual  structural 
relations,  and  as  to  their  position  in  the  geological  scale,  there  has  been 
the  widest  divergence  of  views. 

The  sandstones  of  the  series  appear  to  have  been  taken  by  all  as  of  the 
usual  sedimentary  origin,  but  the  conglomerates  into  which  they  shade  have 
been  regarded  both  as  eruptive  and  as  sedimentary.     Messrs.  Foster  and 
Whitney,  whose  work,  having  been  for  thirty  years  the  most  widely  recog-  . 
nized  authority  upon  Lake  Superior  geology,  may  appropriately  be  men- 


8         COPPEE-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 

tioned  first,  maintained  the  eruptive  origin  of  the  bowlder-conglomerates  of 

Keweenaw  Point  in  the  following  language : 

The  conglomerate  of  Keweenaw  Point  and  Tsle  Eoyale  consists  of  rounded  peb- 
bles of  trap,  almost  invariably  of  the  variety  known  as  amygdaloid,  derived  probably 
from  the  contemporaneous  lavas,  and  rounded  fragments  of  a  jaspery  rock  which  may 
have  been  a  metamorphosed  sandstone,  the  whole  cemented  by  a  dark -red  iron  sand. 
This  cement  may  be  regarded  as  a  mixture  of  volcanic  ash  and  arenaceous  particles, 
the  latter  having  been  derived  from  the  sandstone  then  in  the  progress  of  accumula- 
tion. It  is  not  unusual  to  meet  with  strata  composed  entirely  of  arenaceous  particles 
associated  with  the  conglomerate  beds ;  and  where  these  expand  to  a  considerable 
thickness,  the  associated  sandstone  appears  in  alternating  belts  of  white  and  red, 
and  exhibits  few  traces  of  metatnorphism ;  but  where  the  belts  of  sedimentary  rock 
are  thin,  and  come  in  contact  with  the  trappean  rocks,  the  sandstone  is  converted  into 
a  jaspery  rock,  traversed  by  divisional  planes,  and  breaking  with  a  conchoidal  fracture. 

The  trappean  pebbles  often  attain  a  magnitude  of  eighteen  inches  in  diameter. 
Their  surfaces  do  not  present  that  smooth,  polished  appearance  which  results  from  the 
attrition  of  water ;  in  fact,  a  close  observer  can  readily  distinguish  between  those 
which  have  been  recently  detached  from  the  rock  and  those  which  have  for  a  time  been 
exposed  to  the  recent  action  of  the  surf. 

The  conglomerate  appears  to  have  been  formed  too  rapidly  to  suppose  that  the 
masses  were  detached  and  rounded  by  the  action  of  waves  and  currents,  and  depos- 
ited with  silt  and  sand  on  the  floor  of  the  ancient  ocean  ;  for,  while  the  contemporane- 
ous sandstone  remote  from  the  line  of  volcanic  foci  does  not  exceed  three  hundred  or 
four  hundred  feet  in  thickness,  the  united  thickness  of  the  conglomerate  bands  in  the 
vicinity  of  the  trappean  range  on  Keweenaw  Point  exceeds  five  thousand  feet.  As  we 
recede  for  a  few  miles  from  the  line  of  the  volcanic  fissure,  these  amygdaloid  pebbles 
disappear,  and  are  replaced  by  arenaceous  and  argillaceous  particles.  We  are,  there- 
fore, disposed  to  adopt  the  theory,  as  to  the  origin  of  such  masses,  first  suggested  by 
Von  Buch :  "  When  basaltic  islands  and  trachytic  rocks  rise  on  fissures,  friction  of  the 
elevated  rock  against  the  walls  of  the  fissures  causes  the  elevated  rock  to  be  inclosed 
by  conglomerates  composed  of  its  own  matter.  The  granules  composing  the  sand- 
stones of  many  formations  have  been  separated  rather  by  friction  against  the  erupted 
volcanic  rock  than  destroyed  by  the  erosive  force  of  a  neighboring  sea.  The  existence 
of  these  friction  conglomerates,  which  are  met  with  in  enormous  masses  in  both  hemi- 
spheres, testifies  the  intensity  of  the  force  with  which  the  erupted  rocks  have  been 
propelled  from  the  interior  through  the  earth's  crust.  The  detritus  has  suddenly  been 
taken  up  by  the  waters,  which  have  then  deposited  it  in  the  strata  which  it  still  covers." 

Those  pebbles  having  a  highly  vesicular  structure  may  have  been  ejected  through 
the  fissures,  in  the  form  of  scorise,  while  in  a  plastic  state,  and  have  received  their 
rounded  shape  from  having  been  projected  through  water — on  the  same  principle  as 
melted  lead,  when  dropped  from  an  elevation,  assumes  a  globular  form. 

In  the  jaspery  fragments  included  in  the  conglomerate,  we  often  observe  a  struct- 
ure analogous  to  the  woody  fibre  of  trees.  These  fragments  are  composed  of  laminae, 
more  or  less  contorted,  and  furrowed  longitudinally,  like  the  markings  in  the  extinct 
plants  of  the  genus  Sigillaria.  A  series  of  striae,  as  fine  as  the  engraver's  lines,  ran 
parallel  with  the  larger  ones.    These  can  be  traced  on  some  of  the  specimens,  and  gen- 


FOEMEE  VIEWS  AS  TO  OEIGIN  OF  THE  TEAPS  AND  AMTGDALOIDS.  9 

erally  extend  through  the  different  folds;  while  others  possess  a  structure  like  the 
cellular  tissue  of  wood.  We  have  no  confidence  in  the  vegetable  origin  of  these  mark- 
ings, nor  have  we  any  theory  to  oft'er  in  explanation.' 

The  same  views  are  as  strenuously  maintained  in  other  publications  by 
the  same  authors,^  who  were,  however,  preceded  in  them  by  Dr.  Douglas 
Houghton.  The  latter  speaks  of  the  conglomerates  under  the  name  of  "  trap 
tuff,"  and  states  distinctly  that  they  are  made  up  almost  exclusively  of  pebbles 
and  bowlders  of  "greenstone  and  amygdaloidal  trap."^  Even  those  conglom- 
erates in  which  there  is  evident  more  or  less  sandy  material  he  speaks  of  as 
made  up  entirely  of  .comminuted  greenstone,  while  the  sandstone  and  shale 
which  with  so  great  thickness  form  the  upper  part  of  the  series  on  Kewee- 
naw Point,  he  considers  to  be  of  sedimentary  origin,  the  material  having 
been  worn  from  pre-existing  granitic  rocks. 

So  far  as  I  have  noticed,  none  of  the  other  writers  on  Lake  Superior 
geology  have  accepted  these  peculiar  views  as  to  the  origin  of  these  con- 
glomerates—all regarding  them  as  made  up  of  water-worn  pebbles  detached 
from  some  pre-existing  rock — and  for  the  excellent  reason  that  Dr.  Hough- 
ton's and  Messrs.  Foster  and  Whitney's  facts,  as  well  as  their  theory,  failed 
to  stand  closer  study.  The  included  pebbles  are  only  in  very  subordinate 
quantity  of  "trap"  or  amygdaloid,  being  almost  wholly  of  some  sort  of  acid 
eruptive  rock,  i.  e.,  felsite,  quartziferous  porphyry,  quartzless  porphyry, 
granitic  porphyry,  augite-syenite,  or  granite.  The  fundamental  difference 
between  such  pebbles  and  the  associated  basic  massive  rocks  is  alone 
enough  to  overthrow  the  theory,  even  were  there  not  other  sufficient  argu- 
ments against  it.  Further,  the  pebbles  are  just  as  plainly  water-worn  as 
those  of  any  other  conglomerates,  though  they  may  have  in  some  cases  had 
the  polish  removed  by  surface  alteration. 

The  "trap"  and  amygdaloid  are  by  some  writers  considered  as  having 
had  a  common  origin,  but  by  others  are  separated.  Under  the  generic  term 
of  "trap,"  Messrs.  Foster  and  Whitney  included  "greenstone,  granular  and 
amygdaloidal  trap,  basalt,  etc.,"  and  held  to  their  origin  in  part  as  lava 
flows,  contemporaneous  with  the  deposition  by  ordinary  sedimentation  of 

1  Op.  cit,  pp.  99, 100. 

3  Am.  Jour.  Sci.  (2),  XVII,  1854,  pp.  11-38,  181-194. 

3  Joint  Documents,  Mich.,  1841,  pp.  472-607. 


10  COPPER  BEAErNG  ROCKS  OF  LAKE  SUPERIOR. 

the  associated  sandstones,  and  in  part  as  protrusions  "in  vast  irregular 
masses,  forming  conical  or  dome-shaped  mountains,"  or  "continuous  lines 
of  elevation."^  While  they  class  the  amygdaloids  and  traps  together,  they 
do  not  appear  to  have  distinctly  recognized  that  the  former  are  the  vesicular 
upper  portions  of  the  lava  flows. 

Houghton  regarded  all  the  traps  as  intrusive,  and  the  amygdaloids 
as  semi-fused  sedimentary  matter  or  interfused  eruptive  and  sedimentary 
matter,'^  in  which  view  he  was  followed  by  Jackson.^  Similar  views  as  to 
the  intrusive  origin  of  the  traps  were  held  by  a  few  others  at  an  early  date, 
but  the  only  one  of  these  holding  these  views  who  had  made  any  extensive 
field  explorations  was  Norwood,*  who  in  his  account  of  the  Minnesota  coast 
represents  a  large  proportion  of  the  bedded  crystalline  rocks  of  that  coast 
as  intrusive;  but  even  he  does  not  regard  all  of  the  traps  as  having  had  this 
origin.  The  amygdaloids  he  appears  to  look  on,  for  the  most  part,  as  sedi- 
ments altered  by  igneous  action,  his  general  term  for  them  being  "meta- 
morphic  shales."  A  few  of  these  rocks  he  seems  to  regard  as  volcanic 
tufas,  an  origin  which  was  somewhat  doubtfully  assigned  by  Hunt  to  all 
the  amygdaloids.^  N.  H.  Winchell  has  recently  revived  the  peculiar  views 
of  Norwood  as  to  the  origin  of  the  amygdaloids  of  the  Minnesota  coast.^ 

Of  others  who  have  written  on  Lake  Superior  geology  the  larger 
number  have  maintained  the  origin  of  the  traps  as  lava  flows.  Among 
them  may  be  mentioned  Logan,^  Hunt,^  Macfarlane,®  Bell,^"  Pumpelly,^^ 
Marvine,^^  Irving, ^^^  Chamberlin,^*  and  Sweet.^^     Rivot^^  only  has  maintained 

'  Op.  cit.,  p.  58. 

=  Joint  Documents,  Micli.,  p.  490. 

3  Am.  J.  Sci.  (1)  XLIX,  pp.  62-72. 

4  "Geology  of  tlie  Western  and  Northwestern  Portions  of  the  Valley  of  Lake  Superior,"  in  Owen's 

Geological  Survey  of  Wisconsin,  Iowa,  and  Minnesota,  pp.  333-424. 
s  Geology  of  Canada,  1863,  pp.  698,  699. 

«  Seventh  and  Ninth  Annual  Reports  of  the  Geological  and  Natural  History  Survey  of  Minnesota. 
'Geology  of  Canada,  1863,  pp.  71,72. 

^Second  Geol.  Survey  of  Penn.,  Azoic  Rocks,  Part  I,  p.  256. 
^Report  of  Progress  of  Geol.  Survey  of  Canada,  1863-'66,  pp.  115-164. 
lo Report  of  Progress  of  Geol.  Survey  of  Canada,  1866-'69,  pp.  313-364. 
"Proc.  Amer.  Acad.  1878,  XHI,  pp.  253,254. 
12  Geological  Survey  of  Michigan,  1873,  Part  II,  pp.  109-112. 
"Geology  of  Wisconsin,  Vol.  Ill,  p.  7. 
"Geology  of  Wisconsin,  Vol.  Ill,  p.  391. 
'^Geology  of  Wisconsin,  Vol.  Ill,  p.  336. 
"iAnnales  des  Mines  (5),  VIII,  173-328,  and  364, 374. 


FOEMEE  VIEWS  AS  TO  IfATUEE  OF  THE  FELSITIC  EOCKS.       11 

a  metamorphic  origin  for  them.  Pumpelly  and  Marvine  first  recognized 
definitely  that  the  amygdaloids  are  but  the  upper  portions  of  the  beds 
of  "trap,"  and  the  distinction  between  the  true  vesicular  amygdaloids  and 
the  non-vesicular  pseud-amygdaloids,  which  distinction  the  latter  subse- 
quently made  still  plainer  by  his  microscopic  investigations,^  to  which  we 
are  indebted  for  our  first  knowledge  as  to  the  nature  of  the  so-called 
"  traps."  Marvine's  conclusions  from  his  examination  of  the  beds  of  the 
Eagle  River  section  of  Keweenaw  Point,  made  before  the  use  of  the  micro- 
scope in  the  study  of  these  rocks,  deserve  mention  as  containing  the  first 
plainly  and  thoroughly  worked-out  argument,  from  structural  characters 
alone,  in  favor  of  the  lava-flow  origin  of  the  "  traps,"  and  of  the  connection 
with  the  trap  beds  of  the  amygdaloids.  The  results  of  the  work  of  the 
Wisconsin  Survey,  as  given  in  the  third  volume  of  the  Greology  of  Wis- 
consin, by  Chamberlin,  Sweet,  Strong,  and  myself,  fully  sustained  the  con- 
clusions of  Pumpelly  and  Marvine. 

The  most  recent  writers  on  the  copper  rocks  of  Lake  Superior  have 
been  N.  H.  Winchell  and  Wadsworth.  Winchell's  views  as  to. the  sedi- 
mentary and  metamorphic  origin  of  the  amygdaloids  of  the  Minnesota 
coast  have  already  been  mentioned.  In  the  same  category  he  appears  to 
include  a  large  part  of  the  bedded  traps,  the  more  highly  crystalline  kinds 
only  being  regarded  as  of  eruptive  origin.^  Wadsworth's  paper  maintains 
the  lava-flow  origin  of  the  traps,  although  no  points  are  advanced  in  favor 
of  this  conclusion  that  had  not  already  been  fully  covered  by  Pumpelly 
and  Marvine.  It  is  worthy  of  note  that  while  maintaining  that  Foster  and 
Whitney  are  the  only  authors  who  have  written  correcth^  on  Lake  Supe- 
rior geology,  Wadsworth  should  yet  depart  from  them  in  several  very  im- 
portant points — such  as  the  origin  of  the  conglomerates — and  this  without 
a  word  of  comment. 

The  reddish,  acid,  eruptive  rocks,  which  I  describe  in  subsequent 
chapters  as  constituting  so  important  a  feature  of  the  copper-bearing  series, 
have  been  almost  wholly  overlooked  heretofore.     A  number  of  writers  have 

'Metasomatio  Development  of  the  Copper-Bearing  RockB  of  Lake  Superior.  Proo.  Am.  Acad. 
Sci.,  1878,  XIII,  pp.  253-309. 

^Ninth  Annual  Report  of  the  Natural  History  and  Geological  Survey  of  Minnesota,  1880.  Prelimi- 
nary list  of  rocks,  pp.  10-114. 


12  coppee-bbarhstg  eocks  op  lake  superior. 

recognized  such  rocks  among  the  pebbles  of  the  conglomerates,  but  as 
massive  rocks  they  have  only  been  noticed  by  Foster  and  Whitney,  Logan, 
N.  H.  Winchell,  Macfarlane,  and  Bell.  Foster  and  Whitney,  who  noticed 
them  on  Mount  Houghton  and  in  the  Porcupine  Mountains,  looked  on  them 
as  baked  sandstones,  the  baking  being  regarded  as  due  to  the  heat  of  the 
molten  traps.^  Winchell  has  noticed  them  on  the  Minnesota  coast,  but  re- 
gards them  again  as  altered  sedimentary  rocks.^  Logan^  and  Macfarlane* 
have  observed  them  only  on  Michipicoten  Island,  where  Macfarlane  seems 
to  regard  them  as  eruptive  and  Logan  as  fused  sedimentary  material.  Bell 
merely  mentions  the  existence  of  quartziferous  porphyry  on  Lake  Nipigon.^ 
The  failure  to  recognize  the  importance  and  eruptive  origin  of  these  rocks 
is  peculiarly  strange  in  the  face  of  the  almost  universal  association  in  vol- 
canic regions  of  the  two  types  of  acid  and  basic  eruptives. 

Widely  divergent  views  have  been  held  with  regard  to  the  geological 
relations  of  the  series  as  a  whole,  as  well  as  with  regard  to  the  origin  and 
structural  relations  of  its  constituent  rocks.  In  the  earlier  days  of  the  study 
of  Lake  Superior  geology  the  general  lithological  similarity  between  these 
rocks  and  the  Triassic  sandstones  and  eruptives  of  the  eastern  states  led 
to  the  view  that  they  were  of  the  same  age.  This  view  was  held  by 
Houghton,"  and  at  one  time  by  Jackson,''  and  latterly  has  been  advo- 
cated by  Bell.^  Later,  when  the  Cambrian  age  of  the  so-called  "Eastern 
Sandstone",  which  forms  the  south  shore  of  Lake  Superior  from  the  Sault 
westward  to  the  east  side  of  Keweenaw  Point,  came  to  be  established,  the 
copper-bearing  rocks,  being  regarded  as  belonging  to  the  same  formation, 
were  considered  to  be  the  equivalents  of  the  Potsdam  sandstone  of  New 
York.     This  is  the  position  taken  by  Foster  and  Whitney,^  Owen,"  Ro- 

'  Op.  cit.,  pp.  64-65. 

i^Eiglitli  Annual  Report  of  the  Geological  Survey  of  Minnesota,  pp.  23-26.    Ninth  Annual  Report, 

pp.  12,  17,  31,  32,  etc. 
3  Geology  of  Canada,  1863,  pp.  81-82. 

••Geological  Survey  of  Canada,  Report  of  Progress,  1863-66,  p.  142. 
6  Report  of  Progress  of  the  Geological  Survey  of  Canada,  1866-'69,  p.  348. 
<iAm.  J.  Sci.,  1843  (1),  XLV,  160. 
'Am.  J.  Sci.  (1),  XLIX,  pp.  81-93. 
■  "Report  of  Progress  Geological  Survey  of  Canada,  1866-'69,  p.  321. 
9  Op.  dt.,  p.  99. 
'"Geological  Survey  of  Wisconsin,  Iowa,  and  Minnesota,  pp.  187-196. 


FOEMEE  VIEWS  ON  GEOLOGICAL  POSITION  OP  THE  SERIES.         13 

minger/  Winchell,^  and  Wadsworth,^  although  these  writers  differ  greatly 
among  themselves  as  to  the  exact  structural  relations  subsisting  between  the 
Eastern  Sandstone  and  the  trappean  series. 

As  early  as  1846  Logan  regarded  the  copper-bearing  rocks  of  Kewee- 
naw Point  as  the  equivalents  of  the  Huronian  of  the  north  shore  of  Lake 
Huron,*  and  when,  later,  he  abandoned  this  view,  still  regarding  them  as 
older  than  the  Eastern  Sandstone,  he  made  them  the  equivalents  of  the  so- 
called  "Quebec  Group"  of  Canada  East.^  In  1872  Pumpelly  and  Brooks 
advanced  excellent  reasons  for  placing  the  Keweenaw  Point  rocks  below 
the  Eastern  Sandstone,  and  as,  on  the  whole,  nearer  to  conformity  with 
the  Huronian.*'  In  1876  Brooks  changed  his  views  so  far  as  to  abandon 
the  conformity  with  the  underlying  Huronian,  but  he  still  maintained  the 
unconformity  with  the  Eastern  Sandstone.''  In  1880  the  third  volume  of 
the  Geology  of  Wisconsin  presented  new  and  weighty  evidence  of  the 
pre-Cambrian  age  of  the  copper-bearing  rocks,  which  are  in  Northern  Wis- 
consin found  to  be  separated  from  the  basal  fossiliferous  Cambrian  sand- 
stones of  the  Mississippi  Valley  by  a  great  intervening  erosion,  while 
from  the  underlying  Huronian  the  separation  did  not  appear  to  be  so 
great.  In  that  volume  the  copper-bearing  rocks  were  described  under 
the  term  of  the  Keweenaw  or  Keweenawan  Series,  following  the  previous 
suggestions  of  Hunt*  and  Brooks,^  and  the  same  term  will  be  used  in  this 
memoii-.  A  few  months  later  appeared  the  volume  of  Mr.  M.  E.  Wads- 
worth,  in  which  the  copper-bearing  rocks  are  placed  as  the  upper  part  of  a 
series  of  which  the  Eastern  Sandstone  is  regarded  as  the  basal  member — a 
view  which  can,  I  think,  be  easily  shown  to  be  untenable. 


'Geological  Survey  of  Michigan,  Vol.  I,  Part  III,  pp.  80-81. 
^Eiglitli  Annual  Eeport  Geological  Survey  of  Minnesota,  p.  25. 
»  Oj3.  cit,  pp.  115-127. 
••Am.  J.  Sci.  (2),  1857,  XXIII,  305-314. 

5  Geology  of  Canada,  1863,  pp.  67-86. 

6  Am.  J.  Sci.  (3),  III,  428-432. 
'Am.  J.  Sci.  (3),  XI,  206-311. 

8  Trans.  Am.  Inst.  Mln.  Eng.,  I,  331-342. 

9  Am.  J.  Sci.  (3),  XI,  206-211.     Op  cit.,  p.  66. 


14  COPPBR-BEAETNG  EOCKS  OF  LAKE  SUPEEIOE. 


LITBEATUEB. 

The  following  list  of  works  includes  all  of  any  importance  that  embrace 
references  to  the  Keweenawan  rocks  by  geologists  who  have  examined  them 
in  the  field.  The  arrangement  is  a  chronologic  one,  so  that  any  one  wish- 
ing to  examine  the  history  of  the  subject  may  do  so  conveniently.^ 

1824. 

BiGSBT  (John  J.).  Notes  on  the  Geography  and  Geology  of  Lake  Superior,  Quarterly 
Journal  of  Science  and  Arts,  1824-'25,  xviii,  1-34,  228-269. 

1829. 

Bayfield  (H.  W.).  Outlines  of  the  Geology  of  Lake  Superior.  Transactions  of  the 
Literary  and  Historical  Society  of  Quebec,  1829,  i,  1-43. 

1831. 

HotTGHTON  (Douglas).  A  Eeport  on  the  Existence  of  Deposits  of  Copper  in  the 
Geological  Basin  of  Lake  Superior.  l^Tovember  14,  1831.  In  the  Discovery 
of  the  Source  of  the  Mississippi,  by  H.  E.  Schoolcraft,  526-531. 

1839. 

HoTTGHTON  (DouGLAS).  First  Annual  Eeport  on  the  Geology  of  Michigan,  1838, 
Lanman's  History  of  Michigan,  1839,  347-366. 
Second  Annual  Eeport  of  the  State  Geologist.    Senate  Docs.,  Michigan,  1839, 
264-294. 

1840. 

Houghton  (Douglas).  Third  Annual  Eeport  of  the  State  Geologist,  1840.  Senate 
Docs.,  Michigan,  1840,  66-157. 

1841. 

Houghton  (Douglas).  Fourth  Annual  Eeport  of  the  State  Geologist.  Joint  Docs., 
Michigan,  1841,  472-607. 

1842. 

Houghton  (Douglas).  Fifth  Annual  Eeport  of  the  State  Geologist.    Joint  Docs., 
Michigan,  1842,  436-441. 
Metalliferous  Veins  of  the  Northern  Peninsula  of  Michigan.    American  Journal 
of  Science  and  Arts,  1841  (1),  xli.,  183-186. 


1  For  the  names  of  some  of  tlie  foreign  works  I  am  d  irectly  indebted  to  Wadsworth's  bibliography 
of  Lake  Superior  geology,  without  which  aid  I  might  have  overlooked  them.     Op.  cit.  pp.  133-157. 


LITEEATUEE.  15 

1844. 

Houghton  (Douglas).  Copper  on  Lake  Superior.    Am.  Jour.  Sci.,  1844  (1),  xlvii., 

107,  132. 
Locke  (John).  Geology  of  Porter's  Island  and  Copper  Harbor.    Transactions  of  the 

American  Philosophical  Society,  1844,  ix.,  311,  312,  with  maps. 

1845. 

Jackson  (Charles  T.).  On  the  Copper  and  Silver  of  Keweenaw  Point,  Lake  Su- 
perior.   Am.  Jour.  Sci.,  1845  (1),  xlix.,  81-93. 
Sur  le  Gisement  de  Cuivre  et  d' Argent  natifs  des  Bords  du  Lac  Sup6rieur. 
Comptes  Eendus,  1845,  xx.,  593-595 ;  Bulletin  de  la  Soci6t6  G^ologique  de 
Prance,  1844-'45  (2),  ii.,  317-319. 

Shepabd  (Charles  U.).  On  the  Copper  and  Silver  of  Keweenaw  Point,  Lake  Su- 
perior. Proceedings  of  the  Sixth  Annual  Meeting  of  the  Association  of 
American  Geologists  and  Naturalists,  New  Haven,  1845,  60,  61. 

1846. 

Houghton  (Jacob,  Jr.),  and  Bristol  (T.  W.).  Mineral  Eegion  of  Lake  Superior. 

Detroit,  1846,  109  pp.  and  map. 
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of  the  Boston  Society  of  Natural  History,  1846,  ii.,  110-114. 
Logan  (William  E.).  North  Shore  of  Lake  Superior.     Geological  Survey  of  Canada, 

Eeport  of  Progress,  1846-'47,  5-46. 
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Hist.,  1846,  ii.,  1C4,  125. 

1847. 

DuTTON  (T.  E.).  Observations  on  the  Basaltic  Formation  on  the  Northern  Shore  of 

Lake  Superior.    Am.  Jour.  Sci.  (2),  1847,  iv.,  118,  119. 
Jackson  (Charles  T.).  Eeport  on  the  Mineral  Lands  of  Lake  Superior.     Senate 

Docs.,  1st  sess.  30th  Cong.,  1847-'48,  ii..  No.  2,  175-230. 
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259,  260. 
Logan  (William  B.).  Eemarks  on  the  Mining  Eegion  of  Lake  Superior,  and  a  Eeport 

on  Mining  Locations  claimed  on  the  Canadian  Shores  of  the  Lake.     Montreal, 

1847,  31  pp.,  with  maps. 
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Progress,  1846-'47,  47-57. 

1848. 

Jackson  (Charles  T.).  Age  of  the  Lake  Superior  Sandstone.  Proc.  Bost.  Soc.  Nat. 
Hist.,  1848,  iii.,  76,  77. 

EOGERS  (William  B.).  On  the  Origin  of  the  Actual  Outlines  of  Lake  Superior.  Pro- 
ceedings of  the  American  Association  for  the  Advancement  of  Science,  1848, 
First  Meeting,  79,  80. 


16  COPPEE-BEAEINa  EOCKS  OF  LAKE  SUPEEIOE. 

1849. 

Burt  (William  A.),  and  Hubbaed  (Bela).  Geology  and  Topography  of  the  District 
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842-933. 
HoixJE  (James  T.).  On  the  Mineral  Eegion  of  Lake  Superior.    Proc.  Am.  Assoc.  Adv. 

Sci.,  1849,  Second  Meeting,  301-308. 
Jackson  (Charles  T.).  Mineral  Lauds  of  Lake  Superior.    Senate  Docs.,  1848-'49, 2d 
sess.  30th  Cong.,  ii.,  No.  2,  153-163. 
Eeport  on  the  Progress  of  the  Geological  Survey  of  the  Mineral  Lands  of  the 
United  States  in  Michigan.     Senate  Docs.,  1848-'49,  2d  sess.  30th  Cong.,  ii., 
]S"o.  2,  185-191. 
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Assoc.  Adv.  Sci.,  1849,  Second  Meeting,  283-288. 
On  the  Geological  Structure  of  Keweenaw  Point.    Proc.  Am.  Assoc.  Adv.  Sci., 

1849,  Second  Meeting,  288-301. 

LOG-AN  (William  E.).  Eeport  on  the  North  Shore  of  Lake  Huron.    Geol.  Surv.  of 

Canada,  Eep.  of  Progress,  Montreal,  1849,  51  pp. 
Whitney  (J.  D.).  The  Lake  Superior  Copper  and  Iron  District.    Proc.  Bost.  Soc.  Nat. 

Hist.,  1849,  iii.,  210-212. 

1850. 

Ag-assiz  (Louis).  Lake  Superior;  its  Physical  Characters,  Vegetation,  and  Animals. 

Boston,  1850,  428  pp. 
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Corjjs  in  the  Lake  Superior  Land  District  in  the  Northern  Peninsula  of  Mich- 
igan.    Senate  Docs.,  1st  sess.  31st  Cong.,  1849-'50,  iii..  No.  5,  605-626. 

Eeport  on  the  Geology  and  Topography  of  a  Portion  of  the  Lake  Superior  Land 
^  District  in  the  State  of  Michigan.    Part  I.,  Copper  Lands.    Executive  Docs., 

1st  sess.  31st  Cong.,  1849-50,  ix..  No.  69,  224  pp.,  with  map. 

Aper9u  de  I'ensemble  des  Terrains  Siluriens  du  Lac  Sup^rieur.    Bull.  Soc.  G6ol. 
France,  1850  (2),  89-100. 
Jackson  (Charles  T.).  Eemarques  sur  la  Geologic  du  District  M^tallif6re  du  Lac 
Sup6rieur.     Bull.  Soc.  Geol.  France,  1849-'50,  vii.,  667-673. 

Eeport  on  the  Geological  and  Mineralogical  Survey  of  the  Mineral  Lands  of  the 
United  States  in  the  State  of  Michigan.  Senate  Docs.,  1st  sess.  31st  Cong., 
1849-'50,  No.  5,  iii.,  371-503. 

Eemarques  sur  la  Geologic  du  District  M6talliffere  dii  Lac  Sup6rieur,  suivies  d'une 
courte  Description  de  quelques-unes  des  Mines  de  Cuivre  et  d' Argent.  Ex- 
trait  par  M.  Delesse.    Annales  des  Mines,  1850  (4),  xvii.,  103-115. 

Age  of  the  Lake  Superior  Sandstone.     Proc.  Bost.  Soc.  Nat.  Hist.,  1850,  iii.,  p.  228. 

On  the  Age  of  the  Sandstones  of  the  United  States.     Proc.  Bost.  Soc.  Nat.  Hist., 

1850,  iii.,  335-339. 

Marcou  (Jules).  Espouse  h  la  Lettre  de  MM.  Foster  et  Whitney  sur  le  Lac  Sup6rieur. 
Bull.  Soc.  G^ol.  France,  1850  (2),  viii.,  101-105. 


LITERATUEE.  17 


1851. 


Desoe  (Edward).  Lake  Superior.  On  the  Silurian  Eocks  of  the  Lake  Superior  Land 
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Foster  (J.  W.)  and  Whitney  (J.  D.).  On  the  Age  of  the  Sandstone  of  Lake  Supe- 
rior, with  a  Description  of  the  Phenomena  of  the  Association  of  Igneous  Eocks. 
Proc.  Am.  Assoc.  Adv.  Sci.,  1851,  Fifth  Meeting,  22-38. 
On  the  Different  Systems  of  Elevation  which  have  given  Configuration  to  ]S"orth 
America,  with  an  Attempt  to  Identify  them  with  those  of  Europe.  Proc.  Am. 
Assoc.  Adv.  Sci.,  1851,  Fifth  Meeting,  136-151. 

Jackson  (Charles  T.).  Analyses  of  Pitchstone  Porphyry  from  Isle  Eoyale,  etc.    Am. 
Jour.  Sci.,  1851  (2),  xi.,  401, 402. 
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ary 17,  iv.,  131, 132. 

1852. 

BiGrSBY  (John  J.).  On  the  Physical  Geography,  Geology,  and  Commercial  Eesources 
of  Lake  Superior.    Edinburgh  'New  Philosophical  Journal,  1852,  liii.,  55-62. 

Jackson  (Charles  T.).  Geology,  Mineralogy,  and  Topography  of  the  Lands  around 
Lake  Superior.     Senate  Docs.,  1851-'52,  1st  sess.  32d  Cong.,  xi.,  232-244. 

Logan  (William  E.).  On  the  Age  of  the  Copper-bearing  Eocks  of  Lakes  Superior 
and  Huron.    Am.  Jour.  Sci.,  1852  (2),  xiv.,  224-229. 

Norwood  (Dr.  J.  G.).  Geology  of  the  Western  and  Northwestern  Portion  of  the  Val- 
ley of  Lake  Superior;  in  Owen's  Geological  Survey  of  Wisconsin,  Iowa,  and 
Minnesota.    Philadelphia,  1852,  333-418. 

Owen  (David  D.).  On  the  Age,  Character,  and  True  Geological  Position  of  the  Lake 
Superior  Eed  Sandstone  Formation,  in  the  Eeport  of  a  Geological  Survey  of 
Wisconsin,  Iowa,  and  Minnesota.     Philadelphia,  1852,  187-193. 

Shumard  (B.  F.).  Local  Details  of  Geological  Sections  on  the  Saint  Peter's,  Wiscon- 
sin, Mississippi,  Baraboo,  Snake,  and  Kettle  Elvers ;  in  Owen's  Geological 
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Whittlesey  (Charles).  Description  of  Part  of  Wisconsin  South  of  Lake  Superior. 
With  maps.  In  Owen's  Geological  Survey  of  Wisconsin,  Iowa,  and  Minne^ 
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1853. 

Jackson  (Charles  T.).  Igneous  Origin  of  Calcite  Veins.    Proc.  Bost.  Soc.  Nat.  Hist., 
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of  North  America,  with  an  Explanatory  Text,  Geological  Sections,  etc.    Bos- 
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2  L  S 


18  COPPER-BE AEtNG  EOCKS  OP  LAKE  SUPEEIOE. 

1854. 

Foster  (J.  W.).  Catalogue  of  Rocks, Minerals,  etc.,  collected  by  J.  W.  Foster.    Smith- 
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Hall  (James).  On  the  Silurian  System  of  the  Lake  Superior  Region.  Am.  Jour.  Sci., 
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Jackson  (Charles  T.).  Catalogue  of  Rocks,  Minerals,  and  Ores  Collected  During 
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Sup^rieur.     Comptes  Rendus,  1854,  xxxix.,  803-807. 

Locke  (John).  Catalogue  of  Rocks,  Minerals,  Ores,  and  Fossils  Collected  by  John 
Locke.     Smithsonian  Report,  1854,  367-383. 

1855. 

DUPEE  (J.  A.).  Ashbed  of  the  Phoenix  Mine.    Proc.  Bost.  Soc.  Nat.  Hist.,  1855,  v., 

279, 280. 
Emmons  (Ebenezbr).  Copper  Mines  of  Lake  Superior.    American  Geology,  Albany, 

1855,  171-173. 
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Soc.  Nat.  Hist.,  1855,  v.,  280,  281. 
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Rendus,  1855,  xl.,  1306-1309. 
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Whitney  (J.  D.).  Remarks  on  Some  Points  Connected  with  the  Geology  of  the  North 

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1856. 

Blake  (Willl5lM  P.).  Review  of  a  portion  of  the  Geological  Map  of  the  United  States 
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Jackson  (Charles  T.).  Trap  Dikes.    Proc.  Bost.  Soc.  Nat.  Hist.,  1856,  vi.,  23, 24. 

RiVOT  (L.  E.).  Notice  sur  le  Lac  Superieur.    Annales  des  Mines,  1853  (5),  x.,  365-474. 

Uber  die  Kupfererz-Lagerstatten  am  Obern  See  in  den  Nordamerikanisclien  Frei- 

staateu.    Berg-  und  Hiittenmauuische  Zeituug,  1856,  261-263,  269-271,  293- 

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365-367,  381,  382. 

1857. 

Dawson  (John  W.).  On  the  Geological  Structure  and  Mineral  Deposits  of  the  Pro- 
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MtJLLER  (Alf.).  Ueber  die  Kupferminen  am  Obern  See  in  Nordamerika.  Neues 
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H.  B.  Geinitz,  1857,  79-82,  589,  590. 


LITERATUEE.  19 

1858. 

DiEFPENBACH  (Otto).  BemerkuDgen  iiber  den  Kupferbergbau  in  den  Vereinigten 
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1859. 

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(2),  xxvii.,  134^137. 

Jackson  (Chaelbs  T.).  On  the  Ashbed  and  the  Origin  of  the  Copper.  Proc.  Bost. 
Soc.  Nat.  Hist.,  1859,  vii.,  31. 

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Society  Transactions,  1858-'59,  v.,  391-400,  with  map. 

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Whittlesey  (Charles).  On  the  Origin  of  the  Azoic  Eocks  of  Michigan  and  Wis- 
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1860. 

Jackson  (Charles  T.).  Age  of  the  Lake  Superior  Sandstone.    Proc.  Bost.  Soc.  Nat. 

Hist.,  1860,  vii.,  396-398. 
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vii.,  394,  395. 
Whitney  (J.  D.).  Note  on  the  Geological  Position  of  the  Lake  Superior  Sandstone. 

Mining  Mag.,  1860  (2),  i.,  435-446. 

1861. 

Dbroux  (H.).  Die  Kupfergruben  des  Oberen  See's  (Lake  Superior).    Berg.  u.  Hutten. 

Zeit.,  1861,  305-307,  329-331. 
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1863. 

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1863. 

Logan  (William  E.).  Geology  of  Canada.    Montreal,  1863,  983  pp.,  with  atlas. 
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20  COPPEEBBAEING  EOCKS  OF  LAKE  SUPEEIOE. 

1864. 

Logan  (William  E.).  Kupfererzeflihrende  Gesteine  am  Obern  See.     Leonhard's 

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Jour.  Sci.,  1864  (2),  xxxvii.,  226-232. 

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Counties  of  Minnesota.    St.  Paul,  1866,  58  pp. 
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1867. 

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1868. 

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LITEEATUEE.  21 

1870. 

Ceednbr   (Hermann).  Gewaltige  Kupfermassea  am  Lake  Superior.     Leonhard's 
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UeBer  Nordamerikanische  Schieferporphyroide.     Leonhard's  Jahrbuch,  1870, 970- 
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1871. 

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PuMPELLY  (Eaphael).  The  Paragenesis  and  Derivation  of  Copper  and  its  Associ- 
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1872. 

Brooks  (T.  B.)  and  Pumpelly  (E.).  On  the  Age  of  the  Copper-bearing  Eocks  of 
Lake  Superior.    Am.  Jour.  Sci.,  1872  (3),  iii,  428-432. 

1873. 

Bell  (Egbert).  Eeport  on  the  Country  between  Lake  Superior  and  Lake  Winnipeg. 
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Brooks  (T.  B).  Geological  Survey  of  Michigan,  with  maps,  1869-'73,  i.,  Part  L,  Iron- 
bearing  Eocks,  319  pp. 

Huis^T  (T.  Sterry).  The  Geognostical  History  of  the  Metals.    Trans.  Am.  Inst.  Min. 
Eng.,  1873,  i.,  331-34  . 
The  Origin  of  Metalliferous  Deposits.    Transactions  of  the  American  Institute  of 
Mining  Engineers,  1873,  i.,  413-426. 

Marvine  (A.  E).  Geology  of  Michigan,  1873,  i..  Part  II.,  Copper-bearing  rocks.  Chap- 
ters IV,  Vil,  and  VllL 

Nicholson  (H.  Alleyne).  On  the  Geology  of  the  Thunder  Bay  and  Shebandowan 
Mining  Districts  on  the  North  Shore  of  Lake  Superior.  Quarterly  Journal 
of  the  Geological  Society,  1873,  xxix.,  17-24. 

Pumpelly  (E).  Geological  Survey  of  Michigan,  i..  Part  II.,  Copper-bearing  Eocks, 
1-96,  and  62-95,  with  an  atlas  of  maps. 

EoMiNGER  (Charles).  Geological  Survey  of  Michigan,  1873,  i..  Part  III.,  Paleozoic 
Eocks,  105  pp. 

Selwyn  (Alfred  E.  C).  Notes  of  a  Geological  Eeconnoissance  from  Lake  Superior 
to  Port  Garry.    Geol.  Surv.  Canada,  Eep.  of  Prog.    Montreal,  1872-'73,  8-18. 

1874. 

Douglas  (James).  The  Native  Copper  Mines  of  Lake  Superior.  Quarterly  Journal 
of  Science,  1874,  xi.,  162-180. 

Hunt  (T.  Sterry).  The  Geology  of  the  North  Shore  of  Lake  Superior.  (Supplement- 
ary note).    Trans.  Am.  Inst.  Min.  Eng.,  1873,  ii.,  58,  59. 

Irving  (E.  D.).  On  some  Points  in  the  Geology  of  Northern  Wisconsin.  Transactions 
Wisconsin  Academy  of  Science,  1873-'74,  ii.,  107-119. 


22        COPPEE-BEAEING  EOCKS  OP  LAKE  SUPEEIOE. 

Ikving  (E.  D.).  On  tlie  Age  of  the  Copper-bearing  Eocks  of  Lake  Superior,  and  on  the 

Westward  Continuation  of  the  Lake  Superior  Synclinal.    Am.  Jour.  Sci.,  1874 

(3),  viii.,  46-56. 

1875. 

Bell  (Egbert).  The  Mineral  Eegion  of  Lake  Superior.  Canadian  Nat.,  1875  (2),  vii., 
49-51. 

N'iCHOLSON  (H.  Alletnb).  On  the  Mining  District  of  the  North  Shore  of  Lake  Supe- 
rior. Trans,  of  the  North  of  England  Inst,  of  Mining  and  Mechanical  Engi- 
neers, Newcastle-on-Tyne,  1874^'75,  xxiv.,  237-249,  with  maps. 

Whittlesey  (Charles).  Physical  Geology  of  Lake  Superior.  Proc.  Am.  Assoc. 
Adv.  Sci.,  1875,  xxiv.,  60-72. 

1876. 

Bell  (Egbert).  Eeport  on  an  Exploration  in  1875,  between  James  Bay  and  Lakes 

Superior  and  Huron.    Geol.  Surv.  of  Canada,  Eep.  of  Prog.  1875-1876, 294-342. 
Brooks  (T.  B).  On  the  Youngest  Huronian  Eocks  South  of  Lake  Superior,  and  the 

Age  of  the  Copper-bearing  Series.    Am.  Jour.  Sci.,  1876  (3),  xi.,  206-211. 
Classified  List  of  Eocks  observed  in  the  Huronian  Series  south  of  Lake  Superior. 

Am.  Jour.  Sci.,  1876  (3),  xii.,  194-204. 
EoMiNaER  (Charles).  Observations  on  the  Ontonagon  Silver  Mining  District  and  the 

Slate  Quarries  of  Huron  Bay.     Geol.  Surv.  Michigan,  1876,  iii.,  163-166. 
Sweet  (E.  T).  Notes  on  the  Geology  of  Northern  Wisconsin.    Trans.  Wis.  Acad. 

1875-'76,  iii.,  40-55. 
Whittlesey  (Charles).  On  the  Origin  of  Mineral  Veins.    Proc.  Am.  Assoc.  Adv. 

Sci.,  1876,  XXV.,  213-216. 

1877. 

Bell  (Egbert).  Eeport  on  Geological  Eesearches  North  of  Lake  Huron  and  East 
of  Lake  Superior.    Geol.  Surv.  of  Canada,  Eep.  of  Prog.,  1876-'77,  193-220. 

EgtLESTGN  (Thomas).  Copper  Mining  on  Lake  Superior.    Trans.  Am.  Inst.  Min.  Eng., 
1877,  vi.,  275-312. 
The  Conglomerates  of  Lake  Superior,  and  the  Methods  of  Dressing  Copper.  Trans. 
Am.  Inst.  Min.  Eng.,  1877,  v.,  606-611. 

iRViNa  (E  D.).  Note  on  the  Age  of  the  Crystalline  Eocks  of  Wisconsin.  Am.  Jour. 
Sci.,  1877  (3),  xiii.,  307-309. 

-Steeng  (A.)  and  Klggs  (J.  H.).  tJber  die  Krystallinischen  Gesteine  von  Minnesota  in 
Nord-Amerika.    Leonhard's  Jahrbuch,  1877,  31, 113,  225. 

Whittlesey  (Charles).  On  the  Iron  Eiver  Silver  District.  Engineering  and  Min- 
ing Journal,  April  21,  1877,  xxiii.,  254-255,  278-279. 

1878. 

Credner  (Hermann).  Elemente  der  Geologic,  4th  ed.,  1878,  726  pp. 

Hunt  (T.  Sterry).  Azoic  Eocks.  Parti.,  1878.  Second  Geological  Survey  of  Penn- 
sylvania, E.  253  pp. 

PuMPELLY  (Eaphael).  Metasomatic  Development  of  the  Copper-bearing  Eocks  of 
Lake  Superior.    Proc.  Am.  Acad.,  1878,  xiii.,  253-309. 


LITEEATCTEE.  23 

Spencee  (Joseph  William).  On  the  Nipigon  or  Copper-bearing  Eocks  of  Lake 
Superior,  with  Notes  on  Copper  Mining  in  that  Eegion.  Canadian  Nat.,  1878 
(2),  viii.,  55-81. 

1879. 

Hall  (C.  W.).  Field  Eeport.  In  Seventh  Annual  Eeport  of  the  Geological  and  Natu- 
ral History  Survey  of  Minnesota  for  the  year  1878,  26-29. 

Ikving  (E.  D.),  Note  ou  the  Stratigraphy  of  the  Hurouian  Series  of  Northern  Wiscon- 
sin ;  and  on  the  Equivalency  of  the  Huronian  of  the  Marquette  and  Penokee 
Districts.    Am.  Jour.  ScL,  1879  (3),  xvii.,  393-398. 

Macfarlane  (Thomas).  Eemarks  on  Canadian  Stratigraphy.  Canadian  Nat.,  1879 
(2),  ix.,  91-102. 

MoSLEE  (Che.).  Der  Kupferbei'gbau  am  Obern  See  in  Nordamerika.  Zeitschrift  fur 
das  Berg-  Htitten-  und  Salinenvresen,  1877.  Abhandlungen,  xxv.,  203-221; 
1879,  xxvii.,  77-97. 

Selwyn  (Alpeed  R.  C).  The  Stratigraphy  of  the  Quebec  Group  and  the  Older  Crys- 
talline Eocks  of  Canada.     Canadian  Nat.,  1879  (2),  ix.,  17-32. 

WiNCHELL  (N.  H.).  Sketch  of  the  Work  of  the  Season  of  1878.  In  the  Seventh  An- 
nual Eeport  of  the  Geol.  and  Nat.  Hist.  Survey  of  Minnesota  for  the  year 


1878,  9-25. 


1880. 


Chambeelin  (T.  C.)  and  Steong-  (Moses).  Geology  of  the  Upper  Saint  Croix  District. 

Geology  of  Wisconsin,  iii.,  1880,  363-428. 
Hall  (C.  W.).  Eeport  of  Professor  C.  W.  Hall.    In  Eighth  Annual  Eeport  of  the 

Geol.  and  Nat.  Hist.,  Survey  of  Minnesota  for  the  year  1879,  126-138. 
Hunt  (T.  Steeey).  The  History  of  Some  Pre-Cambrian  Eocks  in  America  and  Europe. 

Am.  Jour.  Sci.,  1880  (3),  xix.,  268-283. 
Ieying  (E.  D.).   Geological  Structure  of  Northern  Wisconsin,  with  maps.     Geol.  of 

Wisconsin,  iii.,  1880,  Part  I.,  1-25. 
Geology  of  the  Eastern  Lake  Superior  District,  with  atlas  maps,  and  plates. 

Geol.  of  Wisconsin,  Part  III.,  1880,  iii.,  51-238. 
JxjLiEN  (Alexis  A.).  Microscopical  Examination  of  Eleven  Eocks  from  Ashland  Co., 

Wisconsin.     Geol.  of  Wisconsin,  iii.,  224-238. 
Pumpellt  (Eaphael).  Lithology  of  the  Keweenawan  System.    Geol.  of  Wisconsin, 

iii..  Part  II.,  27-49. 
Sweet  (E.  T.).  Geology  of  the  Western  Lake  Superior  District,  with  maps.    Geol.  of 

Wisconsin,  iii.,  303-362. 
Wadswoeth  (M.  E.).  Notes  on  the  Iron  and  Copper  Districts  of  Lake  Superior. 

Bulletin  of  the  Museum  of  Comparative  Zoology  at  Harvard  College.    Whole 

series,  vii..  Geological  series,  i..  No.  I.  157  pp. 
WiNCHELL  (N.  H.).  The  Cupriferous  Series  at  Duluth.    In  the  Eighth  Annual  Eeport 

of  the  Geol.  and  Nat.  Hist.  Survey  of  Minnesota,  for  the  year  1879,  22-26. 

1881. 

WiNCHELL  (N.  H.).  Preliminary  List  of  Eocks.    In  the  Ninth  Ann.  Eep.  of  the  Geol. 
and  Nat.  Hist.  Surv.  of  Minnesota  for  the  year  1880,  10-114. 


CHAPTER  II. 

EXTENT    AND    GENERAL    NATURE    OF    THE    KEWEENAW 

SERIES. 

General  statement  as  to  the  scope  of  the  terra  Keweenawan.— General  statement  as  to  the  geographical 
extent  of  the  series.— More  detailed  description  of  its  extent.— Extent  of  the  series  underneath 
the  -waters  of  Lake  Superior.— Its  entire  geographical  extent  in  square  miles.— Constancy  of  its 
general  characteristics.— Basic  crystalline  rocks.— Detrital  rocks.— Porphyry-conglomerates.— 
Other  conglomerates.— Sandstones.— Source  of  the  pehhles  of  the  porphyry-conglomerates  now 
found  in  the  original  acid  rocks  of  the  series  itself. — General  characteristics  of  these  original  acid 
rocks. — Recapitulation. 

That  my  statements  as  to  the  geographical  extent  of  the  Keweenawan 
rocks  may  be  understood,  it  is  necessary  to  say  at  the  outset  that  I  exclude 
from  the  Keweenaw  Series  the  slaty  rocks  of  the  region  of  Thunder  Bay 
and  Pigeon  River — the  so-called  "Lower  Group"  of  Logan,  and  Animikie 
Group  of  Hunt.  The  nature  and  general  relations  of  these  slates  are  dis- 
cussed on  a  subsequent  page.  It  should  also  be  stated  that  I  include  in  the 
Keweenaw  Series  the  white  and  red  dolomitic  sandstones  with  accom- 
panying crystalline  rocks,  which  are  so  largely  developed  in  the  peninsula 
between  Black  and  Thunder  bays,  and  stretch  thence  a  long  distance  north- 
ward in  the  valleys  of  the  Black-Sturgeon  and  Nipigon  rivers,  and  occupy 
a  large  area  about  Lake  Nipigon.  Again,  I  exclude  the  horizontal  sand- 
stones which  form  the  South  Shore  east  of  Bete  Grise  Bay,  on  Keweenaw 
Point,  and  westward  from  Clinton  Point,  in  Wisconsin,  to  Fond  du  Lac,  in 
Minnesota.  The  Keweenaw  or  copper-bearing  series,  then,  as  considered 
in  the  following  pages,  is  made  to  include  only  the  succession  of  interbedded 
"traps,"  amygdaloids,  felsitic  porphyries,  porphyry-conglomerates,  and  sand- 
stones, and  the  conformably  overlying  thick  sandstone,  as  typically  devel- 
oped in  the  region  of  Keweenaw  Point  and  Portage  Lake  on  the  south  shore 
of  Lake  Superior. 

The  series  of  rocks  under  consideration  is  almost  entirely  restricted  to 
the  Lake  Superior  basin,  whose  limits  it  passes  only  at  the  southwest,  where 

(24) 


UNITED  STATES   GEOLOGICAL  SURVEY 


COPPEP -BEARING   ROCKS  OF  LAKE  SUPERIOR   PL. 


DBVUMAS 
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and  lowra'  diriMnn cf Uii-  KpwmsiiliT  mivs 


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IfSmnd^l ..fSJfnvpod Xr\V Logim-Jlejfitui^Afiiirtir.Tlri'inii.^ ifat  ra'iajir  .I'lil  Ri'btj-t BeB .nf>in 


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DISTETBUTIOE"  OE  THE  KEWEENAW  SEEIES.  25 

there  is  an  extension  for  a  short  distance  into  the  west  side  of  the  valley  of 
the  Upper  Mississippi.  The  bottom  of  the  lake  itself  is,  throughout  most  of 
its  extent,  composed  of  these  rocks,  as  are  also  the  immediate  shores,  for  a 
great  part  of  the  lake's  circumference.  In  much  of  its  course  around  the 
lake  basin  this  group  does  not  reach  many  miles  inland,  in  some  places  occu- 
pying only  projecting  headlands,  the  older  rocks  forming  the  intervening 
shores.  Towards  the  west  end  of  the  lake,  however,  in  the  stretch  across 
the  state  of  Wisconsin,  there  is  a  very  wide  surface-spread,  the  area  in  Wis- 
consin alone  being  about  five  thousand  square  miles. 

To  convey  a  more  definite  idea  of  their  distribution  in  the  circuit  about 
Lake  Superior,  it  may  be  said  that,  besides  underlying  the  greater  part  of 
the  lake,  the  Keweenawan  rocks  form  the  larger  part  of  Keweenaw 
Point,  probably  also  underlying  the  horizontal  sandstone  of  the  remainder; 
constitute  the  Michigan  shore  from  Keweenaw  Point  to  the  Montreal  River, 
extending  back  into  the  country  8  to  20  miles  ;  in  all  probability  underlie 
the  sandstone  country  still  further  away  from  the  lake,  since  they  appear 
again  in  a  few  places  on  its  southern  edge,  which  would  carry  them  inland 
from  the  Michigan  shore  as  much  as  30  to  35  miles ;  underlie  all  of  north- 
ern Wisconsin  north  of  a  line  from  the  Montreal  River  at  a  point  15  miles 
from  Lake  Superior  to  Numakagon  Lake,  and  thence  to  Saint  Croix  Falls, 
or  the  west  boundary  of  the  state ;  stretch  in  Minnesota  over  two-thirds  of 
the  triangular  area  included  between  the  state  boundary,  the  Saint  Croix 
River,  and  the  Saint  Paul  and  Duluth  Railway;  constitute  the  entire  Minne- 
sota shore  of  the  lake,  from  Duluth  to  Grand  Portage  Bay,  running  back 
into  the  interior  about  midway  in  the  coast  as  much  as  30  miles ;  make  up 
the  outer  ones  of  the  Lucille  group  of  islands  off  Pigeon  Point,  and  the 
whole  of  Isle  Royale;  form  the  entire  peninsula  between  Black  and  Nipi- 
gon  bays,  with  all  outlying  islands,  and  also  the  whole  group  of  islands, 
large  and  small,  south  of  Nipigon  Bay ;  spread  over  a  very  wide  area  in 
the  valleys  of  Black-Sturgeon  and  Nipigon  rivers,  north  of  Lake  Superior; 
after  a  long  interval,  during  which  older  rocks  only  appear  on  the  coast, 
come  up  again  in  Michipicoten  Island,  which  they  entirely  compose ;  ap- 
pear again  on  the  east  coast  of  the  lake  at  Cape  Choyye,  Cape  Gargantua, 
Pointe  aux  Mines,  Mamainse,  Batchewanung  Bay,  and  Gros  Cap — at  nearly 


26  COPPEE-BEARESTG  EOCKS  OF  LAKE  SUPERIOR. 

all  of  which  places  they  form  mere  edgings  between  the  lake  and  the  older 
rocks,  the  only  exception  being  at  Mamainse,  where  a  great  thickness  is 
exposed ;  and,  finally,  constitute  the  isolated  reef  between  Keweenaw 
Point  and  Mai-qiiette,  known  as  Stannard's  Rock.  Between  Gros  Cap  and 
Stannard's  Rock  the  rocks  of  this  series  do  not  appear,  but  there  can  be 
little  doubt  that  they  are  continuous  underneath  the  lake  and  underneath 
the  newer  horizontal  beds  of  the  eastern  part  of  the  upper  peninsula  of 
Michigan. 

The  main  dimensions  of  Lake  Superior  are  as  follows,  in  air-line  dis- 
tances:^ from  Fond  du  Lac  to  Sault  Sainte  Marie,  377  miles;  from  Duluth 
to  Batchewanung  Bay,  the  extreme  straight-line  length  of  the  lake,  368.5 
miles;  extreme  width  from  South  Bay,  near  Grand  Island,  to  the  mouth  of 
Nipigon  River,  195.5  miles;  width  from  Presq'isle  River,  on  the  Michigan 
coast,  to  Poplar  River,  on  the  Minnesota  coast,  73.5  miles;  from  the  eastern 
extremity  of  Keweenaw  Point  to  Nipigon  Point,  116  miles;  from  the  same  to 
Otter  Head,  91.5  miles;  from  the  same  to  Mamainse  Point,  140.5  miles ;  from 
the  same  to  Whitefish  Point,  137.3  miles;  from  the  same  to  Sault  Sainte 
Marie,  170.5  miles;  and  from  the  same  to  Duluth,  212  miles.  A  measure- 
ment, first,  on  a  line  from  Fond  du  Lac,  N.  66J°  E.  to  a  point  in  the  middle 
of  the  lake  on  the  national  boundary  line,  about  45  miles  north  of  Kewee- 
naw Point,  and  thence  S.  55J°  E.  to  Sault  Sainte  Marie,  gave  for  the  two 
distances,  respectively,  229  miles  and  191  miles,  or,  for  the  extreme  length  of 
the  lake,  measured  along  its  middle  line,  420  miles.  The  approximate  length 
of  the  coast  line  of  the  lake,  exclusive  of  the  more  minute  irregularities,  but 
including  all  the  larger  bays,  is  1 ,700  miles.  The  coasts  of  the  greater  islands 
will  increase  this  to  1,900  miles.  The  total  area  of  the  lake,  exclusive  of 
all  the  larger  islands,  but  inclusive  of  all  bays,  is  about  30,880  square  miles. 
Deducting  fi-om  the  last  figure  2,500  square  miles  for  lake  area  underlaid 
by  older  rocks  in  the  extreme  northeast,  in  Thunder,  Black,  and  Nipigon 
bays,  and,  again,  between  Keweenaw  Bay  and  Marquette,  we  obtain 
28,380  square  miles  as  the  approximate  total  extent  of  the  group  under  the 
waters  of  Lake  Superior. 

To  obtain  its  whole  geographical  extent  we  must  add  to  the  last  figure 

^  All  of  these  figures  were  obtained  by  careful  measurements  on  the  V.  S.  Lake  Survey  charts. 


AREA  OP  THE  KEWEENAW  SERIES.  27 

as  follows:  for  Michigan,  in  the  area  "between  Keweenaw  Point  and  the 
Montreal  River  (the  "Main  Trap  Range,"  and  Porcupine  Mountains  region) 
1,744  square  miles;  for  the  area  of  horizontal  sandstone  south  of  the  Main 
Range  of  Michigan,  and  west  of  the  head  of  Keweenaw  Bay,  which  area 
is  undoubtedly  underlaid  by  rocks  of  this  group,  1,400  square  miles;  for 
Wisconsin,  including  the  area  in  the  immediate  vicinity  of  Lake  Superior, 
and  again  on  the  Saint  Croix  River,  where  the  K©weenawan  rocks  are  cov- 
ered with  horizontal  sandstones,  about  5,000  square  miles;  for  Minnesota, 
south  of  the  Saint  Louis,  including  portions  buried  beneath  horizontal  sand- 
stone, 1,000  square  miles;  for  Minnesota,  north  of  Lake  Superior,  about 
3,200  square  miles ;  for  Isle  Royale,  210  square  miles ;  for  Isle  Saint  Ignace 
and  adjoining  islands,  180  square  miles;  for  Michipicoten  Island,  75  square 
miles ;  and  for  small  areas  along  the  east  shore  between  Cape  Gargantua 
and  the  Sault,  90  square  miles.  These  rocks  are  also  undoubtedly  buried 
beneath  the  horizontal  formations  of  the  south  shore  of  the  lake  west  of 
the  Sault,  but  not  to  any  very  great  extent.  So  that  we  may  be  safe  in 
placing  the  entire  geographical  extent  of  the  series  at  41,000  square  miles 
for  the  immediate  basin  of  Lake  Superior.  This  is  exclusive  of  an  exten- 
sion northward  of  some  of  the  lower  beds  into  the  basin  of  Lake  Nipigon 
through  the  valleys  of  Black-Sturgeon  and  Nipigon  rivers.  The  thick- 
ness of  this  extension,  judging  from  Bell's  descriptions,^  is  inconsiderable, 
the  rocks  lying  often  nearly  horizontal.  In  the  valleys  of  Black-Sturgeon 
and  Nipigon  rivers  they  appear  to  form  strips  between  older  rocks  on  either 
side,  but  in  the  Nipigon  Lake  basin  to  have  a  wider  extent.  Bell's  map 
makes  the  total  area  in  this  basin  as  much  as  5,000  square  miles. 

Throughout  all  of  this  wide  extent,  though  local  peculiarities  are  to  be 
noted,  the  general  characteristics  of  the  group  are  wonderfully  constant. 
The  predominating  rocks  belong  to  the  basic  crystalHne  class.  They  are, 
as  a  rule,  in  distinct,  but,  for  the  most  part,  heavy  layers,  their  bedded 
structure  being  due,  as  I  believe,  to  their  having  been  spread  out  as  suc- 
cessive molten  flows,  and  more  rarely,  perhaps,  as  injections  between  the 
previously  formed  layers.     These  basic  rocks  belong  wholly  to  the  augite- 

»  Report  of  Progress  of  Geological  Survey  of  Canada,  1866-'69 ;  1872-'73. 


28        COPPER-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 

plagioclase  class,  hornblende  occurring  only  very  rarely  and  then  always  as 
an  alteration-product.  Pumpelly  has  heretofore  recognized  the  three  types 
of  diabase,  melaphyr,  and  gabbro  as  characterizing  the  Keweenaw  Point 
district,  and  after  a  study  in  the  field  of  the  entire  North  Shore,  and  the 
examination  of  large  numbers  of  specimens  from  all  portions  of  the  Lake 
Superior  basin,  I  am  able,  thovxgh  having  discovered  a  number  of  interest- 
ing new  varieties,  to  add  only  two  kinds  deserving  of  distinct  names,  viz. 
diabase-porphyrite  and  anorthite-rock,  and  these  are,  after  all,  so  closely 
allied  to  the  others  as  to  be  hardly  more  than  varieties.  Indeed,  the  three 
kinds  first  named  grade  into  each  other  in  the  field,  and  are  themselves 
merely  phases  of  an  ancient  class  of  rocks,  for  which  the  science  has  as 
yet  established  no  common  name,  but  which  are  the  old  equivalents  of  the 
post-Cretaceous  basalts. 

The  diabase  is  a  plagioclase-augite  rock,  with  or  without  olivine,  and 
without  unindividualized  base ;  the  melaphyr  carries  more  or  less  of  this 
base  with  olivine,  and  is,  throughout  the  Lake  Superior  region,  everywhere 
characterized  by  the  presence  of  relatively  large  individuals  of  augite,  in- 
cluding numbers  of  minute  plagioclases ;  while  the  gabbro  has  part  or  all  of 
the  augitic  ingredient  as  diallage,  is  orthoclase-bearing  or  not,  and  is  either 
olivine-bearing  or  not.  The  diabase-porphyrite  is  an  olivine-free  diabase, 
with  a  strong  porphyritic  development  and  a  more  or  less  thoroughly  unin- 
dividualized base ;  and  the  anorthite-rock  is  merely  a  coarse  gabbro,  in  which 
all  ingredients  but  the  feldspar  are  wanting.  The  nomenelatiire  adopted  for 
tliese  rocks  is  Rosenbusch's.  There  are  numbers  of  peculiar  phases  of  the 
three  kinds  named,  due  to  amygdaloidal  and  compact  conditions,  relative 
abundance  of  the  several  ingredients,  coarseness  of  grain,  the  presence  of 
unusual  constituents,  and,  especially,  internal  molecular  rearrangements. 
But  the  same  types  constantly  recur  in  the  circuit  of  the  lake,  and  there 
are  only  one  or  two  subordinate  varieties  that  have  not  been  seen  again  and 
again,  and  at  points  widely  removed  from  each  other.  Some  arrangement 
of  the  kinds  as  to  horizon  also  is  to  be  observed.  One  of  the  most  inter- 
esting results  of  my  work  is  the  finding  of  gradation  phases,  not  only 
between  all  the  kinds  named,  but  from  the  most  basic  kinds,  with  less  than 


CHAEACTEES  OF  THE  KEWEENAW  AN  EOCKS.  29 

46  per  cent,  silica,  to  the  most  acid  of  the  acid  kinds  subsequently  men- 
tioned. 

Intel-stratified  with  these  basic  crystalline  rocks,  at  many  different 
horizons,  but  genei'ally  greatly  more  abundant  above,  are  detrital  beds, 
chiefly  reddish  conglomerate  and  sandstone.  The  conglomerates  are  for  the 
most  part  made  up  of  pebbles  of  one  or  more  of  three  kinds  of  acid  rocks, 
viz :  (1)  a  red  to  bi'own  or  purple  felsite,  nearly  or  quite  without  either 
quartz  or  orthoclase  as  porphyritic  ingredients ;  (2)  a  true  quartziferous 
porphyry,  usually  brick-red  in  color ;  (3)  a  non-quartziferous  porphyry, 
with  bright-red  striated  feldspar  crystals ;  and  (4)  fine-grained  to  coarse- 
grained granitic  porphyry  and  augite-syenite,  of  which  the  several  phases 
verge  towards  quartziferous  porphyry  or  gi'anite  on  the  one  hand,  and 
quartzless  porphyry  or  an  orthoclase-bearing  gabbro  on  the  other.  The 
first  two  kinds  are  commonly  without  structural  lines,  but  occasionally  show 
faint  and  wavy  bandings.  One  or  two  of  these  kinds  will  be  found  to  pre- 
dominate greatly  at  any  one  place,  the  same  conglomerate  belt  showing  at 
different  points  along  its  course  great  differences  in  this  respect. 

Pebbles  of  the  basic  rocks  also  occur  in  the  porphyry- conglomerates, 
but  they  are  relatively  very  rare.  Pebbles  of  altogether  different  rocks  are 
occasionally  seen ;  as,  for  instance,  on  the  peninsula  of  Mamainse,  on  the 
■east  coast,  where  granitic  and  gneissic  pebbles  are  abundant.  The  matrix 
of  these  conglomerates  appears  to  be  of  the  same  material  as  the  pebbles 
themselves,  and  in  the  coarser  kinds  can  easily  be  seen  to  be  so.  It  is  fre- 
quently permeated  by  calcite,  which  at  times  has  completely  replaced  the 
matrix,  yielding  a  striking  looking  combination  of  red  and  brown  pebbles 
with  a  background  of  pure  white  cleavable  calcite. 

An  altogether  different  conglomerate  from  those  just  described,  and  one 
of  much  more  restricted  distribution,  has  a  red  shaly  matrix,  often  finely 
laminated,  in  which  the  pebbles  are  wholly  of  the  common  diabase  and  dia- 
base-amygdaloid. Of  these  conglomerates  there  seem  to  be  again  two 
kinds;  one  in  which  the  jjebbles  are  distinctly  waterworn,  and  another  in 
which  there  is  no  such  distinct  evidence  of  water  action,  and  in  which  the 
vesicular  exteriors  of  the  balls  suggest  their  possible  origin  as  volcanic  scoriae 
that  have  become  buried  in  the  accumulating  detritus.     The  first  of  these 


30  eOPPER-BEAEINa  EOCKS  OF  LAKE  SUPERIOR. 

varieties  has  been  noted  on  the  North  or  Minnesota  Shore  only.  The  other 
has  been  observed  on  both  the  South  and  North  Shores,  and  is  often  hard  to 
distinguish  from  a  kind  in  which  the  red  shaly  material  is  most  confusedly 
mingled  with  the  vesicu.lar  amygdaloidal  diabase,  which  at  times  seems  to 
grade  into  the  detrital  matrix,  and  again  to  be  separated  from  it  in  more  or 
less  distinctly  defined  balls;  an  appearance  suggesting  the  deposition  of 
detrital  material  upon  and  within  the  extremely  scoriaceous  upper  portion 
of  a  lava  flow. 

Sandstones  make  up  much  the  greater  portion  of  the  detrital  members 
of  the  series,  reddish  sandstones  prevailing.  These  run  from  earthy  and 
shaly  to  quite  coarse  granular,  but  are  always  aluminous  from  the  presence 
of  a  more  or  less  decomposed  feldspathic  constituent.  They  vary  from 
brick-red  to  quite  dark-red  in  color,  and  are  made  up  in  large  measure  of 
the  detritus  of  the  same  acid  rocks  that  have  supplied  the  pebbles  of  the 
conglomerates,  as  was  first  shown  by  Pumpelly.  In  the  darker  kinds  more 
or  less  basaltic  detritus  is  included.  Quartz  is  never  an  exclusive,  nor 
often  even  a  very  prominent,  ingredient  in  any  of  the  sandstones  belonging 
without  dispute  to  this  group.  Certain  sandstones  forming  the  eastern  side 
of  Keweenaw  Point,  and  again  the  Wisconsin  shore  as  far  west  as  Fond 
du  Lac  in  Minnesota,  are  highly  quartzose  in  their  uppermost  portion,  but 
these  do  not  belong  to  the  Keweenaw  Series.  Many  of  the  red  sandstones 
are  highly  charged  with  secondary  calcite.  Those  kinds  of  sandstone 
which  are  dark-gray  to  nearly  black  in  color  are  made  up  of  basic  detritus, 
usually  mingled  with  more  or  less  of  the  common  porphyry  detritus,  and 
cemented  by  secondary  calcite.  These  sandstones  often  contain  only  a 
rare  quartz  grain,  having  then  not  over  fifty  per  cent,  of  silica.  They  grade 
into  finer  varieties,  which  at  times  pass  into  an  earthy  black  shale  or  slate. 
These  gray  sandstones  and  accompanying  black  shales,  with  a  thickness  of 
several  hundred  feet,  have  been  recognized  in  a  single  belt,  running  from 
the  neighborhood  of  the  Gratiot  River,  on  Keweenaw  Point,  to  Bad  River, 
in  Wisconsin — a  total  distance  of  150  miles. 

The  source  of  the  materials  which  make  up  the  porphyry  conglom- 
erates and  red  sandstones  has  been  a  matter  of  speculation  to  all  writers 


SOUECE  OF  THE  DETEITAL  MATERIAL.  31 

on  Lake  Superior  geology.  Foster  and  "Whitney  supposed  them  to  come 
from  the  friction  of  the  ascending  igneous  rocks  against  the  rocks  pene- 
trated/ but  they  ignored  the  totally  different  natures  of  the  porphyry  of 
the  pebbles  and  of  the  diabases  forming  the  greater  part  of  the  series.  Be- 
sides, it  has  long  been  plain  that  the  pebbles  of  these  conglomerates  are 
simply  waterworn  fragments  of  some  massive  acid  rocks,  which  could  never 
have  been  far  removed  from  where  the  pebbles  now  are.  It  has  been  sup- 
posed by  some  that  the  original  massive  rocks  were  to  be  looked  for  in  the 
older  so-called  Huronian,  although  wherever  this  Huronian  is  exposed  in 
the  Lake  Superior  country  such  acid  rocks  are  noticeably  wanting. 

I  find  the  source  of  the  pebbles  in  the  massive  acid  rocks  of  the  series 
itself,  and  recognize  now  for  the  first  time  that  these  original  acid  rocks  are 
one  of  its  most  prominent  features,  characterizing  it,  as  they  do,  through- 
out its  entire  extent,  although  always  subordinated  in  quantity  to  the  basic 
kinds.^  I  find  it  even  possible  to  trace  some  of  the  pebbles  of  the  con- 
glomerates to  their  immediate  sources.  Pumpelly  has  shown^  that  the 
same  belt  of  conglomerate  will  vary  in  its  predominant  pebbles  in  differ- 
ent portions  of  its  longitudinal  extent,  while  several  conglomerates  in  one 
section  will  often  show  the  same  characteristic  pebbles,  facts  which  are  to 
be  explained  by  the  differences  in  the  original  rocks  at  different  points  along 
the  trend  of  the  formation,  and  the  derivation  of  the  pebbles  of  the  several 

1  Op.  cit.,  p.  99. 

=  Foster  and  Whitney  (Report  on  the  Lake  Superior  Land  District,  Vol.  T,  pp.  65  and  70)  speak 
of  "quartzose  porphyry"  and  "jasper"  as  occurring  at  Mount  Houghton,  on  Keweenaw  Point,  and  in 
the  PorcupineMonntains,  but  they  do  not  seem  to  have  appreciated  the  true  nature  of  these  rocks,  which, 
moreover,  they  regarded  as  merely  alterations  of  the  red  sandstones  by  the  heat  of  the  intrusive  rocks. 
Macfarlane  distinctly  recognizes  the  existence  of  true  quartzose  porphyry  and  of  "trachyte"  and 
"phonolite,"  on  Miohipicoteu  Island  (Report  of  the  Geological  Survey  of  Canada  for  18C6,  pp.  137-143), 
but  he  does  not  appear  to  have  realized  the  importance  of  his  observation. 

Hunt  has  recently  spoken  of  a  true  quartzose  porphyry  as  occurring  on  a  small  island  near  Saint 
Ignace  Island,  on  the  North  Shore,  but  he  appears  to  regard  this  as  Huronian,  though  it  is  undoubtedly 
merely  one  of  the  numerous  instances  of  the  occurrence  of  this  rock  within  the  Keweenaw  Series. 
(Second  Geological  Survey  of  Pennsylvania.  Special  Report  on  the  Trap-Dykes  and  Azoic  Rocks  of 
S.  E.  Penna.,  ^  373,  p.  193,  and  §446,  p.  229.)  In  my  work  in  the  Bad  River  country  of  northern  Wis- 
consin, in  the  years  from  1873  to  1877,  I  myself  had  also  recognized  true  granites  cutting  gabbro  at 
the  base  of  the  Keweenaw  Series,  and  also  noted  and  mapped  two  or  more  belts  of  apparently  massive 
quartzose  porphyry  and  felsite ;  but  these  latter  were  so  poorly  exposed— the  deceptively  massive  ap- 
pearance of  some  of  the  conglomerates  being  well  known  to  me — that  I  only  provisionally  announced 
the  existence  of  massive  acid  rocks  in  my  published  results  (Vol.  Ill,  Geology  of  Wisconsin,  pp.  11  and 
193-198.) 

!*  Geological  Survey  of  Michigan,  Vol.  I,  Part  II,  p.  16. 


32        OOPPEE-BEAEING  EOOKS  OF  LAKE  SUPERIOE. 

conglomerates  of  one  section  from  a  common  source.  Thus  the  Portage 
Lake  conglomerates  all  carry  a  great  predominance  of  non-quartziferous 
porphyry  pebbles,  while  further  northeast  a  granitic  porphyry  or  augite- 
syenite  becomes  very  abundant,  and  still  further,  in  the  region  of  the  Cal- 
umet mines,  a  true  quartz-porphyry  prevails.  In  the  latter  case  the  source 
was  a  quartz-porphyry  mass  to  the  southeast,  of  which  small  exposures  are 
still  to  be  seen.  In  the  Eagle  River  conglomerate  again  a  common  pebble  is 
a  quartzless  porphyry,  much  like  the  massive  rock  exposed  at  the  old  Suffolk 
location  to  the  southeast.  The  same  association  is  to  be  noted  in  the  Onto- 
nagon and  Porcupine  Mountains  regions. 

These  original  acid  rocks  embrace  all  of  the  kinds  included  as  pebbles 
in  the  conglomerates,  viz:  true  quartziferous  porphyries,  with  large  doubly 
terminated  quartzes  and  orthoclases  as  porphyritic  ingredients;  a  non- 
quartziferous  porphyry;  compact  felsite;  granitic  porphyry  and  augite- 
sy enite ;  and  true  granite.  Most  of  these  rocks  are  of  some  sort  of  reddish 
hue,  running  from  a  pale  pink  to  a  bright  brick-red;  whence  in  large 
measure  the  red  colors  of  the  conglomerates  and  sandstones  derived  from 
them. 

In  recapitulation,  then,  it  is  to*  be  said  that  the  Keweenaw  Series  con- 
sists of  eruptive  flows  and  beds  of  detrital  rocks  interstratified  with  one 
another,  the  eruptive  rocks  occurring  also  subordinately  in  dike  form. 

The  eruptive  rocks  include  basic,  intermediate,  and  acid  kinds,  as  is 
commonly  the  case  with  volcanic  regions  of  moi'e  modern  activity,  but 
there  is  no  such  chronological  relation  between  these  three  kinds,  as  is  so 
often  found  to  be  the  rule  in  Tertiary  and  post-Tertiary  volcanic  regions. 

An  extraordinary  thing  is  the  complete  absence  from  the  series  of  any- 
thing like  volcanic  ash;  another  point  of  difference  between  it  and  the 
rocks  of  regions  of  more  recent  volcanic  activity,  and  one  which  helps 
to  support  the  view  that  the  eruptive  rocks  of  this  region  have  come  through 
open  fissures,  and  not  after  the  manner  of  the  volcanic  flows  of  the  present 
day,  as  the  extreme  uniformitarians  would  have  us  believe. 

The  detrital  rocks  of  the  series  are  all  composed  of  water-derived  frag- 
ments, broken  for  the  most  part  from  the  acid  rocks  of  the  series  itself. 


EECAPITULATIOK.  33 

Such  viscous  materials  as  these  acid  rocks  must  have  been  when  molten 
would  solidify  into  more  or  less  bulky,  erect  masses  of  relatively  small 
area;  and  their  degradation  into  geologically  contemporaneous  conglomer- 
ates seems  to  have  been  chiefly  rendered  possible  by  this  mode  of  occur- 
rence. Strewn  as  they  were  around  the  rim  of  a  basin  whose  middle  por- 
tion was  steadily  depressing  during  the  growth  of  the  series  and  whose 
edges  were  at  the  same  time  rising,  each  of  these  bulky  masses  was  able 
to  supply  the  materials  for  a  number  of  different  horizons  of  sandstone  and 
conglomerate. 
3  L  s 


OHAPTEK  III. 
LITHOLOGY  OF  THE  KEWEENAW  SERIES. 

Section  I.  Basic  original  rocks.— Pumpelly's  investigations  on  the  basic  rooks  of  Keweenaw  Point; 
of  Northern  Wiaoonsin.— Status  of  the  subject  at  the  close  of  the  Wisconsin  Survey.— Nomencla- 
ture.—General  statement  of  results. — Classiiication. 
Coarse-grained  basic  rocks.— Orthoelase-free  diabase  and  gablro,  and  olivine-gablro :  general  macroscopic 
description  of ;  olivine  of;  plagioclase  of ;  magnetite  of;  augite  or  diallage  of ;  accessory  ingre- 
dients of ;  appearance  in  the  field  of;  typical  localities  of;  tabulation  of  microscopic  observa- 
tions on.— Orthoclase-iearing  gabbro:  distinctions  from  the  non-orthoclastic  gabbros;  orthoclase 
of;  plagioclase  of ;  magnetite  of ;  augite  and  diallage  of ;  accessory  ingredients  of ;  occurrence  in 
the  field  of;  typical  localities  of ;  tabulation  of  microscopic  ohservihtioiis  on.— Hornblende-gabbro: 
distinctions  of,  from  preceding  kinds ;  tabulation  of  microscopic  observations  on.— Anorthite-rock : 
general  description  of;  tabulation  of  microscopic  observations  on. 
riKE-GRAiHED  BASIC  KocKB.— Quantitative  relations.— OZij!iMe-/)-€e  diabase,  of  the  ordinary  type:  Pumpelly's 
study  of;  occurrence  in  the  field  of ;  amygdaloidsandpseudamygdaloidsof ;  alterations  of ;  micro- 
scopic characters  of;  typical  localities  of;  tabulation  of  microscopic  observations  on.— Olivin- 
itic  fine-grained  diabase  ana  melapliyr  :  Pumpelly's  descriptions  of ;  occurrence  in  the  field  of ;  tab- 
ulation of  observations  on. — Ashled-diabase  and  diabase-porpTtyrUe :  macroscopic  characters  of; 
-  microscopic  description  of :  wide  range  in  acidity  of ;  typical  localities  of ;  tabulation  of  observa- 
tions on.— ^m2/prfaZoi(?s ;  relations  to  the  other  basic  rocks  of ;  microscopic  characters  of ;  macro- 
scopic characters  of;  Pumpelly's  study  of;  altered  forms  of;  Pumpelly's  conclusions. 

Section  II.  Acid  ohiginai  rocks  :  Quantitative  relations  of;  classification  oi.—  Quarisless  porphyry : 
relations  of,  to  the  diabase-porphyrites  and  quartziferous  porphyries  ;  macroscopic  characters  of; 
microscopic  characters  of ;  porphyri tic  ingredients  of ;  typical  localities  of ;  tabulation  of  micro- 
scopic observations  on.— Quartzife)-ous  porphyry  andfelsite:  quantitative  relations  and  occurrence 
of-  difSoulties  in  the  study  of ;  Eosenbusch's  nomenclature;  macroscopic  characters  of  matrix 
of-  microscopic  characters  of  matrix  of;  macroscopic  and  microscopic  characters  of  porphyritic 
ingredients  of ;  origin  of ;  typical  localities  of ;  t&hnlation  of  ohaervntions  on.— Augite  syenite  and 
granitell  or  granitic  porphyry:  macroscopic  characters  of;  microscopic  characters  of;  gradation 
phases  of,  into  other  kinds;  difficulty  in  finding  a  name  for;  relations  of,  to  the  modern 
trachytes;  typical  occurrences  of ;  tabulation  of  microscopic  observations  on.— Granite :  occur- 
rence of,  in  the  Bad  River  country  of  Wisconsin ;  macroscopic  and  microscopic  characters  of; 
absence  of,  in  the  rest  of  the  extent  of  the  formation. 

Section  III.  Summary  view  of  the  original  rocks  of  the  Keweenaw  series. 

Section  IV.  Detbital  TiOC^s.—Conglovierates.— Sandstones.— Basic  sandstones.— Quartzose  sandstones. 
—Secondary  minerals  in  sandstones.— Tabulation  of  microscopic  observations. 

The  natural  grouping  of  all  of  the  rocks  of  the  Keweenaw  Series  into 
the  three  classes  of  basic  original,  acid  original,  and  detrital  rocks  has 
already  been  indicated.^  In  what  follows,  each  prominent  variety  is  taken 
up  in  some  detail. 

1  See  Chapter  II. 
(34) 


UNITED  STATES   GEOLOGICAL  SURVEY 


COPPER-BEARING   ROCKS  OF  LAKE  SUPERIOR    PL. 


A.HoenA  Co.HUi..HiiltiiiK>rp. 


OLIVINE -FREE  GABBROS 


35 


^•^BwsSantt  .1.^3^ 


to   viXlsfi 


scribed 
r 


in  the  same  volum* 


These 


soft's,  -f ^--tn^S. -roan    .-^ojTso^aT.  a-S^ni \<s  Vino5    wo-^X   o-^iSv^n^i  -^o   5^r.Sa'sG.   .&  ttrtn.  T,<}'5\ 


Titfsia-nii..  Hzalate   or  ffcOb'^To  from    coasi  of  laTre.  Suptrior.  near- Braver  Say,    -J^zn 
Jno-rtTi-iiefVi   aztfiie^ll. 


^7.1.  orclznary 


Fi'ffs.  s  n-ni^.      Ga.iiTofro-m.  cnasi  of  laJre  Superior  near  2>u?tci7).,    JffirtTi  Ft'^.  S    ordinary  li'yA^ .         ■Ti.'y.  ^ 

po7arxr.tcl    Iz^Tii  Scale,   so   c7za.7neie.rs' 

-^inorihticlJ^;  ctzalZaytc   Qzi.^tte(±j ;  tzianiferouj  mcLyrteiiit  (S)j  iron.  oxi'6lt/(^.> 


LITHOLOGT.  35 


Section  I.— BASIC  OEIGINAL  BOOKS. 

In  his  paper  on  "The  Metasomatic  Development  of  the  Copper-bearing 
Eocks  of  Lake  Superior,"  published  in  1878/  Pumpelly  first  showed  the 
true  nature  of  the  prevalent  basic  rocks  of  the  copper  region  of  Michigan. 
His  work  was  chiefly  on  specimens  from  the  section  displayed  on  the  lower 
Eagle  River,  Keweenaw  Point.  Subsequently  (1880),  the  same  geologist 
published^  the  results  of  an  examination  of  a  suite  of  specimens  collected 
for  the  Wisconsin  Geological  Survey  by  Messrs.  M.  Strong,  E.  T.  Sweet, 
and  myself  in  the  districts  severally  under  our  charge.  These  Wisconsin 
rocks  he  was  able  for  the  most  part  to  throw  into  the  same  groups  whose 
existence  he  had  already  determined  in  his  Michigan  woi'k,  the  only  im- 
portant addition  being  one  or  two  varieties  of  true  gabbro,  now  first  proved 
to  exist  in  the  Lake  Superior  country.  These  gabbros  were  further  de- 
scribed in  the  same  volume  by  Mr.  A.  A.  Julien,  and  in  some  detail  by 
myself 

At  the  close  of  the  investigations  of  the  Wisconsin  Survey  the  list  of 
the  basic  rocks  of  the  Keweenaw  Series  stood  as  follows :  gabiro,  including 
gabbro  proper,  olivine-gabbro,  uralitic  gabbro,  and  an  orthoclase-bearing 
gabbro ;  diabase,  including  the  ordinary,  prevalent,  fine-grained  type,  one 
or  two  coarse-grained  varieties,  a  type  designated  as  "  ashbed  "  diabase,  a 
pseud-amygdaloidal  diabase,  and  true  diabase-amygdaloids ;  and  melaphyr, 
including  melaphyr  proper,  as  also  its  pseud-amygdaloidal  and  true  amyg- 
daloidal  phases.  The  names  gabbro,  diabase,  and  melaphyr  were  given  in 
accordance  with  the  usage  of  H.  Rosenbusch,  who  has  classed  the  pre- 
tertiary  plagioclase-augite  rocks  as  indicated  in  the  following  scheme:^ 
I.  Geanulae. 

a.  Plagioclase-augite  =  diabase. 

b.  Plagioclase-augite-olivine  =  olivine-diabase. 

II.  PoEPHYEiTic,  contaiuing  more  less  of  an  insoluble  base. 

a.  Plagioclase-augite  =  diabase-porphyrite. 

b.  Plagioclase-augite-olivine  =  melaphyr. 

'Proceedings  of  the  American  Academy  of  Arts  and  Sciences,  Vol.  XIII,  pp.  253-309. 
'Geology  of  Wisconsin,  Vol.  II,  pp.  29-49. 

'Microscopische  Physiographie  der  Massigen  Gesteine,  von  H.  Eoaenbuscli.  Stuttgart,  1877,  pp. 
317,  458. 


36        COPPER-BEAEING  ROCKS  OF  LAKE  SUPERIOR. 

III.  Glassy,  known  only  as  subordinate,  vitreous  modifications  of  diabase-porphyrite 

not  deserving  of  a  special  name. 

IV.  Gkanui.ak  Plagioclasb-Diallage  Rocks. 

a.  Plagioclasc-diallage  =  <7a&6ro. 

b.  Plagioclase-diallage-olivine  =  olivine-gabhro. 

For  the  present  work  I  have  studied  some  eight  hundred  sections  made 
from  specimens  collected  from  all  parts  of  the  entire  extent  of  the  Kevree- 
navp-  Series,  including  the  typical  localities  of  Pumpelly's  Michigan  descrip- 
tions. I  have  also  had  in  my  hands  the  original  sections  made  by  Pumpelly 
for  his  study  of  the  Wisconsin  rocks.  Although  my  microscopic  work  has 
been  extended  over  so  much  wider  a  field  than  his — covering,  as  it  has,  not 
only  the  already  partly  studied  districts  of  Keweenaw  Point  and  northern 
Wisconsin,  but  also  the  hitherto  wholly  unstudied  rocks  of  the  north  and 
east  shores  of  the  lake,  of  the  Bohemian  Range  on  Keweenaw  Point,  of  the 
Ontonagon,  Porcupine  Mountain,  and  "  South  Range"  districts  of  western 
Michigan,  of  the  Snake  and  Kettle  River  region  of  Minnesota,  and  of  Mi- 
chipicoten  Island — its  chief  result,  so  far  as  the  basic  rocks  are  concerned, 
has  been  to  extend  the  application  of  Pumpelly's  conclusions  over  the  entire 
spread  of  the  Keweenaw  Series.  I  have  been  able  to  recognize  a  number  of 
new  varieties  of  his  gabbro,  melaphj'r  and  diabase,  and  two  new  kinds — 
anorthite-rock  and  diabase-porphyrite — not  to  be  included  under  any  of 
these  three,  though  closely  related  to  them,  and  have  found  that  there  is  a 
large  class  of  rocks  which  are  intermediate  in  point  of  acidity. 

Tlie  classification  given  below  of  the  basic  original  rocks  of  the  Ke- 
weenaw Series  is  based  not  only  on  microscopic  differences  and  resem- 
blances, but  also  upon  the  characters  and  relations  of  the  several  kinds  as 
seen  in  the  mass,  and  on  their  prominence  and  persistence  in  the  field.  For 
instance,  the  first  group  given  of  the  coarser  grained  rocks  is  made  to  in- 
clude orthoclase-free  gabbro  and  diabase,  olivine-diabase,  and  olivine-gab- 
bro,^  because  all  of  these  in  the  hand  specimens  bear  the  closest  resemblance 
to  each  other,  and  in  the  field  are  seen  to  constittite  parts  of  the  same  con- 
tinuous mass  or  bed,  forming  together  one  of  the  most  prominently  occur- 
ring types. 

'  The  distinction  between  diallage  and  augite  is  a  valueless  one,  since  not  only  are  both,  often 
found  in  the  same  section,  but  every  gradation  is  found  in  the  rooks  of  this  class  from  augite  to  diallage. 


BASIC  ORIGINAL  EOCKS.  37 

It  will  be  seen  at  once  that  all  of  the  kinds  named  are  very  closely 
related.  Except  one  extreme  phase — the  anorthite-rock — they  are  all  pla- 
gioclase-augite  rocks  (the  diallage  being  but  a  phase  of  ordinary  augite) 
and  all  carry  magnetic  or  titanic  iron,  while  olivine  is  a  common  ingre- 
dient. The  differences  consist  only  in  variations  in  coarseness  of  grain, 
relative  amounts  of  the  several  ingredients,  presence  or  absence  of  olivine, 
presence  or  absence  of  unresolvable  base,  presence  or  absence  of  ortho- 
clase,  variations  produced  by  metasomatic  changes,  and  variations  in  texture 
from  granular  to  vesicular  (amygdaloids).  All  of  the  kinds,  though  dis- 
tinct enough  in  the  field,  are  in  fact,  lithologically  considered,  but  phases  of 
one  kind  of  rock,  for  which  usage  has  not  established  any  common  name. 
The  term  "basalt,"  restricted  byRosenbusch  to  the  younger  equivalents  of 
the  pre-Tertiary  olivine-diabase  and  melaphyr,  has  been  much  used  with  a 
more  extended  signification,  and  might,  perhaps,  be  not  improperly  used  as 
a  general  term  for  all  plagioclase-augite  rocks,  young  and  old. 

BASIC  OEIGIKAL  EOCKS  OF  THE  KEWEENAW  SERIES. 

I.  COAKSE-GRAINED. 

1.  Gabbro  and  diabase;  olivine-gabbro  and  olivine-diabase;  all  free  from  ortho- 

clase. 

2.  Orthoclase-bearing  gabbro. 

3.  Hornblende-gabbro. 

4.  Anorthite-rock. 

II.   FiNE-GEAINED. 

5.  Diabase  of  the  "ordinary  type." 

6.  Olivinitic  fine-grained  diabase  and  melaphyr. 

7.  "Ashbed  "-diabase  and  diabase-porphyrite. 

8.  Amygdaloids  (vesicular  diabase  and  melaphyr). 

COARSE-GKAINED   BASIC    ROCKS. 

Ortliodase-free  diabase  and  gahhro,  olivine-diabase  and  olivine-gabbro. — The 
rocks  included  here  have  a  prevailing  very  dark-gray,  often  black,  shade. 
More  rarely  they  are  light-gray,  when  the  plagioclastic  ingredient  becomes 
greatly  predominant,  as  is  apt  to  be  the  case  in  the  coarsest  kinds.  Not 
unfrequently  a  brownish  film  (ferrous  oxide)  over  the  shining  black  augite 
produces  a  resinous  hue,  and  a  somewhat  similar  effect  is  occasionally 
produced   by  a  relatively  large  proportion  of  olivine.     The  texture  is  a 


38        COPPBE-BEAKING  EOCKS  OP  LAKE  SUPEEIOE. 

very  highly  crystalline  one,  causing  a  rough-surfaced  fracture.  The  sev- 
eral primary  ingredients,  except  olivine,  can  nearly  always  be  recognized 
with  a  lense,  and  in  the  coarser  kinds  with  the  naked  eye.  The  specific 
gravity  ranges  from  2.8  to  3.1. 

The  olivine,  which  is  a  common,  but  not  constant,  ingredient  of  these 
rocks,  is,  when  present,  alwaj^s  the  oldest  of  the  chief  ingredients,  as  is 
plainly  enough  shown  by  its  relation  to  the  others  in  the  thin  section. 
Occasionally  in  the  fresher  rocks  it  may  be  detected  with  the  lens  in  char- 
acteristic glassy,  green  grains,  and  at  times  is  even  of  so  large  a  size  as  to 
attract  the  unaided  eye;  as,  for  instance,  in  a  resinous- hued,  rather  coarse, 
and  a  good  deal  weathered  rock,  which  forms  a  low  cliff  on  the  north  shore 
of  Lake  Superior  (Sec.  34,  T.  57,  E.  3  E.,  Minnesota),  a  short  distance  east 
of  the  mouth  of  the  Brul^  River.  In  this  rock  the  olivine  occurs  in  abundant, 
black,  glassy  particles,  from  one-sixteenth  to  one-third  inch  in  diameter, 
with  a  scaly  structure  from  commencing  decomposition,  and  is  evidently, 
from  its  high  iron  content,  close  to  the  variety  hyalosiderite.  In  specimens 
from  the  vicinity  of  Bladder  Lake,  in  Ashland  County,  Wisconsin,  a  light- 
green,  glassy  olivine  is  very  noticeable  to  the  unaided  eye. 

.  As  seen  under  the  microscope,  the  olivine  occurs  nearly  always  in 
irregularly-outlined,  rounded  particles,  from  a  fraction  of  a  millimeter  to 
two  or  three  millimeters  in  length.  Only  very  rarely  does  it  present  crys- 
talline outlines.  Commonly,  it  is  largely  fresh,  presenting  a  grayish  or 
nearly  colorless  section,  with  the  characteristic  rough  surface.  It  is,  how- 
ever, very  rarely  so  fresh  as  to  be  without  some  traversing  rifts,  edged  with 
a  greenish-brown  or  brownish-yellow  alteration-product.  In  less  fresh 
kinds  this  brown  alteration  has  affected  the  whole  area,  and  in  such  cases 
the  rock  has  macroscopically  a  pronounced  resinous  appearance.  Less  fre- 
quently, but  still  often,  the  iron  oxide,  instead  of  being  deposited  about  and 
within  the  olivine,  has  been  leached  out,  and  then  the  mineral  is  more  or 
less  completely  represented  by  a  greenish  material,  supposed  to  be  serpen- 
tine. In  a  number  of  sections  in  which  this  greenish  alteration  was 
observed  magnetite  was  noticed  in  small  particles,  associated  with  the 
green  in  such  a  way  as  to  suggest  that  it  also  was  an  alteration-result  from 
the  olivine.     In  some  of  the  very  coarse-grained  gabbros  of  Bad  River, 


rt.^&/tLn.cfJL    Coarse.   olnr^ne-Aaiaje.  or  oiiJ'jne.-yaiip'o  J^rorn.    J^re/tjC-A. 
Hirer    j^tnn&jota-  Scaler  ■i^  <£'i  a.  yn^ie-rj 

Fiff./    OTclzTtary  t^yd        Fvoa-  po2a-i-zxa^    H^At.  See- ^afC-tXT.Af/ 


Fzff.3.  OxKii-je^  oZi-ri  ?i&.-<iza2at&  or  oliyiiie.^-oai'iro  ^roTri, 
fior^A.  Jjia-ro  JjaTfe^ Simper z'oT^  -near  SzicJ^er  7?t'yfr,  ^^?7i/*; 
•ScaZe.   S.f  iiz.<t-7'Leifers.  Orc?tna.irY  Zz^^ .  See-iiy!  zjaittiv/: 


Fzf-V.  Jizrye,  oYz-nyteJ    fro?rt.  scLTttc  T'oc.?r  at  i7to/r>*  z'n.  J\^JJ unSj, 
Sca.7c.  J-s-^zamjefers.  Oralznary  iio/U.  JeC'p  Z(,  zil 

7^A&s-&.   v?s'yzne.f    a?Z  ao^ea-r  au   tf  ca^tf?t,  o?' Tforj^,    a?t2y ^orCzo^S 
arc  eT^i^rcly  fres^  17/ ;    ^Acy  are.  ira-yersed.  iy  r^ii{3.jj  o2o}l^ 
yr^Lzc^  tTzCTC^  iif  «'?«'    aJtem^zojo  to  •yerpe7z.tz7ze,(jj ^  ari^i/ 
oxz^e.   ojiro7v(j^jj     /?at?i.  o^  rr?i'Z''c\  a/^Gro-izaro pro ^zco-^s 


iic  olivine  occurs  in  abundant, 

.■      „  .       ,^        •  ■.  ,  .^ric-lliird  inch  in  diameteaj. 


Viio-'.aXiisa'is"^  ■^T^aSs'^a  ■os'^sEs'iss'fci  \S S.\ b^ si\ "idol's a  xV'^svi^iW 


UNITED  STATES   GEOLOGICAL  SURVEY 


COPPER-BEARING   ROCKS  OF  LAKE  SUPERIOR   PL. 111. 


OLIVINE -GABBROS 


ORTHOOLASE-FEEE  GABBEO.  39 

"Wisconsin,  as  first  shown  by  Julien,^  the  ohvine  shows  a  very  interesting 
and  unusual  mode  of  change,  namely,  into  biotite,  viridite,  and  talc,  the 
two  former  replacing  the  interior  of  the  olivine  grain,  the  latter  forming 
a  sort  of  shell  of  minute  flakes  around  the  outer  part  of  the  grain.  Parti- 
cles may  be  seen  in  all  stages  of  this  change. 

Next  to  the  olivine,  in  order  of  age,  is  the  plagioclastic  ingredient.  Its 
crystals  are  usually  in  elongated  forms,  running  from  under  a  sixteenth  of 
an  inch  in  length  to  two  or  three  inches  in  the  coarsest  kinds.  The  outlines 
of  these  crystals  are  commonly  linear,  or  at  least  partly  so,  but  in  some 
sections  the  mutual  interruptions  have  produced  completely  rounded  con- 
tours. In  composition — to  judge  from  measurements  of  the  angle  contained 
between  the  maximum  extinction  positions  of  adjacent  hemitropic  bands,  in 
sections  cut  at  random  in  the  zone  0:  il,  as  viewed  between  the  crossed 
nicols — the  plagioclase  appears  always  to  be  near  the  basic  end  of  the  feld- 
spar series.  The  sections  measured  were  known  to  be  in  this  zone  by  their 
giving  equal,  or  nearly  equal,  angles  between  the  position  of  coincidence 
of  a  nicol  plane  and  the  lines  of  junction  of  the  hemitropic  bands,  and  the 
maximum  extinction  position  of  the  one  set  of  hemitropic  bands  on  the  one 
side  of  this  position  of  coincidence  and  that  of  the  other  set  on  the  other 
side.  The  method  is  Pumpelly's  modification  of  Des  Cloizeaux's  method 
of  distinguishing  between  the  plagioclase  feldspars.^  According  to  it,  those 
feldspars  from  which  the  largest  angles  obtainable,  after  measuring  a  number 
of  individuals,  are  below  36°,  are  classed  as  oligoclase;  those  whose  largest 
angles  lie  between  36°  and  62°,  as  labradorite;  and  those  giving  angles 
above  62°,  as  anorthite — the  size  of  the  angles  increasing  with  the  basicity.* 

'  Vol.  Ill,  Geology  of  Wisconsin,  p.  235. 

2  Metasomatio  Development  of  the  Copper- bearing  Eocks  of  Lake  Superior ;  Proo.  Am.  Acad.  Sci., 
Vol.  XIII,  pp.  30,  31.     (1878. ) 

3  The  method  is,  of  course,  open  to  the  objection  that  it  determines  the  presence  of  only  the  high- 
angled  or  basic  feldspars.  In  a  rock  of  which  a  number  of  sections  refused  always  to  give  any  but  low 
angles  this  would  not  be  any  objection,  but  in  those  giving  high  angles  it  leaves  room  for  doubt.  Nev- 
ertheless, in  the  present  case  I  have  little  doubt  that  there  is  only  one  feldspar  concerned.  This  is 
indicated  by  the  exact  similarity  in  all  respects  in  any  one  thin  section  between  the  high-angled  and 
low-angled  particles,  the  latter  of  which  are,  indeed,  but  few  in  number.  So  far  as  the  Lake  Superior 
rocks  are  concerned,  I  have  always  found  those  feldspars  which  refuse,  through  a  number  of  sections, 
to  give  high  angles  to  have  strongly-marked  peculiarities,  among  which  greater  tendency  to  decompo- 
sition, and  relatively  very  narrow  lineations  between  the  nicols  are  most  prominent.  It  is  also  my 
experience  that  these  low-angled  feldspars  are  always  associated  with  orthoclase,  or,  if  they  occur  with- 
out orthoclase,  that  they  are  only  large-sized  porphyritic  ingredients  in  an  aphanitic  base. 


40        COPPEE-BEAEmG  EOCKS  OF  LAKE  SUPEEIOE. 

In  the  table  given  below  the  angular  measurements  always  represent 
the  largest  angles  that  could  be  found.  It  will  be  noted  at  once  that  two- 
thirds  of  these  measurements  are  high  enough  for  anorthite — the  most  basic 
of  the  plagioclases — while  in  all  the  remaining  sections  entered  as  contain- 
ing labradorite,  angles  were  obtained  not  far  below  the  upper  limit  for  that 
mineral.  In  some  of  these  cases  there  was  difficulty  in  finding  enough 
plagioclase  sections  in  the  right  zone,  and  other  slices  might  have  given 
higher  angles. 

The  plagioclastic  ingredient  of  these  rocks  is  commonly  quite  fresh, 
and  always  fresher  than  the  olivine  or  augite.  When  alteration  occurs,  it  is 
nsually  but  a  slight  cloudiness.  In  the  angular  measurements  made  on 
over  two  hundred  sections  of  the  several  kinds  of  basic  rocks,  it  was  ob- 
served that  the  freshness  of  the  plagioclase  bears  a  distinct  relation  to  the 
size  of  the  angle  between  the  maximum-extinction  positions  of  the  adjacent 
hemitropic  bands,  the  freshest  feldspar  always  giving  the  largest  angles, 
and  vice  versa.  Pumpelly  gives  an  instance  (that  of  the  rocks  of  bed  96  of 
the  Eagle  River  section,  Keweenaw  Point)  of  a  change  of  the  plagioclase 
to  prehnite  in  a  rock  of  this  class.  This  is  an  alteration  commonly  noted 
in  the  finer  diabase,  but  in  the  rocks  of  the  class  now  under  description  it  is 
well-nigh  unknown. 

In  sections  of  the  very  coarse-grained  gabbro  of  Bad  River,  Wisconsin, 
large  areas  are  often  seen  which  polarize  monochromatically,  and  are  hence 
parallel  to  the  brachy-pinacoid.  These  areas  are  thickly  crowded  with 
minute  black  needles,  arranged  in  several  directions.  The  needles,  whose 
nature  is  in  doubt,  are  characteristic^  of  the  gabbros  of  many  other  regions. 
In  the  Bad  River  rock  the  set  of  the  needles  lying  parallel  to  the  vertical 
axis  includes  very  much  the  larger  number.  Other  needles,  making  an 
angle  of  112°  with  the  first,  and  evidently  placed  parallel  to  the  strong 
basal  cleavage,  are  fewer  in  number,  but  much  longer.  Still  others,  very 
numerous,  lie  oblique  to  the  plane  of  the  section,  and  are  parallel  to  pyra- 
midal planes.^     The  occurrence  of  the  large  monochromatically  polarizing 

•  Geology  of  New  HampsMre,  Vol.  Ill,  Part  IV,  pp.  94. 

2  Compare  Geology  of  New  Hampshire,  Vol.  Ill,  Part  IV,  Plate  V,  Fig.  5;  also  Fig.  3,  Plate  XV  D, 
Geology  of  Wisconsin,  Vol.  Ill;  also  Fig.  58,  Plate  X,  Eosenbusch's  " Microscopische  Physiograpkie 
der  massigen  Gesteine." 


OF  LAKE  SUPERIOR   PL 


Coarse.     Oltrtne-GoJlro    fvo^     SZadaer  Lc7(&,   ma./  0,'clzna.ry  2zyAir .  Settle.  JC  diavn^6e^-s 


In  the  ta' 

.rest  ang 


JR. 


M)v.   that  two- 
most  basic 


i||x  '>*>>^  .^uii^for  that 


of  the  pla.g^L^^^:^^\^^Si^^^^VHI' 
ing  labradoijt('A^)»  ^..V^^'e^B]^^^ 
mineral.     1 1 1  K^'ri;^.j7^ijl^tt\\js^,>^^se|^^ 

file  'rW\ 


enough 
given 

fresh, 

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jvr-rnis  is^arrrfknnniyri  coi^jj.oniy'ajVyi.-''' 


■      "   Ar',ias_^*'-  ttm:k^^9wdeii-W7^\^ 
:5i ',on«>;   Ibe  ne'edles, "whosev, j:' 


XV  D, 


►^v 


^;:^::.  ,».i!:;r:-:..  ::i;::f::rLrl,:-!-- 


UNITED  STATES   GEOLOGICAL  SURVEY 


COPPER-BEARING   ROCKS  OF  LAKE  SUPERIOR   PL.  IV 


n  a  Co.  Udi    BalHmoi 


COARSE  OLIVINE  -  GABBRO 


OETHOGLASE-FEEE  GABBEO.  41 

areas  which  contain  these  needles,  and  the  irregularity  and  frequent  great 
breadth  of  the  lineations,  as  a  result  of  which  crystals  are  found  at  times 
giving  only  two  bands,  have  led  to  the  idea  that  such  rocks  from  other  re- 
gions contain  orthoclase,^  and  the  same  conclusion  has  been  announced  for 
the  Bad  River  gabbro  by  Julien,'  who  also   supports   his   decision   by 
an  appeal  to  the  cross-barred  twinning  of  the  grains  as  seen  in  the  polarized 
light.     This  appearance  was  for  a  long  time  supposed  to  be  characteristic  of 
orthoclase,  but  the  supposed  orthoclase  has  since  been  shown  to  be  micro- 
cline,  and  a  cross-barred  twinning  is  now  known  to  be  common  in  labradorite 
as  well.     Moreover,  a  careful  examination  of  the  cleavage  directions,  so  often 
emphasized  by  inclusions,  makes  it  certain  that  we  have  to  do  with  a  tri- 
clinic  feldspar.     The  plagioclase  of  these  rocks  is  often  the  most  plentiful 
ingredient,  but  in  the  darker  colored  varieties  is  dominated  by  the  augite, 
which  in  some  of  the  very  black  kinds  constitutes  nearly  the  whole  section. 
The  iron-oxide  constituent  of  these  rocks  appears  commonly  to  stand 
between  the  plagioclase  and  the  augitic  constituent  in  point  of  time  of  crys- 
tallization.    This  is  not  always  evident,  even  for  the  magnetite  that  is  a  pri- 
mary constituent,  while  there  is  undoubtedly  at  times  a  magnetite  resulting 
from  the  alteration  of  the  augite.     The  distinction  between  magnetite  and 
titanic  iron,  still  more  between  magnetite  and  titaniferous  magnetite,  in  rock 
sections,  is  a  difBcult  one  always,  and  often  is  well-nigh  impossible  without 
a  quantitative  analysis,  when  the  characteristic  crystalline  outlines  of  mag- 
netite and  the  equally  characteristic  white  alteration-product  of  titanic  iron 
and  of  highly  titaniferous  magnetite  are  both  lacking.     The  powder  of  the 
finer  grained  of  these  rocks  generally  yields  a  considerable  beard  to  the 
magnet,  and  in  the  coarser  kinds  the  plainly  visible  metallic-lustered  part- 
icles are  always  strongly  magnetic.     A  number  of  qualitative  tests  made 
yielded,  about  half  and  half,  negative  results  and  feeble  reactions  for  titan- 
ium.    On  the  whole,  I  am  inclined  to  consider  that  in  these  rocks  the  iron- 
oxide  ingredient  is  always  a  titaniferous  magnetite,  although  the  titanium  is 
at  times  in  very  minute  quantity.     In  size  this  ingredient  runs  from  mere 
dust  to  particles  a  quarter  of  an  inch  across,  and  usually  is  in  as  large 

1 H.  Eosenbusch,  op.  dt,  p.  460. 

2  Geology  of  Wisconsin,  Vol.  Ill,  p.  234. 


42        COPPEE-BEAEmG  EOCKS  OF  LAKE  SUPEEIOE. 

particles  as  tlie  olivine.  It  occurs  commonly  in  very  irregularly  outlined 
forms,  much  more  rarely  in  elongated  parallel  rods,  in  which  no  distinct 
crystalline  outlines  can  be  made  out. 

The  augitic  constituent,  which  varies  from  augite  to  highly  fibrous 
diallage,  is  invariably  of  later  formation  than  the  olivine  and  plagioclase; 
and  commonly,  perhaps  always,  is  later  than  the  magnetite  also.  It  often  in- 
cludes the  olivine  grains,  and  has  always  its  outlines  determined  by  those  of 
the  previously  formed  plagioclase.  As  seen  with  the  naked  eye,  on  a  fresh 
fracture,  it  is  of  a  lustrous  black  color,  more  rarely  having  a  tendency  to  a 
metallic  luster.  On  a  weathered  surface  the  diallage  is  at  times  of  a  bril- 
liant, brassy,  metallic  luster.  It  is  commonly  the  coarsest  ingredient  present, 
its  particles  at  times  reaching  as  much  as  one  or  two  inches  across,  even 
when  the  rest  of  the  rock  is  not  unusually  coarse.  Very  often  one  crystal 
will  present  in  the  thin  section  a  number  of  wholly  detached  areas,  which 
are  proved  to  be  parts  of  one  individual  by  their  common  cleavage  direc- 
tions, and  common  behavior  between  the  crossed  nicol  prisms. 

In  some  cases,  when  the  augitic  constituent  is  diallage  and  reaches  the 
extraordinarily  large  sizes  above  noted,  it  will  include  a  large  number  of 
plagioclase  crystals,  and  then  the  rock  presents  externally  a  peculiar  appear- 
ance, analogous  to  the  luster-mottling  described  by  Pumpelly  as  character- 
istic of  the  Michigan  and  Wisconsin  melaphyrs,  only  on  a  far  grander  scale. 
The  body  of  the  rock  in  these  cases  is  no  coarser  than  usual,  nor  does  the 
hand  specimen  present  any  peculiar  appearance  until  it  is  held  in  a  certain 
position,  when  a  brilliantly  flashing,  brassy  surface  is  seen,  from  half  an 
inch  to  two  inches  across,  where  before  seemed  to  be  only  the  usual  mingled 
ingredients.  The  explanation  of  this  peculiar  appearance,  as  Pumpelly  has 
shown  for  the  melaphyrs,  lies  in  the  existence  of  the  diallage  in  extraordi- 
narily large  areas  enveloping  many  plagioclases.  This  appearance  is  pre- 
sented in  a  most  striking  manner  by  the  rock  of  the  cliffs  of  the  north  shore 
of  Lake  Superior,  ten  miles  northeast  of  the  village  of  Beaver  Bay,  Minne- 
sota, and  again  six  miles  northeast  of  the  same  place,  between  the  Palisades 
and  the  mouth  of  Baptism  River.  The  rock  seen  at  these  places  is  of  a 
moderately  coarse  grain  and  black  color.  It  is  very  fresh  within,  but  without 
is  weathered  to  a  light  brown,  and  on  the  wave-worn   surfaces  the  eye 


OETHOCLASE-FEEE  GABBRO.  43 

catches  in  every  direction  the  flashes  of  the  brassy  diallage  faces,  which  are 
often  as  much  as  two  inches  across. 

In  the  thin  slice  the  augitic  ingredient  presents  a  wine-colored  or  violet 
section,  varying  considerably  in  depth  of  tint.  It  often  shows  the  charac- 
teristic prismatic  cleavage,  but  more  commonly  is  traversed  by  irregular 
cracks,  or  is  affected  in  very  varying  degree  by  the  diallage  cleavage,  parallel 
to  the  clinodiagonal.  It  is  often  quite  fresh,  but  often  also  has  undergone 
alteration,  generally  to  some  sort  of  soft,  greenish,  chloritic  substance,  which 
is  feebly  dichroic,  or  non-dichroic,  in  the  section.  This  change  is  at  times 
complete,  no  augite  cores  remaining.  The  alteration-product  is  often 
stained  brown  from  the  peroxidation  of  the  iron  of  the  augite,  but  in  other 
cases  has  a  pale-green  color.  These  light-green  areas  sometimes  contain 
particles  of  magnetite  in  such  a  way  as  to  suggest  that  they,  too,  are  sec- 
ondary products  from  the  augitic  alteration.  The  change  of  augite  to  ura- 
lite,  so  characteristic  of  the  orthoclase-gabbros  of  the  Keweenaw  Series,  is 
only  very  rarely  seen  in  the  rocks  of  this  class,  and  then  with  an  inconsider- 
able development. 

In  only  two  or  three  sections  out  of  the  forty  studied  of  this  class  of 
rocks  was  any  apatite  found,  and  then  only  in  rare  and  minute  crystals, 
although  search  was  made  for  it  in  every  one  of  the  sections.  A  few  scales 
of  biotite  were  occasionally  found. 

The  existence  in  these  rocks  of  serpentine,  chlorite,  viridite  and  the 
brown  and  red  oxides  of  iron  as  secondary  or  alteration-products  has  been 
mentioned.  These  ingredients  rarely  exert  any  considerable  influence  on  the 
outward  appearance,  the  rocks  of  this  kind  being  on  the  whole  remarkably 
fresh.  Prehnite  as  an  alteration-product  of  the  plagioclase  is  exceptional, 
as  stated  above. 

The  rocks  of  this  class  present  in  the  field  very  massive  exposures,  often 
with  very  marked  columnar  structure.  They  occur  in  very  heavy  beds,  and 
are  never  furnished  with  an  upper  amygdaloidal  or  vesicular  portion. 

As  typical  and  easily  accessible  localities  for  these  rocks  may  be  men- 
tioned the  cliffs  of  the  North  Shore,  between  Sucker  and  Knife  River  Bays; 
again  for  some  five  miles  southward  and  four  miles  northeast  of  the  hamlet 
of  Beaver  Bay ;  and  again  for  long  distances  between  the  mouth  of  Brul^ 


44        COPPEK-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 

Eiver  and  Grand  Portage  Bay — all  on  the  Minnesota  coast.  Other  places 
are  the  Lake  shore  at  Dulutli  (small  areas);  Brewery  Creek,  Duluth;  the 
north  shore  of  Nipigon  Bay,  near  the  mouth  of  Nipigon  River,  Ontario,  Can- 
ada; the  west  side  of  the  mouth  of  Nipigon  Straits;  and  the  beds  immedi- 
ately over  "The  Greenstone,"  in  the  Eagle  River  section  of  Keweenaw 
Point,  especially  the  beds  numbered  107  and  96  by  Marvine.^  These  rocks 
are  also  common  in  the  country  back  of  the  Minnesota  coast,  where  they 
form  most  of  the  steep-backed  ridges  so  commonly  encountered  in  trav- 
ersing the  woods  of  this  region.  The  Duluth  gabbro  falls  for  the  most  part 
under  the  next  head,  i.  e.,  is  orthoclase-gabbro ;  but  in  the  western  extension 
of  the  Duluth  gabbro  belt  towards  Fond  du  Lac,  and  again  to  the  north- 
ward, on  the  Cloquet  River,  very  coarse  olivine-gabbros  are  abundant. 
There  is  again  an  immense  development  of  these  rocks,  often  with  exces- 
sively coarse  grain,  in  the  Bad  River  country  of  Wisconsin,  where  they 
underlie  the  greater  part  of  a  belt  of  country  forty  miles  in  length  and  two 
to  four  in  width. 

The  following  tabulation  presents  in  a  compact  way  the  observations 
made  on  a  number  of  specimens  of  these  rocks  brought  from  different  parts 
of  the  Lake  Superior  basin.  The  angular  measurements  by  which  the  feld- 
spars were  determined  are  included.  The  numbers  of  the  specimens,  when 
unaccompanied  by  any  letter,  are  the  collection  numbers  of  the  specimens 
gathered  especially  for  this  work.  Numbers  with  the  letter  "W  attached 
belong  to  a  collection  made  by  Col.  Charles  Whittlesey  in  the  Bad  River 
region  of  Wisconsin,  and  now,  with  the  thin  sections,  in  the  cabinet  of 
the  Wisconsin  State  University.  Those  with  the  letters  S,  SW,  and  I 
attached  were  collected  respectively  by  Messrs.  Strong,  E.  T.  Sweet,  and 
myself,  for  the  Wisconsin  State  Geological  Survey,  and  are  also  now,  with 
the  thin  sections,  in  the  cabinet  of  the  Wisconsin  State  University.  For 
further  statements  as  to  the  four  last-named  collections,  with  descriptions  of 
many  specimens,  see  the  Geology  of  Wisconsin,  Vol.  III.,  pp.  27  to  49,  53 
to  238,  305  to  362,  and  365  to  428. 

1  Geological  Survey  of  Michigan,  Vol.  I,  1869-1873,  Chapter  VIII.,  pp.  117-140. 


OETHOCLASB-FEEE  GABBEO. 


45 


Tabulation  of  observations  on  coarse  orthoclase-free  gabbro  and  diabase. 


1 

§ 

Place. 

1 

§ 

o 
CO 

6 

bJC 
1 

Macroscopic  char- 
acters. 

Constituents  as  determined  by 
microscope,  in  order  of  age. 

Angle    between 
maximum    ex. 
tinctions   of 
adjacent  heml- 
tropic  banda  of 
the  plagioclase 
in  sections  cut 
at   random   in 
the  zone  0;  ii. 

Angles  on 
opposite 
sides   of 
cross-hair. 

o 

o 

0 

Bed    107,     Eagle 
Eiver      section, 



Medium- grained; 
black-and-white- 

Anorthite;  Tnagnetite,  or  titanic 
iron;  augite.    (Pampelly.)  ^ 

49 

60 

Keweenaw  Point. 

mottled. 

70 
75 
59 

93  "W. 

NearPotato  Elver, 

SE. 

6 

45 

IE. 

Medium. grained; 

Anorthite ;  augite  tending  to  di. 

28 

31 

Ashland  County, 

dark- gray ;  rough- 

aUage ;  a  little  altered  magma. 

33 

35 

68 

Wisconsin. 

textured.  Sp.  gr., 
2.95. 

(Pumpelly.)2 

32 

29 

61 

1661.. 

Ashland  County, 

NW. 

15 

45 

IW. 

Coarse-grained; 

Olivine,  in  large  areas  reaching 

26 

23 

49 

"Wisconsin. 

very  light-colored, 
gray,  mottled  wi  th 
mucfe       white; 
rough-textured. 
Sp.  gr.,  2.80. 

2min   in   length,  largely  quite 
fresh,  but  also  in  part  altered 
to  ochre,  biotite  and  talc ;  an- 
orthite, greatly  predominating ; 
diallage. 

23 
18 

34 

21 

67 
39 

31661 

Ashland  Connty, 

NE. 

34 

45 

2  W. 

Eine-grained; 

Olivine,  largely  quite  fresh ;  an- 

31 

30 

61 

Wisconsin. 

dark-gray. 

orthite  abundant;  titaniferous 
magnetite;  aMSftte,only  slightly 
diallagio. 

31 

32 

63 

2028  S. 

Bad   Elver,  Ash. 

NW. 

31 

45 

2W. 

Tery  ooarse-graui- 

Olivine,  very  abundant  and  coarse. 

34 

30 

64 

landConnty,  Wis- 

ed;       light-gray ; 

making  up  over  one-third  of  the 

25 

28 

53 

consin. 

mostly  made  up  of 
very  coarse    plar 
gioclase,  in  which 
are     embedded 
very     numerous 
large      olivines. 
Sp.  gr.,  2.82. 

section,  the  particles  elongated, 
reaching   one-fourth   inch   in 
length,  wholly  altered  to  vlri. 
dite,  with  some  biotite  in  the 
interior  and   a  shell   of  talc 
scales;  anorthite;    magnetite, 
sparse ;  augite ;  diallage,  sparse. 

37 

38 

75 

121... 

Bad   Elver,  Ash* 

NE. 

31 

45 

2  W. 

Coarse-grained; 

Olivine,    partly   fresh,    partly 

28 

30 

58 

land    County, 

dark-gray,  some- 

alteied,   reaching    1.3»"    in 

19 

18 

37 

Wisconsin. 

what    stained 
with    brown; 
rough -textured. 
Sp.  gr.,  2.81. 

length ;    lairadorite    greatly 
predominating;  augite;  biotite 
in  minute  scales  between  the 
plagioclase   grains;    viridite, 
biotite  and  talc  secondary  to 
olivine;  viridite  iiuimagnetite 
secondary  to  augite ;  ochre.^ 

18 

19 

37 

701... 

Ashland  County, 

NW. 

6 

44 

3W. 

Very  coarse-grain. 

Olivine,    very   abundant,    and 

32 

33 

65 

Wisconsin. 

ed;   lightgray; 
shows  much 
olivine  to  naked 
eye. 

coarse,    partly  fresh,    partly 
altered  to   biotite  and  talc; 
labradorite  or  anorthite 
crowded  with  black  needles, 
constitutlBg  most  of  the  sec 
tion;    magnetite,  sparse; 
augite,  only  slightly  diallaglc. 

28 
18 

28 
21 

56 
39 

'  Metasomatio  Development  of  the  Copper.bearlng  Eocks  of  Lake  Superior,  Proc.  Am.  Acad.  Sci.,  Vol.  XIII.,  pp.  256, 259. 

=  Geology  of  Wisconsin,  VoL  III.  p.  38. 

'  See  also  A.  A.  Julien,  Vol.  Ill,  Geology  of  Wisconsin,  pp.  235  to  237. 


46 


COPPEE-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 


Tabulation  of  observations  on  coarse  orthocluse-free  gabbro  and  diabase — Continued. 


a 
1 

to 

Place. 

§ 
1 

s 

0 

•J 
§ 

CO 

•g 

o 
H 

Macroscopio  char- 
acters. 

Constituents  as  determined  by 
microscope,  in  order  of  age. 

Angle    between 
maximum    ex- 
tinctions of 
a^acent  hemi- 
tropic  bands  of 
the  plagioclase 
in  sections  cut 
at  random    in 
the  zone  0:zz. 

Angles  on 
opposite 
sides  of 
cross-hair. 

o 

o 

o 

MS... 

Brunschweiler'a 

sw. 

11 

44 

4-W. 

Coarse-grained; 

Olivine,  partly  altered;   labra- 

22 

25 

47 

Eiver,  Asliland 

dark-gray,  mot- 

dorite; titaniferous  magnetite; 

28 

29 

57 

County,    "Wis- 

tled with -white. 

diaUagic    augite;    secondary 

18 

18 

36 

consin. 

Sp.  gr.,  2.92. 

biotite,  viridite,  ochre. 

158  W. 

Ashland  County, 
■Wiaconsin. 

NE. 

15 

45 

4W. 

Coarse-grained; 
light-gray.    Sp. 
gr.,  2.81. 

Labradorite;  titaniferous  mag- 
netite; diailajje  sparse,  partly 
changed  to  uraXite. 

IMW 

B  r  u  n  s  chweiler's 

22 

45 

4W. 

Eine-grained; 

Anorthite;  magnetite;  diaUage; 

30 

37 

67 

Hiver,  Ashland 

dark-gray;  lus- 

a, little  biotite. 

33 

36 

69 

County,     "W  i  3- 

trous. 

36 

32 

68 

consin. 

86P... 

Ashland  County, 
Wisconsiu. 

SE. 

14 

44 

5W. 

Tory  coarse.grain- 
ed;  light-gray. 

Labradorite;  titaniferous  mag- 
netite: diaUagic  augite,  uralite. 

1132... 

King's  Creek,  near 
Duluth,  Minn. 

STE. 

12 

49 

15  W. 

Apatite;    olivine  more  altered 
than    in    the    last    section; 

32 
34 

27 
38 

59 
72 

anorthite ;    titaniferous  mag- 

29 

35 

64 

netite;    diallage;    Moiite  in  a 

31 

33 

64 

few  flakes ;  ferrite. 

12 

Brewery     Creek, 

SE. 

22 

50 

14 -W. 

Medium -grained; 

Olivine,  highly  altered ;  anorth- 

29 

32 

61 

near  Duluth, 

dark-gray  to 

ite;    titaniferous    magnetite; 

30 

37 

67 

Minn. 

nearly  black; 
rough-textured. 
Sp.  gr.,  2.91. 

aMjtte  in  part  diaUagic;  altera- 
tion-product of  augite. 

18 
33 
30 

23 

27 
37 
27 
21 

45 
70 
57 

44 

1103... 

Palls  of   Cloquet 

SE. 

34 

53 

14  "W. 

Coarse-grained; 

Apatite;   olivine,  mostly  fresh. 

26 

25 

49 

Eiver,     Minne. 

Tery  Ught-gray. 

partly  altered  to  green  sub- 

35 

33 

68 

sota.    4001f.,200 

stances,  and  red  and  brown 

24 

26 

50 

W.3 

iron  oxides;  anorthite;  titan- 
iferous   magnetite;    diallage; 
biotite  in  a  few  small  flakes; 
ferrite. 

33 

34 

67 

621.... 

Duluth,  Minn., 

27 

50 

14W. 

Medium  -  grained ; 

Anorthite;  titaniferous  magne- 

34 

38 

72 

near  center  of 

nearly  black; 
rough-textured. 

tite,  in  elongated  parallel  rods ; 
augite,  in  part  diallagic ;  light 
green    alteration-product    of 
augite. 

33 
38 

31 
40 

64 

78 

628.... 

North  shore  Lake 
Superior. 

NW. 

24 

50 

14  W". 

Medium-grained  . . 

Olivine;  anorthite;  titaniferous 
magnetite;  augite,  in  part  dial- 
lagic, in  large   crystals,  en- 
closing many  feldspars. 

23 
21 
20 
39 
17 
28 

24 
19 
21 
32 
16 
25 

47 
40 
41 
71 
33 
63 

'  VoL  III,  Geology  of  Wisoonsin,  p.  39.  '  Vol.  HI,  Geology  of  Wisconsin,  p.  40. 

^  400  paces  north  and  200  paces  west  of  the  sontheast  comer  of  the  section ;  2,000  paces  to  the  mile. 


OETHOOLASE-FEEE  GABBEO.  47 

Tabulation  of  observations  on  coarse  ortJioclase-free  gabbro  and  diabase— GonimueA. 


679. 


580... 


582.... 


588... 


658... 


63. 


Place. 


Korth  shore  Lake 
Superior  j  east 
point  of  Sucker 
KiverBay,  Min- 
nesota. 

do 


583... 


North  shore  Lake 

Superior,    near 

east    point     of 

I      Sucker      Eirer 

Bay,  Minnesota. 

do 


North  shore  Late 
Superior,  be- 
tween Knife  and 
Sucker  Eivers, 
Minnesota. 

North  shore  Lake 
Superior,  be- 
tweenmontha  of 
Knife  and  Suck- 
er Rivera,  Min- 
nesota. 

Encampment 
Bluff,  north 
ahore  Lake  Su- 
perior, Minne- 
sota. 

Encampment 
River,  Minne- 
sota. 


SE. 


SE. 


SE. 


SE. 


NE. 


SE. 


NW. 


51 


nw. 


Macroscopic  char, 
acters. 


Constituents  as  determined  by 
microscope,  in  order  of  age. 


51 


2     51 


,2 


12-W. 


51 


51 


NE. 


10 


53 


53 


12  W. 


12  "W, 


12  ■W". 


12 'W. 


low. 


low. 


Medium  -  grained ; 
light  -gray; 
rough-textnred. 


Medium  -  grained 
light-  gray 
rough-textured 
close  to  579. 


Medium-grained ; 
dark  -  gray; 
rough-textured ; 
altered. 

Medium-grained ; 

light  -  gray; 

rough-textured ; 

near  579.     SiOj 

46. 44  per  cent. 
Medium  -  grained 

light-  gray 

rongh-texturedi 

near  579. 

Medium-  grained; 
dark-gray 
rough-textured 
near  579. 


Medium  •  grained ; 
dark-gray; 
rough-textured. 


Medium  to  coarse- 
grained; black; 
rough-textured. 
Sp.  gr.,  2.89. 


Olivine,  mostly  very  fresh,  in 
part  altered  to  a  green  sub- 
stance; anorthite;  titaniferous 
magnetite;  diallage,  in  large 
areas,  including  many  feld- 
spars.   A  very  fresh  rock. 

Olivine,  mostly  very  fresh,  in 
part  altered  to  a  green  sub. 
stance ;  anorthite ;  titanifer- 
ous magnetite;  diallage,  in 
large  areas,  including  many 
feldspars. 
Olivine,  very  large  and  abun- 
dant, wholly  altered  to  a  green 
substance  and  magnetite ;  an- 
orthite; titaniferous  magne- 
tite; augite;  diallage. 
Olivine,  mostly  very  fresh;  an- 
orthite; titaniferous  magne- 
tite; diallage  in  large  areas. 

A  very  fresh  rock. 

Olivine,  very  large,  abundant 
and  fresh;  anorthite;  titan- 
iferous magnetite;  diaUage; 
green  alteration -products  of 
olivine  and  diallage. 

Olivine,  very  large  and  abun 
dant;  anorthite;  titaniferous 
magnetite ,-  augite,  in  part  di- 
allagio;  green  alteration-pro- 
ducts of  olivine  and  augite; 
brown  iron  oxide. 

Olivine;  labradorite;  titanifer- 
ous magnetite ;  augite,  in  pait 
diallagic,  very  coarse ;  serpen- 
tine as  alteration-product  of 
oKvine;  a  green  alteration- 
product  of  augite. 
Olivine,  partly  altered;  anor- 
thite ;  titaniferous  magnetite  ; 
augite  non-dlallagio;  altera- 
tion-products of  olivine  and 
augite. 


Angle  between 
maximum  ex- 
tinctions  of 
adjacent  hemi- 
tropic  bands  of 
the  plagioclase 
in  sections  cut 
at  random  in 
the  zone  O;  ii. 


Angles  on 
opposite 
s  1  d  e  s  of 
cross-hair. 


20 


30 

30 

27 

38 

32 

33 

24 

26 

26 

28 

25 

27 

26 

24 

24 

23 

37 

42 

27 

37 

23 

44 

37 

33 

60 


48 


COPPEE-BEAEING  EOCKS  OP  LAEIB  SUPEEIOE. 


Tabulation  of  ohservations  on  coarse  orthoclase-free  gabhro  and  diabase — Coulinued. 


■a 

i 

Pi* 
m 

Place. 

1 

% 

m 

o 
B 

i 

Macroscopic  char, 
aoters. 

Constituents  as  determined  by 
microscope,  in  order  of  age. 

Angle    between 
maximum    ex- 
tinctions  of 
adjacent  hemi- 
tropic  bands  of 
the  plagioclase 
in  sections  cut 
at   random  in 
the  zone  0:  it. 

Angles  on 
opposite 
siaes  of 
cross.hair. 

o 

o 

o 

67 

do 

NU. 

10 

53 

low. 

Medium-  grained ; 
hrownish,  with 

Olimme,  wholly  altered  to  green 
substance;  labradorite;  titan- 

21 

24 

45 

25 

25 

50 

diallage;  much 

iferoics    magnetite    in    large 

29 

20 

49 

altered ;  seamed 

areas ;  diaUage,  mostly  altered 

by  laumontite. 

to  a  green  substance. 

69 

do 

NE. 

10 

53 

low. 

Medium  -grained ; 
hlack :     rough- 

Olivine^  wholly  altered  to  green 

33 

39 

72 

substance ;    anorthite ;   titan- 

32 

31 

63 

textured. 

iferousmagTieiite;  augite,  non- 
diallagic,  greatly  predominat- 

682.... 

Uorfh  shore  lake 

s-w. 

5 

54 

8W. 

Medium  .grained ; 

LabradoHte;  titaniferous  mag- 

33 

27 

60 

Superior,  'below 

hlack ;    heavy ; 

netite;   augite;  diallage;   al- 

30 

26 

56 

Split  Eock 

rough-textured. 

teration-product  of  augite  and 

25 

27 

52 

Biver,     Minne- 

Si02 46.17p.C6nt. 

diallage. 

sota. 

» 

779.... 

North  shore  Lake 

NTV. 

3 

55 

8W. 

Coarse-grained ; 

Anorthite;  titaniferoits  magne- 

32 

33 

65 

Superior,     one- 

hlack,     stained 

tite,  sparse;    awgrite,  in   very 

35 

36 

71 

quarter  mile 

with     brown; 

large    crystals,    rarely   dial- 

34 

28 

62 

southwest  of 

rough.textured. 

lagic,  very  fresh,  and  greatly 

south  point   of 

Sp.  gr.,  2.87. 

predominating. 

Beaver   Bay, 

Minnesota. 

785.... 

North  shore  Lake 

SW. 

12 

55 

8W. 

Coarse.grained; 

Olivine,  represented  by  green. 

40 

33 

73 

Superior;  south 

black ;     rough- 

ish.y  ello  w  alteration-product ; 

31 

30 

61 

side  of  Beaver 

textured. 

anorthite;   titaniferous   mag- 

28 

31 

59 

Bay  Point,  Min. 

netite,  sparse  and  smaH ;  dial- 

31 

36 

67 

neaota. 

lage. 

789 

....do  

SB. 

12 

55 

8W. 

Coarse-grained; 
dark-gray; 

Labradorite ;   iitaniferous  mag- 
netite; diallagio  auffite,  highly 

20 

20 

40 

24 

29 

53 

rough-textnred. 

altered. 

20 

27 

26 
20 

46 
47 

793.... 

Falls    of  Beaver 

sw. 

12 

55 

8W. 

Coarse-grained; 

Olivine,  in  large  areas,  largely 

32 

32 

64 

"R.ivp.T       ATinnp.. 

nearly     Mack ; 

altered    to    brown   ochreous 

29 

29 

58 

sota. 

rough-textured. 
Sp.gr.,  2.93, 

substance;  anorthite;  titanif- 
erous     magnetite;     diaUagio 
augite  in  very  large  areas. 

25 
31 

26 
31 

51 
62 

794.... 

Beaver  Eiver, 

sw. 

12 

55 

8W. 

Coarse-grained; 

Olivine,  in   large  areas,  much 

33 

40 

73 

Minnesota. 

black ;     rough- 
textured. 

altered;  anorthite;   titanifer- 
0U8  magnetite;   non-diallagic 
augite. 

33 

38 

31 
35 

64 
73 

782.... 

Falls   of  Beaver 

sw. 

12 

55 

8W. 

Coarse-grained; 

Labradorite;   titaniferous   mag- 

18 

24 

42 

Eiver,  Minne- 

black;     rough- 

neKte,  not  abundant;  diallage; 

17 

17 

34 

sota. 

textured. 

viridite  as  alteration-product 

24 
29 

23 
28 

47 
57 

OETHOOLASE-PEEE  GABBEO. 


49 


Tabulation  of  observations  on  coarse  orthoclase-free  gabbro  and  diabase — Continued. 


a 
S 

Flfloe. 

a 

o 

•.g 

CI 

n 
o 

o 

ft 

e 

n 
0i 

Macroscopic  char- 
acters. 

Constituents  as  determined  by 
microscope,  in  order  of  age. 

Angle    between 
maximum    ex- 
tinctions of 
adjacent  hemi. 
tropic  bands  of 
the  plagioclase 
in  sections  cut 
at   random    in 
the  zone  0 :  ii. 

Angles  on 
opposite 
sides  of 
croas-hair. 

0.2 

0 

0 

0 

95 

Beaver  Eiver, 

sw. 

12 

55 

8W. 

Medium  -  grained ; 

Anorihite,  greatly  predominat- 

28 

27 

55 

Minnesota,    875 

red-and-black- 

ing;    titaniferous  magnetite; 

29 

25 

54 

N.,  1,600  W. 

mottled ;     very 
large      diallage 
luster-mottlings. 

diallage. 

30 
28 
33 

30 
24 
35 

60 
52 
68 

1007... 

Fonr  miles  north 

NW. 

25 

56 

8-W. 

Medium-grained  to 

Olivine,  large,    abundant    and 

30 

27 

57 

of  Beaver  Bay, 

coarse-grained; 

very  fresh;  anorthite;  titanif- 

23 

25 

48 

Minnesota. 

gray;     rough- 
textured. 

erous  magnetite;  augitein-part 
diallagio. 

35 
37 

39 
37 

74 
74 

1012... 

Six    miles    north 

NE. 

13 

56 

8W. 

Medium-grained  to 

Anorthite ;  titaniferous  magnet- 

32 

38 

70 

of  Beaver  Bay, 
Minnesota. 

coarse-grained; 
black ;     rough- 
textured. 

ite;  awjite,  very  much  altered; 
diallage;  much  ochre  and  far- 
rite. 

31 
25 

33 
30 

63 
55 

1021... 

Eight  miles  noi-th 
of  north  line  of 
Beaver  Bay, 
Minnesota. 

NB. 

2 

56' 

8-W. 

"VVhite-and-green- 
mottled ;  rough- 
textured. 

Labradorite,     much     clouded ; 
titaniferous    magnetic,    very 
abundant  and  coarse ;  diallage, 
largely   altered   to    greenish 
dichroic  uralite.^ 

24 

25 

49 

819.... 

North  shore  Lake 

NW. 

7 

55 

7^. 

Medium-grained  to 

Olivine,    very    abundant    ,ind 

24 

24 

48 

Superior;  north 
point  of  Beaver 

coarse-grained; 
black ;     rough- 

large,  largely  altered  to  the 
characteristic     brown      sub- 

23 
31 

30 
25 

53 
56 

Bay,  Minnesota. 

textured. 

stance;  labradorits;  titanifer- 
ous magnetite;   non-diaUagio 
augite  largely  .altered  to  green- 
ish material. 

31 

29 

60 

821.... 

Island    northeast 

NW. 

5 

55 

7W. 

Medium -grained; 

Anorthite ;  titaniferous  magnet- 

33 

35 

68 

of  Beaver  Bay, 
Minnesota. 

black ;     rough- 
textured. 

ite;  oujriie  in  very  large  areas; 
diallage;  mridite  as  alteration- 

32 
32 

44 
37 

76 
69 

1515... 

North  shore  Lake 

NE. 

34 

62 

3E. 

Coarse-grained; 

product  of  augite. 

36 
20 

35 
22 

71 
42 

Superior ;    near 
mouth  of  Brul6 

nearly      black ; 
rough-texturedj 

36 
38 

31 
35 

67 
73 

Kiver,     Minne- 

shoTvs   brassy 

36 

33 

69 

sota. 

diallage     on 
■weathered   sur- 
face. 

152S... 

North  shore  Lake 

(ahou 

t)5 

62 

5E. 

Medium-grained  to 

Labradorite;  titaniferous  mag- 

25 

29 

54 

Superior;  ahout 
IJ  miles  east  of 

(un 

eur 

vey 

ed). 

fine-grained; 
very  dark-griiy ; 

netite;   augite,  non-diallagic, 
much  altered  to  viridite  mate- 

15 
20 

14 
18 

29 

38 

Mawskeqnaw. 

rough-textured. 

riaL 

camaw     Eiver, 

Minnesota. 

>  The  green  appearance  of  the  rook  to  the  naked  eye  is  evidently  connected  with  this  nralitlc  change. 
5  L    S 


50 


COPPEE-BBAEING  EOCKS  OF  LAKE  SUPEEIOE. 


TahulaUon  of  observations  on  coarse  orthoclase-free  gabbro  and  diabase — Continued. 


o 

1 

a 
§ 

Place. 

o 
1 

I- 

i 

EC 

ft 

CD 

to 

Macroscopic  char- 
acters. 

Constituents  as  determined  by 
microscope,  in  order  of  age. 

Angle    between 
maximum    ex- 
tinctions of 
adjacent  hemi- 
tropic  bands  of 
the  plagioclase 
in  sections  cut 
at  random   in 
the  zone  0:  ii. 

Angles  on 
opposite 
sides  of 
cross-hair. 

1752... 

NortbTvest     shore 
Nipigon   Bay, 
near   mouth  of 
Niplgon   Kiver, 
Ontario,  C.inada. 

Medium  -  grained ; 
Tery  light,  gray, 
mottled  with 
white ;  rough- 
textured.  Sp. 
gr.,  3.10. 

Olivine,  very  fresh,  abundant, 
and  large;  apatite;  labrador- 
ite ;  titaniferous  magnetite ; 
augite,  only  slightly  diallagic. 
A  very  fresh  rook. 

o 

19 
36 
25 

o 
22 
17 
23 

0 

41 
33 

48 

Orthoclase-hearing  gabhro. — The  rocks  of  this  class  are  less  common 
than  those  of  the  class  last  described,  but  are,  nevertheless,  often  met  with 
in  all  of  the  different  districts  crossed  by  the  Keweenaw  Series  in  its  course 
around  Lake  Superior.  They  are  distinguished  from  the  ordinary  gabbros 
and  coarse-grained  diabases  by  the  presence  of  more  or  less  orthoclase  feld- 
spar, and  of  a  plagioclase  near  oligoclase;  by  the  abundance  of  coarse- 
gi-ained  apatite,  often  large  enough  to  be  readily  seen  with  a  lens,  or  even 
with  the  naked  eye ;  by  the  invariable  absence  of  oHvine ;  by  the  compar- 
atively greater  tendency  to  decomposition  of  the  feldspars,  the  results  of 
which  are  a  clouding  and  reddening  of  the  constituents,  and  the  introduc- 
tion of  a  greater  or  less  amount  of  secondary  quartz ;  by  the  greater  rich- 
ness of  the  magnetite  in  titatiic  acid ;  and  by  the  common  presence  of  a 
uralitic  alteration  of  the  augitic  ingredient.  So  constant  is  the  association 
in  the  Keweenawan  basic  rocks  of  orthoclase,  oligoclase,  coarse  apatite, 
secondary  quartz  and  uralitic  alteration  of  augite  or  diallage,  that  only  in 
rare  cases  is  one  of  these  characters  found  without  most  of  the  others. 

In  grain  these  rocks  cover  about  the  same  range  as  those  of  the  last 
class,  running  from  a  medium-grain  to  an  exceedingly  coarse  one,  when  the 
feldspar  crystals  reach  some  inches  in  length.  The  specific  gravity  is 
lower — 2.7  to  2.8 — than  with  the  more  common  orthoclase-free  gabbros, 


Fiys  laiicij.  ffom    ntaT  Lcstc-r  ?iircr,  •^finne.iala. .  fiy  I  o rdina ry  Zi yTii ,  T'xo  z  polari^.<L3  li^At. 

ZaTm<3ori'ie.(//,  ortTtocZase-i-iJ;  tmnnecl aicriiieJ.3J ;  diaHaye-Wj ;  tilamytious   ■ma^ne.'it.'te.fSI;  fe?-/'Ufi- 


0lvQoclo.se.l7 J ;  orthoclA'^e.lJ-/;  clz'a2la^e^l3l,  mvcA  atfereci  to 


Fvo.  Jf..  f-rom.    STUixscJi-rrc-Llcr  Utrcr^  /Tis.        7^o7o7'zxc4^  Zz^ht 
Scale.  Ji"  ^lutiiaters. 


ii;ir\   Ooiistjtuonta  ns  de'.':;, 
\\'A      microacopo.  in  orO 


dasi^irctirhh^j  -ff^Ut'o. — I'iie  rocks  ot    this  class  a,.  oommo 

Li  •  of  t]ip  class  last  described,  hut  are,  nevertheless,  oiit'n  met  with 

around  Lak%l^^(^^i^■3^■»*aT■^s*'l8i•&^(dt8Sisg^Mli*^i^"^^^  A\\aii^-%o'iis«Ss\ 

and  f'oarse-'<*'i'??>u^u''7|jfu)^a^'^-'^*-'^*^"^^  '^?^^'>■^^  «o'j^»-^i55Sa  *«oj^ 


md  of  a  plagiocL 
o-rained  apatite,  often  la: 


>.la.« 


<TT»(Mas( 


•e  of  coar.se- 

-,  or  even 

the  compai- 


idary  Cjua; 
"•  ^'■"  the  i-'-.Li 
;30  constant 
lase,  oligochji^c 
autnte  or  di 'I 


dingly 


■^\J.^ailHo^<<^l^  ^ao-59\siTOVjS   \^\S-\-3XaSaof.'S'^'J>    \.Vv^3^jMi'i>o'\S»^ 


ob  SS3-s9Vii)  ^assw  ^VsA-3^w>5v'i>s>sS3  \\i-\3'>-fiS3oi\STO    vVMairft^DoaiSQ 


UNITED  STATES   GEOLOGICAL  SURVEY 


COPPER-BEARING   ROCKS  OF  LAKE  SUPERIOR   PL.V 


ORTHOCLASE-  GABBROS 


ORTHOCLASB-BEAEING  GABBEO.  51 

not  only  because  of  secondary  quartz,  but  also  because  of  the  nature  of  the 
feldspar  and  of  the  comparatively  small  amount  of  the  augitic  ingredient. 

The  orthoclase  of  these  rocks  is  always  more  or  less  clouded,  and  is 
often  reddened  and  charged  with  secondary  quartz.  The  two  latter  results 
of  alteration,  however,  vary  greatly  in  the  amount  present,  and  are  at  times 
nearly  or  quite  wanting,  or  at  least  the  reddening  is  diminished  so  as  to  be 
hardly  perceptible  in  the  thin  section,  while  the  quartz  may  be  wholly 
absent.  The  orthoclase  appears  always  to  be  later  in  origin  than  the  pla- 
gioclase,  since  its  contours  are  always  molded  around  those  of  the  latter 
mineral. 

The  plagioclastic  ingredient  of  these  rocks  gives  in  some  sections  angles 
higher  than  the  limit  set  down  for  oligoclase,  but  it  never  reaches  very  high 
upon  the  labradorite  range.  It  is  always  much  less  fresh  than  in  the  rocks 
of  the  previous  class,  in  which  exti'eme  freshness  of  the  feldspars  is  the  rule, 
and  is  often  reddened  as  much  as  the  accompanying  orthoclase.  In  some 
cases  both  it  and  the  orthoclase  show  an  alteration  to  a  greenish  chlorite. 
The  lineation  in  the  polarized  light  is  at  times  very  faintly  seen,  owing  to 
decomposition,  but  is  usually  sufficiently  distinct  for  measurement. 

The  magnetite  of  these  rocks  appears  always  to  be  much  more  highly 
titaniferous  than  that  of  the  ordinary  gabbros.  It  is  generally  in  relatively 
large  fragments,  which  have  at  times  a  marked  pinkish  tint,  and  then  the 
attraction  by  the  magnet  is  very  feeble.  In  the  thin  section  it  is  often 
accompanied  by  the  peculiar  white  alteration-product  indicative  of  the 
presence  of  titanic  acid.  It  appears,  however,  always  to  be  rather  a  very 
highly  titaniferous  magnetite  than  a  true  titanic  iron.  In  the  sections  of 
the  coarse  orthoclase-gabbro  of  Duluth,  the  large,  irregular  areas  of  this 
ingredient  are  commonly  surrounded  by  a  brownish  film,  which  appears 
usually  to  be  merely  ocherous,  but  is  at  times  certainly  made  up  in  part 
of  scales  of  biotite.-'  In  some  of  these  rocks  the  titaniferous  magnetite 
forms  bunches  of  such  size  as  to  have  attracted  attention  as  an  iron  ore, 
as,  for  instance,  at  Duluth,  and  again  at  several  points  in  the  interior  back 
of  Grand  Marais  and  Beaver  Bay.     From  some  of  this  ore  obtained  some 

'  Compare  G.  W.  Hawes,  In  Geology  of  New  Hampshire,  Vol.  II,  Part  IV,  Plate  XI,  Pig.  6. 


52        COPPEEBEAEING  EOCKS  OP  LAKE  SUPEEIOE. 

miles  back  of  Grand  Marais,  I  obtained,  some  years  since,  over  ten  per 
cent,  of  titanic  acid. 

The  augitic  ingredient  of  the  orthoclase-gabbros,  like  that  of  the  more 
ordinary  orthoclase-free  gabbros,  runs  from  augite  to  diallage,  and  in  some 
sections  the  two,  quite  distinct  from  each  othei",  are  present.  In  one  pecu- 
liar class  of  the  orthoclase-gabbros  met  with  at  a  number  of  points  in  the 
country  back  from  the  Minnesota  coast,  and  especially  in  what  I  have  called 
the  "Duluth"  and  "Lester  River"  Grroups,  the  augite  is  in  long  radiating 
blades,  which  are  very  pronounced  macroscopically,  and  which  in  the  thin 
section  are  seen  to  be  twinned.  By  far  the  most  of  the  sections,  however, 
show  a  true  diallage,  much  more  highly  fibrous  than  in  the  orthoclase-free 
gabbros,  and  nearly  always  more  or  less  extensively  changed  to  uralite.  This 
uralitic  alteration,  which,  in  cross  section,  shows  the  true  hornblende  cleav- 
age, is  finely  displayed  in  the  very  coarse  gabbro  of  Duluth. 

Among  the  accessory  ingredients  apatite  is  always  very  prominent,  in 
crystals  which  at  times  reach  an  eighth  of  an  inch  in  length.  Chlorite,  as 
an  alteration  of  augite,  or  more  commonly  of  uralite,  and  biotite  are  often 
present,  and  iron  and  copper  sulphides  are  often  to  be  met  with  in  small 
particles. 

In  the  mass  the  orthoclase-gabbros  do  not  present  any  features  differ- 
ent from  those  of  the  orthoclase-free  kinds,  like  which  they  occur  chiefly  in 
heavy  flows,  without  amygdaloids.  They  may  also  occur  as  intersecting 
masses,  but  this  needs  proof;  that  is  to  say,  it  is  not  always  possible  to  be 
certain  as  to  the  structural  relations  of  rocks  met  with  in  isolated  exposures 
in  the  woods. 

As  instances  of  the  occurrence  of  orthoclase-bearing  gabbros  may  be 
mentioned  the  coarse  syenite-like  rock  of  the  Bohemian  Mountain,  on  the 
north  shore  of  Lac  la  Belle,  Keweenaw  Point;  the  bed  numbered  94  by 
Marvine  in  his  description  of  the  Eagle  River  section  of  Keweenaw  Point;^ 
the  rocks  of  a  belt  or  belts  running  for  many  miles  through  the  Bad  River 
region  of  Wisconsin;^  and  the  rocks  exposed  on  the  Aminicon  River,  in 

'  Geological  Survey  of  Michigan,  Vol.  I,  Part  II,  p.  134. 
2  Geology  of  Wisconsin,  Vol.  Ill,  p.  170. 


UNITED  STATES   GEOLOGICAL  SURVEY 


COPPER-BEARING   ROCKS  OF' LAKE  SUPERIOR   PL,  VI 


fe.; 


^'-.r^ 


ORTHOCLASE  -  GABBRO 


A.  Roan  &  Co.  LIth.  Baltiiiuira  - 


■;*  ■-  r.w.v.r^n. 


53 


_0 


'^^'V 


sU 


Si^- 


^-^^'P  ''"■  ^'    '^"" 


/    :1.' 


niDty,  Wisconsin.     Other 

•  iiis  River  bluffs  at  and 

"  T^ester  River,  Min- 

'he  bed  of  Cascade 

■  'oek  of  the  south 

-named  place; 

ich  the  trail 


"    1l  c 


f'"k-?) 


>**-'-' 


Bi'i'i'*       /     .';S,.'    »       ^' -.'t      ^     ^  .^F 


>  ilot..' 


^■^:/-Jy       •*"'<;-5|' «:%-■.         ■Z^^JtJlUvs  to  the  feldspara; 


lit 

M 

7 

11 

>       4 

6 

-     18 

r 

fl8 

A  '..)i, 


SP^or,  Proc.  Am.  Acail.  " 


■w. 


.\o\\  .9'^ai4i»\  ■^■!5sS-3!ti,i»a'l.  .js.^p    vaSaox- sS'jieaS,  liKSKiB^ 


LRIOR   PLV: 


Very    coAi'Se     ovi7i.ocZa.]e.-fQiiiroJro?n     near   J)zi.2jt-i?i,  ^t'jtnefo'da.. 
<Si.&.  poorer  ji|Ii_  jV,    ^-J'  iif. 

dTaiia^eCs'/^  ofte.n,,iiiie.j^cZ   ^{>uraZtiic.horniie-7tcle.(i/^    invel   to  l^iricittC' 

(7J ;  pyi'xielfj ;    pcAerffJ,   surravndiTiy  ^he,  magnet w.'tt-  and   vyy-i^c   attdi    co>v- 
fat7iz?iy  Aiaiite,  scaZeJ    oy  a.   feouiicitii-y  ttutie.7-t,  f/aj . 


ORTHOCLASE-BEAEmG  GABBRO. 


53 


the  S.  E.  I,  Sec.  32,  T.  48,  E.  12  W.,  Douglas  County,  Wisconsin.  Other 
instances  are  the  coarse  gray  rock  of  the  Saint  Louis  River  blufFs  at  and 
near  Dukith,  Minn. ;  the  exposures  in  the  woods  west  of  Lester  River,  Min- 
nesota, in  Sec.  29,  T.  51,  R.  13  W. ;  the  rock  exposed  in  the  bed  of  Cascade 
River,  in  the  southern  half  of  Sec.  10,  T.  62,  R.  2  W.;  the  rock  of  the  south 
side  of  Eagle  Mountain,  about  four  miles  north  of  the  last-named  place; 
and  the  rock  forming  the  west  wall  of  the  gorge  through  which  the  trail 
passes  from  Eagle  Mountain  to  Bruld  Lake. 

Tabulation  of  observations  on  ortJioclase-bearing  gabbros. 


Angle    between 

maximum    ex- 

tinctions of 

adjacent  hemi- 

tropic  bands  of 

u 

the  plagioclase 

^ 

a 

Macroscopic  char- 

Constituents as  determined  by 

in  sections  cut 

B 

Place. 

§ 

acters. 

microscope,  in  order  of  age. 

at  random    in 

P 

1 
1 

Q 

the  zone  0 :  ii. 

Angles  on 

.—1   03 

B 

m 

o 

a 

0  p  p  osite 

o'bi 

o 

ri 

^ 

f 

sides    0  f 

t?  fl 

CD 

s 

g 

IB 

M 

cross-hair. 

>  el 

o 

o 

o 

1902 

Bohemian  Monnt- 

NE. 

32 

58 

29  W. 

Medium  -  grained 

A'gatiU,  abundant,  often  quite 

14 

17 

31 

ain,  north  side 

to  coarse-grain- 

large crystals;  oligoclase  and 

11 

13 

24 

Lao  la  Belle,  Ke- 

ed ;  mixture  of 

orthoclase,  both  reddened  and 

4 

7 

11 

weenaw  Point, 

red  feldspars 

dulled ;    titaniferous    magne- 

2 

4 

6 

Michigan. 

and  shining  dark 
greenish  -  hlack 
mineral. 

tite  ;  augite ;  diallage,  mostly 
altered  to  uralite ;  amorphous 
green  substance,  secondary  to 
augite;    gray   substance  sec- 
ondary to  titaniferous  magne- 
tite ;  a  little  secondary  quartz. 

20 
4 
15 

IB 
5 
12 

38 
9 
27 

Bed  941 

Eaglo  Eiver  sec-     SE. 

30 

58 

.31 W. 

Medium  -  grained 

Apatite;  oligoclase;  orthoclase; 

22 

tion,  Keweenaw 

to  coarse ;  black, 

titaniferous  inagnetiie;  augite; 

23 

Point,  Michigan. 

thickly  studded 
with   long    red 
feldnpars.      Sp. 
gr.,  2.94. 

a  little  altered  magma;  sec- 
ondary/errite,  chlorite,  quartz, 
from  the  feldspars;  secondary 
yellowish-green    mineral   and 
magnetite,  from  the  augite. 

31 
33 
33 
33 
36 

6-W. 

Old  Ironton  trail, 
AshlandCounty, 
"Wisconsin. 

N.i 

34 

46 

IW. 

Coarse-grained, 
red,  black,  and 
gray  mottled; 
shows  much 
highly  -  magnet- 
ite. Sp.  gr.,2.89. 

Plagioclase;   orthoclase,  highly 
altered,   much   stained  with 
ferrite ;  titaniferous  magnetite, 
coarse  and  abundant;  diallage 
largely  .altered  to  uralite ;  chlo- 
rite secondary  to  the  feldspars ; 
magnetite,  secondary  to  dial- 
lage; alittle  secondary  quartz. 

>  Metaaomatio  DoTelopment  of  the  Copper-bearing  Eocks  of  Lake  Superior,  Proc.  Am.  Acad.  Sci.,  Vol.  Xn,  pp.  253-309. 


54 


COPPEE-BEAEING  EOOKS  OF  LAKE  SUPEEIOE. 


Tabulation  of  observations  on  orthoclase-bearing  gabbros — Continued. 


a 


61. 


641. 


Place. 


112  L 


2021. 


2051. 


Cut  on  ■Wiscon- 
sin Central 
Eailroad,  Bad 
Kiver,  Ashland 
County,    "W  i  s- 


Brunacli'weiler's 
Eiver,  Ashland 
County,  W  i  s- 
consin. 


Bayfield  County, 
"Wisconsin. 


Douglas  County, 
Wisconsin. 


Aminicon  Elver, 
DouglasCounty, 
■Wisconsin. 


CT 


SE. 


30 


45 


SE. 


NE. 


SE. 


SE. 


16 


12 


32 


2W. 


45    IW. 


44 


48 


48 


Macroscopic  char- 
acters. 


Coarse-grained; 
red  -  and-hlack 
mottled ;  rough- 
textured. 


6^. 


12  W. 


12  "W. 


Constituents  as  determined  by 
microscope,  in  order  of  age. 


Coarse-grain|ed, 
dark  greenish- 
gray,  mottled 
■with  dull  red 
and  white. 


Coarse-grained; 
dark  greenish- 
gray,  sparsely 
mottled  with 
red;  dull. 

Medium-  grained ; 
red- and -gray - 
mottled.  Sp. 
gr.,  2.76. 


Coarse-grained, 
red-and  black- 
mottled. 


Angle  between 
maximum  ex- 
tinctions of 
at^iacent  hemi- 
tropic  bands  of 
the  plagioclase 
in  sections  cut 
at  random  in 
the  zone  O ;  z"z. 


Angles  on 
opposite 
sides  of 
cross-hair. 


Oligoclttse,     predominating    in 
large  crystals,  much  clouded, 
but  still  showing  the  banding 
distinctly;    orthoclase;    very 
coarse    titanic    magnetite; 
greenish  hornblende  or  uralite, 
showing    strong    dichroism, 
and  in  a  marked  degree  the 
characteristic  prismatic  cleav- 
age, but  evidently  an  altera- 
tion from  diallage  and  augite, 
as  proved  by  the  existence  of 
a  few  cores  of  the  original 
substance.    This  homblendio 
mineral  fills  in  the  spaces  be- 
tween the  oligoclases  in  the 
manner  characteristic  of  the 
gabbros. 
Apatite,  in  very  large  crystals ; 
labradorite ;  orthoclase,-  titan^ 
iferous  mjignetite,  very  abun- 
dant   and  coarse;   uralite; 
quartz,  ferrite,  secondary  feld- 
spars; chlorite,  secondary  to 
uralite. 
OUgoclase ;    orthoclase ,-    highly 
altered  (i(an^eroi^  magnetite, 
very  coarse  and  abundant;  di- 
allage, almost  wholly  altered 
to  uralite ;  chlorite  secondary 
to  feldspars ;  ferrite. 
Apatite  in  long  crystals ;  oligo- 
clase,  stained  red ;  orthoclase ; 
diallage,    sparse,    mostly   al- 
tered  to   a   dark-green  sub- 
stance;   ferrite,     chlorite, 
quartz,  secondary  to  feldspars. 
Labradorite  (in  sections  cut  at 
random  in  the  zone  O :  ii,  the 
angle  ranged  from  37°  to  50°; 
a  section  cut  independently 
and   carefully   parallel  to  O 
gave  12°  to  14°);  orthoclase; 
augite;   diallage   in  twinned 
plates.  (Pumpelly,  Geology  of 
Wisconsin,  Vol.  III.,  p.  41.) 


14 


14 


10 


10 
17 
10 


0.2 


44 

38 


24 


21 
32 
21 


OETHOCLASE-BEARmG  GABBEO. 


55 


Tabulation  of  observatiom 

on  orthoclase-iearing  ^a66ros— Continued. 

8' 

a 

Place. 

§ 
-.3 
o 

p, 

Macroscopic  char- 
acters. 

Constituents  as  determined  by 
microscope,  in  order  of  age. 

Angle    between 
maximum    ex- 
tinctions  of 
adjacent  hemi- 
tropic  bands  of 
the  plagioclase 
in  sections  cut 
at   random   in 
the  zone  0:  ii. 

1 

1 

1 

o 

^ 

i 

Angles  on 
opposite 
sides  of 
cross-hair. 

:o 

Near  Duluth, 

ms. 

33 

50 

14-W. 

Very  coarse-graln- 

Apatite;  labradm-ite;  orthoclase; 

o 
23 

o 
22 

o 
45 

Minn.,     1,980 

ed;    light-grays 

titaniferou^  magnetite;  diaUage 

9 

10 

19 

north,  0  west. 

much     pintish 
titanic     iron; 
veryrough-text- 
nred. 

mostly  changed  to  uralite. 

3 

Kear    Duluth, 

N.i 

28 

60 

UW. 

Coarse-grained; 

Apatite  in  a  few  large  crystals ; 

22 

21 

43 

Minn.,     1,970 

light-gray  J  very 

labradorite;    titanic   iron, 

27 

20 

47 

north, -680  west. 

rough-textured. 

abundant ;     diaUage,    highly 
fibrous  and  largely  altered  to 
uralite. 

27 

23 

50 

i 

Ifear    Duluth, 

NW. 

28 

50 

14-W. 

Very  coarse-grain- 

Oligoclase;     orthoclase;     little 

12 

11 

23 

Minn.,     2,000 

ed;  light- gray; 

titan^erous  Tnagnetite;    dial- 

8 

13 

21 

north,  1,300  west. 

very  rough-text- 
tured. 

lage  mostly  altered  to  uralite. 

5 

Near    Dnluth, 

NW. 

28 

50 

14  W. 

Very  coarse-grain- 

Apatite, in  quite  large  crystals ; 

13 

16 

29 

Minn.,     J,800 

ed;    light-gray; 

oligoclase;  orthoclase;   titani* 

11 

13 

24 

north,  2, 000  west. 

very  rough- 
textured. 

ferous     magnetite ;    diallage, 
partly  fresh,  partly  altOTed  to 
uralite;  ochre;  biotite. 

12 

13 

25 

509.... 

Top  of  Muff,  Du- 

SW. 

27 

50 

14-W. 

Very  coarse-grain- 

Labradorite; titaniferous  mag- 

20 

24 

44 

luth,  Minn. 

ed;  dark-gray. 

netite;  diallage,  partly  fresh, 
partly  altered  to  uralite  and 
chlorite. 

25 
21 

24 
25 

49 
46 

511.... 

do 

sw. 

27 

50 

14-W. 

Very  coarse-grain- 
ed ;    light-gray ; 
rough-textured. 

Apatite  in  large  crystals ;  labra- 
dorite, much  altered;    titani- 
ferous   magnetite;     diallage, 
much  altered;  chlorite. 

20 
26 
23 

19 
24 
23 

39 
50 
46 

508.... 

Near  quarry,  Du- 

SW. 

27 

50 

14-W. 

Very  coarse-grain- 

Labradorite or  anorthite,  much 

31 

31 

62 

luth,  Minn. 

ed,   the  plagio- 
clases    running 
from  a  haU  inch 
to   an    inch    in 
length. 

clouded;   titaniferous  magne- 
tite; diallage,  mostly  altered 
to  uralite. 

19 

21 

40 

31 

Lester  Eiver,  Min- 

WW. 

4 

50 

13  W. 

Medium-grained; 

Apatite     abundant ;     oligoclase 

14 

15 

29 

nesota,    near 

red-hlack-and- 

much     clouded;     orthoclase; 

8 

10 

18 

northwest    cor- 

green-mottled; 

titaniferous   magnetite;   non- 

13 

13 

26 

ner. 

much  weathered. 

diallagic  augite;    alteration- 
product  of  augite. 

41 

Near  Lester  Eirer, 

NE. 

29 

51 

13 -W. 

Clese  to  39,  less 

Labradorite,  orthoclase,  in  the 

19 

21 

40 

Minnesota. 

weathered. 

characteristic   twins ;    titani- 
ferous  magnetite;   augite  in 
long,  twinned  blades;    dial- 
lage;  a  green  alteration-pro- 
duct  of     augite;    secondary 
quartz. 

29 
16 
18 

30 
14 
15 

59 
30 
33 

56 


COPPER-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 


Tabulation  of  observations  on  orthoclase-bearing  gabbros — Continued. 


Angle    between 

1 

a 

1 

Place. 

1 
1 

1 

'% 

i 

Macroscopic  char- 
acters. 

Constituents  as  determined  by 
microscope,  in  order  of  age. 

maximum    ex- 
tinctions of 
adjacent  hemi- 
tropic  bands  of 
the  plagioclase 
in  sections  cut 
at  random   in 
the  zone  0:  ii. 

Angles  on 
opposite 
sides  of 
cross-hair. 

o 

o 

0 

39 

do 

SE. 

29 

51 

13  AV. 

Medinm-grained 
to  coarsegrain- 

Apatite,  large   and   abundant; 
labradorite;  orthoclam;  titani- 

20 
16 

25 
21 

45 
37 

ed;  resinous; 

ferous  magnetite!    augite  in 

12 

13 

25 

brownish;   an- 

long,    twinned   blades;    diaU 

17 

17 

31 

gite  in  long  radi- 

lage;  ochre;  mnch secondnry 

atingblades.  Sp. 

quartz. 

gr.,  2.82. 

1057... 

Bed   of    Cascade 

S.i 

10 

62 

2W. 

Medium -grained; 

Apatite ;     labradorite ;     ortho- 

21 

18 

39 

Eiver,     Minne- 

black-and-gray- 

elase ;  titaniferous  magnetite  ; 

30 

28 

58 

sota  ;    12   miles 

mottled ;  rough- 

diallagic  augite,  much  altered ; 

23 

18 

40 

from  moutli. 

textured.       Sp. 
gr.,  2.82. 

secondary  quartz. 

1062... 

Sonth  side  Eagle 

(ahou 

t)26 

63 

2^. 

Medium-grained 

Apatite ;     labradorite ;     ortho- 

16 

14 

30 

Mountain. 

(Kot'surveyec 

•) 

to  coarse;  nearly 

elase;  augite,  sparse,  highly 

22 

26 

48 

black;  rough. 

altered,  and  filled  with  dusty 
magnetite     (alteration-pro- 
duct ?) ;  secondary  quartz. 

26 

28 

54 

Hornhlende-gabbro. — Along  a  belt  of  countiy,  some  fourteen  to  twenty- 
miles  in  length,  running  westward  from  Bad  River,  Wisconsin,  through  parts 
of  townships  44  and  45,  ranges  3,  4,  5,  and  6  west,  at  a  horizon  not  far 
above  the  Huronian  slates,  exposures  of  a  peculiar  hornblende-gabbro 
have  been  noticed.  This  rock  differs  from  the  uralitic  gabbros  previously 
described  in  containing,  instead  of  the  fibrous,  greenish,  comparatively 
weakly  dichroic  uralite,  a  deep-brown,  intensely  absorptive,  so-called 
basaltic  hornblende.  Some  of  these  rocks  have  been  described  briefly  by 
Pumpelly  in  the  third  volume  of  the  Geology  of  Wisconsin,^  under  the 
name  of  "augite-diorite."  He  regarded  the  hornblende  as  primary,  and 
the  rocks  as  intermediate  between  diabase  and  diorite,  whence  the  name.^ 
In  the  same  volume  I  suggested  that  the  hornblende  was  secondary,  and 
that  the  rocks  were  merely  altered  gabbros.^  This  opinion  I  find  sustained 
by  a  re-examination  of  Pumpelly's  sections,  and  a  study  of  a  number  of 


•Page  36. 


2  Page  170. 


3  Page  170. 


j;.<iTE.D  STATES   GEOLOGICAL  SLi'-.'  i 


■.,Af.L    ^Ul'LKlJH    r-X.Vi: 


T'xji    ^omhZende.-fcU>iT"0  from    Jtslilancl    Cotiniy  TJ^i'i, 

Jjaljrix<3eTt^e(iy;  orilio c2ase,(A.i i  naft^e-f'yjCfr'fBZi/  alle-reR 
■fo  viriAiin.  a,-nA  tc-r<JHe^j  iTon-n^omiZe-Tiolt'i.'i-J;  'Zzta.nxf- 


rtf.i..  /fornl>1enSe,-yaTjlir'o  -fTom   AtKlficl  Ca-uitiu ,  nv'T. 
0r3ina.Tu  ciiyTii  Sco-Ze,  it,  cZr-a^ne-icrt 

OZz]facZa-ft(iJ ;  ortJiociase^xJ ,  irorr/iJiornhZK-ndtJiaJjuriiii^ie. 
(vfj  aZZcrecifrom-  t^ia7Za,^e^ ,     izieniferovi,s  Tna^nciii^e^CfJ j 


fif.3    /^ornhZe-n^e-faiiro  fr<o?Tt  lE-yi^ixfk  Labile.,       ?fis. 


Ztornhiie-n^e^dj  nrH\  ccret  of  a.icy%ie^s.J;   Zahr'a.aoT'ite.Xs'J 
Tn  u^-»,e-i  lie.  (/ij. 


P'^y-  V    j4norik-Cie.  r-oo/r  from,   narih  r/ioTe  of  ZaTte.  Su.ptr- 

T'oZari'zeci    Zi^At. 

•^acZe  icp  rr7,o7Zy  of  ano-ri\xZe  incZzri'^tiaZt 


RIOE 


ing  gahb 


.-^^^ 


f;raiDed 
cottree-grain* 
A;  TCftinons; 
IjrownUb;  au- 
pte  in  loog  radi- 
atingblades.  Sp. 


Y^Moti  iiKs'vr.i,  S.         ^  I      gr.,  2.83. 


jiSi*r«,v>  >.N    c       c'         .  '■^*'^^   ^'^  ^'^■OoL  VS:  -^     '        T^;*'^        dose;  «tonirer<«a«.ai.tfl.(o,>l    80        88        68 


^pafite,   large   and  ahmi 
lahradoriU; 
/eroua  magu. 
long,   twiniied   \t- 
lage;  ochre  ;  ma* 
i/uarfcr. 

Apatite !     labradoriti'  .• 


21 


18    1    39 


,    ,    ..  '^^■««'^4'  v^iVaiftK-^.ii.  ^""Viwrf-V     diaHoffic ou-7i<e, much 


tT^s 


•'^^i^aVs^Ac 


(Not  sarveyed.) 


tocoarse;  nearlf       daae;  auffite,  sparse,  highly 
blacli;  nineh.  nltf  rcl,  and  flUcd  vrith  dn?*r  ]    26 


/teeji    t 


28 


1  ^      -^^i«^5S3K,^ar„  x«o-v9 


Co    Ol    ii.    j 

s  from  the  ur. 
the  fibrous,  j. 
[•own,    intense; 
rocks  have  bo 
he  Geology  oi 
g,ardecl  the  hornl 
'//^m  diabase 
■.^t  the  I 


-•^^      ^-^^-l^^.W'i«,e. 


■V>».UVjVraj 


r-^ai 


COPPER-BEARING  ROCKS  OF  LAKE  SUPERIOR   PL.Vi 


HORNBLENDE- GABBROS  _WD  ANORTHITE-ROCK 


HOENBLENDB  GABBEO. 


57 


new  sections  from  other  outcrops.  The  rocks  described  in  the  table  have 
been  selected  to  illustrate  the  different  phases  of  this  hornblende-bearing 
rock.  The  first  one  is  a  rock  in  which  the  basaltic  hornblende  is  in  small 
quantity  only.  The  next  three  contain  more  hornblende,  and  are  peculiar 
macroscopically,  being  mottled  black,  white  and  pinkish.  They  also  carry 
a  good  deal  of  original  quartz.  The  last  two  are  black  rocks,  in  which  the 
hornblende  makes  up  most  of  the  section.  Abundance  of  coarse  apatite, 
presence  of  a  low- angled  plagioclase,  of  orthoclase,  and  of  original  quartz 
seem  to  be  prevailing  characteristics. 


Tabulation  of  observations  on  hornblende-gabbro. 


1 

a 
3 

ID 

St 

EC 

Place. 

1 

1 
I 

03 

1 

§ 
1 

1 
1 

i 

Macroscopic  char- 
acters. 

Constitaents  as  determined  by 
microscope,  in  order  of  age. 

Angle     between 
maximum    ex- 
tinctions of 
adjacent  hemi- 
tropic  bands  of 
the  plagioclase 
in  sections  cut 
at   random    in 
the  zone  0:  it. 

Angles  on 
opposite 
sides  of 
cross-hair. 

137  "W. 

AeUand  County, 
Wisconsin. 

N.-W. 
cor. 

35 

45 

AW. 

Medium-grained ; 
dark    greenish- 
gray. 

Apatite,  in  large  crystals,  very 
abundant;  ia&ratioriie,  in  crys- 
tals often  much  rounded,  quite 
fresh;  orthoclase  (?) ;  magnetite 
or  titanic  iron,  very  large  and 
abundant;  aM^rife,  very  abund- 
ant, in  very  large,  rounded  par- 
ticles between   the   feldspar 
grains,  only  rarely  showing 
the  diallage  cleavage,  partly 
fresh,  partly  altered  by  spots 
and  streaks  of  a  greenish  fee- 
bly dichroic  or  non-dichroic 
substance;   brown,  intensely 
absorptive,  and  dichroic  horn- 
blende, not  very  abundant,  oc- 
curring partly  away  from  the 
augite — but  like  it  between 
the  feldspar  gi'ains-and  partly 
in  patches  within  the  areas  of 
the  altered  augite,  in  such  a 
manner  as  to  suggest  its  sec- 
ondary origin;  uralite,  in  little 
streaks   within    the    altered 
augite. 

0 

12 
19 

24 

o 
17 
21 
21 

o 
29 
40 
45 

58 


COPPEE-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 


Tabulation  of  observations  on  hornblende-gabbro — Continned. 


u 

■a 

g 
1 

Place. 

1 

m 

1 

1 

o 

1 

Macroscopic  char- 
acters. 

Constituents  as  determined  by 
microscope,  in  order  of  age. 

Angle    between 
maximum    ex- 
tinctions of 
adjacent  hemi- 
tropic  bands  of 
the  plagioclase 
in  sections  cut 
at   random   in 
the  zone  0 :  ii. 

Angles  on 
opposite 
sides  of 
cross-hair. 

o 

o 

o 

2022  I. 

Ashland  County, 
Wisconsin. 

S.W. 

53 

45 

3W. 

Medium -grained; 
mottled     black 
and     white; 
rough.    Sp.  gr., 
2.83. 

Apatite,  very  abundant,  in  small 
crystals;  labradoriteaaiortho- 
close,  in  much  rounded  parti- 
cles;   diallage,    often   partly 
altered  to  uralite;   non-dial- 
lagic   augite;    basaltic  horn- 
blende; a  few  large  scales  of 
biotite;  9Mar(2,  fllhng  comers 
and    apparently    primary, 
abundant. 

20 

19 

39 

2020  I. 

....do 

2 

44 

3W. 

Medium  -grained ; 

Apatite;    oligoclase  and   ortho- 

7 

5 

12 

nearly  black: 

clase  in  much-rounded  grains ; 

5 

3 

8 

rough.textured. 

magnetite,  or  titanic  iron ; 
augite,    very    abundant,    in 
large,  rounded  grains,  at  times 
diallagic;  hsLaaltic  hornblende, 
not  very  ah  mdant,  secondary, 
and   grading   into    augite; 
quartz,  filling  comers,  not 
abundant. 

1 

4 

5 

106  1.. 

Ashland  County, 

N.W. 

17 

44 

5W. 

Medium -grained; 

Apatite,  very  large  and  abun- 

7 

4 

U 

Wisconsin,  north 

mottled    black. 

dant,  just  as  in  No.  1  of  this 

11 

12 

23 

line  of 

white,  and  yel- 
lowish-pink; 
rough-textured. 

group — often  in  hexagonal  sec- 
tions  which  reach   .01""  in 
diameter;    oligoclase;    ortho- 
close;    magnetite,  or    titanic 
iron;  diallage  almost  whoUy 
changed  to  uralite;    basaltic 
hornblende,  occurring  just  as 
in  137  W.,  but  very  much  more 
abundant;    biotite,  in   a  few 
flakes ;  quartz,  in  a  few  spots, 
filling     corners,     apparently 
original ;   chlorite  and  ferrite, 
secondary. 

187  W. 

English  Lake,  at 

S.i 

5 

44 

3W. 

Medium-grained ; 

Apatite,  in  a  few  large  crystals ; 

15 

13 

28 

outlet,  Ashland 

black;  lustrous. 

labradorite;  magnetite,  small 

24 

23 

47 

county,  Wiscon- 

and  sparse;    augite,    mostly 

13 

14 

27 

sin. 

quite    fresh;     basaltic   horrid 
blende,  making  up  most  of  the 
section,  in  a  few  cases  appear- 
ing to  shade  into  the  augite; 
biotite. 

96W.. 

Ashland  County, 

S.i 

15 

45 

IW. 

Medium-grained 

Oligoclase;  orthoclase;   m,agne- 

14 

17 

31 

Wisconsin. 

to  fine-grained; 
nearly  black. 
Sp.  gr.,  3.08. 

tite;    augite;    basaltic   horn- 
blende ;    biotite.      Eesemhles 
Ko.  5,  but  much  finer  grained. 

13 
11 

16 
13 

29 
24 

ANOETHITE-EOCK. 


59 


AnortJiite- rock — At  several  points  on  the  north  or  Minnesota  shore  of 
Lake  Superior,  between  the  mouth  of  Split  Rock  River  and  the  Great 
Palisades,  and  again  in  the  high  point  near  the  mouth  of  Temperance 
River,  known  as  Carlton's  Peak,  are  to  be  seen  exposures  of  a  very  coarse 
light-gray  to  colorless  or  white  rock,  occasionally  with  a  faint  greenish 
tinge.  This  is  seen  in  the  thin  section  to  be  composed  exclusively,  or  nearly 
so,  of  anorthite  feldspar.  Often  there  is  no  other  mineral  present  except  in 
exceedingly  minute  inclusions,  and  these  are  very  sparse.  In  one  section 
a  few  grains  of  altered  olivine  were  noticed  within  the  anorthite,  and  in  two 
or  three  a  little  augite  between  the  feldspar  grains.  The  feldspar  appears 
in  every  case  to  be  anorthite.  In  no  section  did  it  show  the  peculiar  ar- 
rangement of  needle-like  inclusions  met  with  in  European  gabbros,  and  so 
common  in  the  coarse  gabbros  of  Lake  Superior,  to  which  this  rock  is  very 
nearly  related. 

This  anorthite-rock  presents  very  interesting  occurrences,  as  described 
in  a  subsequent  chapter.  It  appears  both  as  masses  cutting  black  gabbro, 
and  as  included  angular  masses  in  the  same  rock. 

Tabulation  of  observations  on  anorthite-rock.      ^^^K 


1 

a 
§ 
.§ 

O 

CD 

Place. 

1 

» 

C 

§ 

13 

o 
H 

i 

Macroaonpio  char- 
acters. 

Coustitnents  as  determined  hy 
microscope. 

Angle    between 
maximum    ex- 
tinctions of 
adjacent  bemi- 
tropic  bands  of 
the  plagioclase 
in  sections  cut 
at  random    in 
the  zone  O.-ti. 

Angles   on 
opposite 
sides  of 
cross-hair. 

0.2 

729.... 

Korth  shore  Lake 

SW. 

5 

54 

sw. 

Very   coarse- 

Pure anorthite,  without  acces- 

o 
28 

0 

30 

o 
58 

Superior;  IJ 

grained;   the 

sory. 

37 

41 

78 

miles  below 

crystals   reach- 

35 

41 

76 

month  of  Split 

ing  an  inch  in 

Kock  iliver, 

length  and 

Minnesota,  near 

breadth;   color- 

center of 

less   to   white; 
occasionally 
with  a  greenish 
tinge. 

746.... 

North  shore  Lake 

SW. 

5 

54 

8-W. 

Very    coarse- 

Pure anorthite,  without  acces- 

34 

40 

74 

Superior;  IJ 

grained;  the 

sory. 

22 

25 

47 

miles   below 

single    crystals 

29 

24 

53 

mouth  of  Split 

often    reaching 

Kock  River, 

i    inch   across; 

Minnesota. 

light-gray. 

60 


COPPER-BEARING  ROOKS  OF  LAKE  SUPERIOR. 


Tabulation  of  observations  on  anorthite-rocJc — Continued. 


a 
p 

a 
S 

Place. 

1 

d 
o 

CO 

d 

a 
1 

i 

Macroscopic  char- 
acters. 

Constituents  as  determined  by 
microscope. 

Angle     between 
maximum     ex- 
tinctions of 
adjacent  hemi- 
tropic  bands  of 
the  plagioclaae 
in  sections  cut 
at   random    in 
the  zone  0:ii, 

Angles   on 
opposite 
sides  of 
cross-hair. 

o 

o 

0 

750 

Kortli  shore  Lake 

sw. 

5 

fi4 

8W. 

C  0 .1  r  s  e-grained ; 

Pure  cmorthite 

39 

34 

73 

Superior;  li 

nearly  white, 

42 

40 

82 

mileB  below 

with  faint  green- 

35 

33 

63 

montli  of  Split 

ish  tinge. 

Eock  Eiver. 

759.... 

North  shore  Lake 
Superior;    2 
miles   helow 
mouth  of  Split 
Eock  Eiver. 

NE. 

5 

55 

S'W. 

Very  coarse- 
grained;  nearly 
colorless,  with 
faint  greenish 
tinge. 

Anorthitef    in    large    crystals, 
makes  up  most  of  the  section. 
The  large  crystals  are  often 
crushed  into    fragments    at 
their   extremities.    Among 
the  fragments  are  smaller  an- 
orthite  crystals,  and  numer- 
ous small  rounded  grains  of 
augite. 

792.... 

Falls  of  Beaver 
Eiver,  Minne- 
sota. 

SW. 

12 

55 

S'W. 

Very  coarse- 
grained; trans- 
lucent ;    nearly 
colorlesa,     but 
with  faint  green- 
ish tinge. 

Pure  anorihite,  in  very  large, 
unbroken  crystals. 

795.... 

Beaver  Eiver, 
Minnesota. 

sw. 

12 

55 

8W. 

Coarse-grained. 

• 

Anortkitet  in  large  crystals, 
makes  up  most  of  the  section. 
A  few  small  grains  of  oliviiie, 
altered  to  a  brown  ocheroua 
substance,  occur  in  the  feld- 
spars.   A  little  augite  lies  be- 
tween  the    feldspar   grains. 
Bands    of  viridite   cross  the 
feldspars.    The  anorthite  car- 
ries long   rows   of    minute, 
rounded    inclusions,    readily 
seen  with  a  low  power  (70  to 
80    diameters).     These   rows 
run  for  short  distances  paral- 
lel to  the  cleavage,  but  are 
mostly  in  rows  crossing  the 
cleavages.     A  higher  power 
shows  numerous  bubbles  in 
these   inclusions.     The   bub- 
bles  will   not    disappear    or 
move  at  a  temperature  of  112° 
C.    Black  microliths  also  oc- 
cur in  the  particles. 

UNITED  STATES   GEOLOGICAL  SURVEY 


COPPER-BEARING   ROCKS  OF  LAKE  SUPERIOR    PL. VIII 


DIABASE  AND  DIABASE -PSEUDAMTGDALOIDS. 


-5<B^s^  \Vt-)^3^5^«5>  "Yo^^^^o-^t^  Kos^Ji-^as^si  s^'KO'^S.    'AV^ 31. n^oo^sivi 


'|i:>ie,  tL_Ji 


iiiu'l  basic 


•X03r^   ,'^0  5'v5a;^w'L   3-lKil\"*\^  3'^tiVv%   ^"H-^&CS     V^Q'O^  37^1i0.ns^  >f  ^j'Sl 

-So«ioi^5   S^.'iiis   ^»^•^c  3<^ai  s^^  \i.^  b^s^m  o  ■^nos^os-'isSs  ««>j«s^o3 


?as    Co.    ^li.  OrJzna-rtf  l-ioTzi-        6cule,  if  ciia.me.te.r:r 

Ohfec/ascliJ;    Zrojrn  a7'ieTa.iicn  /oroc/zc^i of  aicqt6e^(^j,J ;  it-tan- 
iferous    Trva^T^e^z^B  Cj'J:ps£ii^a7ny^(^u2e^   of  cUorvieit)  ancL 


Ordi^CL-ri^  liffkt.  Scale   fii/xia-n-ieieri' 


7ioit.  JJiaha'Se.  from   TieriTi  sTiore  of  lake  Stcper-zoT,  nta.-r 
Spin  ^ock  Jiiy-er,   Minn.      7^olarii.eel  lzj>^t .     (fca^-/^  t^i'urte^rt 

Lahracloriiefyj  I'tT,  sntalZ   icuin^ar-  cnysi^a.7t  a-rranpe^  nt  a. 
common  diTecizon  ;  OMffi^e  liJ  tniar^e.  areas  ea.cn  tricitcc/- 
(nf  ma-nif  p?a^zoc7ase!j    maone^z^ielSJ  z-niTie.  xyi^e-rfp^cst 
of  i he.  aiz.yi6e.s 


FINE-GRAINED  BASIC  ROOKS. 


61 


Tabulation  of  observations  on  anorthite-rocJc — Continued. 


1 

g 

a 

3 

Place. 

d 

1 

I 

0 
& 

a 
o 

'■s 

ID 
03 

'M 

CO 

a 
o 

a 

Macroscopic  char- 
acters. 

Constituents  as  determined  by 
microscope. 

Angle    between 
maximum    ex- 
tinctions of 
ai^jacent  hemi- 
tropic  bands  of 
the  plagioclase 
in  sections  cut 
at   random    in 
the  zone  0:  il. 

Angle     on 
opposite 
sides  of 
cross-hair. 

If 

0 

o 

o 

803.... 

Nortli  shore  Late 

NE. 

12 

55 

8W. 

Rather  coarse; 

AnortMie  makes  almost  the  en- 

22 

26 

48 

Superior ;   Bea- 

nearly colorless; 

tire  section;  rare  and   small 

27 

31 

58 

ver   Bay,    Min- 

translucent. 

a-ugite   particles  lie  beiween 

24 

28 

52 

nesota. 

the  grains  of  feldspar. 

32 

33 

65 

811.... 

Island   off  sonth 

SE. 

12 

55 

8W. 

Coarse-grained; 

Anortkite,  with  numerous  min- 

34 

46 

80 

Beaver  Bay 

dark-gray. 

ute  ca-vities  and  particles  like 

31 

42 

73 

Point,  Minne- 

those of  ISo.  795,  but  much 

30 

37 

67 

sota. 

smaller,  makes  most  of  the 
section.    A  few  small  augite 
particles,   often  altered  to  a 
greenish    substance,    lie  be- 
tween the  feldspars. 

822.... 

Korth  eliore  Lake 
Superior;    2 

ITE. 

6 

55 

IW. 

Coarse-grained; 
colorless  to 

Pure  anoTthite 

31 

33 

64 

30 

43 

73 

miles  below 

white. 

BeaverBay, 

Minnesota. 

FINE-GRAINED    BASIC    ROCKS. 

Looking  over  the  Keweenaw  Series  as  a  whole,  the  fine-grained  basic 
rocks  are  found  to  prevail  in  extent  of  surface  spread  and  total  thickness  over 
the  coarser  kinds  above  described,  though  these  latter  are  very  frequently 
met  with,  and  constitute  great  thicknesses,  especially  in  the  lower  portions 
of  the  series.  Coarse-graiued  layers  continue  to  quite  high  horizons,  as,  for 
instance,  some  of  the  beds  of  the  Greenstone  Group  of  Keweenaw  Point, 
but  they  are  here  much  less  frequent  than  in  the  lower  portions,  from 
which,  however,  the  finer-grained  kinds  are  never  absent. 

OUvine-free  diabase  of  the  "  ordinary  type." — In  his  descriptions  of  the 
Keweenaw  Point  diabases,  Pumpelly  recognized  two  types,  to  which  he 
gave  the  names,  respectively,  of  "  ordinary  "  and  "  ashbed  "  types.  The 
former  name  applies  to  the  fact  that,  so  far  as  his  acquaintance  with  the 
Keweenaw  Series  went,  that  type  was  the  most  common  and  characteristic 
of  all  its  crystalline  rocks.     The  latter  name  was  given  because  the  type  to 


62        COPPEE-BBAEING  BOOKS  OF  LAKE  SUPBEIOE. 

which  it  was  appHed  makes  up  the  lower  portion  of  the  bed  whose  upper  or 
vesicular  portion  is  the  extraordinarily  scoriaceous  amygdaloid  so  well 
known  on  Keweenaw  Point  as  the  "  ashbed."  The  "ordinary  type"  is  alone 
considered  under  the  present  heading,  the  "ashbed  type,"  according  to  my 
observations,  being  more  closely  related  to  the  diabase-porphyrites.  While 
the  ordinary  type  diabases,  with  their  amygdaloids,  are  the  most  com- 
mon kinds  on  Keweenaw  Point,  they  do  not  appear  to  me,  when  we  look 
the  whole  ground  over,  to  be  any  more  common  than  the  fine-grained  oliv- 
initic  kinds,  and  not  much  more  so  than  those  of  the  ashbed  type. 

The  ordinary  type  diabases  make  up  relatively  thin  flows,  which 
are  almost  invariably  furnished  with  vesicular  or  amygdaloidal  upper  por- 
tions. These  vesicular  upper  portions  have  always  undergone  great  in- 
ternal changes,  both  in  connection  with  the  deposition  of  minerals  in  ves- 
icles, and  in  the  formation  of  pseud-amygdules,  or  minerals  replacing  primary 
constituents  in  such  a  way  as  to  present  macroscopically  very  much  the 
appearance  of  the  true  vesicular  fillings.  The  latter  change,  with  others, 
has  usually  affected  also  the  lower  non-vesicular  portions  of  the  beds  in 
greater  or  less  degree.  So  general  are  these  alterations  that  an  account  of 
these  rocks  has  to  be  taken  up  with  the  internal  changes  that  they  have  un- 
dergone more  than  with  the  nature  and  arrangement  of  the  original  constit- 
uents. Professor  Pumpelly's  studies  have  been  so  exhaustive  that  I  can  do 
no  better  than  quote  largely  from  him  in  this  connection.  His  microscopic 
studies  were  chiefly  made  on  specimens  from  the  Eagle  River  section  of 
Keweenaw  Point  (that  is,  on  beds  lying  between  the  Great  Conglomerate 
and  the  base  of  the  Greenstone  Group),  and  on  specimens  from  the  lower 
horizons  at  Portage  Lake.  I  have  examined,  in  addition,  a  large  number 
of  specimens  from  still  higher  and  lower  horizons  on  Keweenaw  Point, 
and  from  all  other  parts  of  the  extent  of  the  formation.  For  the  most  part, 
my  work  has  merely  served  to  extend  the  geographical  range  to  which  most 
of  his  conclusions  are  applicable. 

"Externally",  according  to  Pumpelly,  "the  different  varieties  of  these 
diabases  are  dark  in  shade,  varying  from  almost  black  in  unaltered  speci- 
mens to  dark-green  or  dark-brown,  or  varying,  minutely  subdivided  mix- 
tures of  these  colors,  according  to  the  relative  proportions  of  chlorite  and 
ferric  oxide  among  the  decomposition  products.     They  vary  in  texture  from 


OLIVINE-FEEE  DIABASE.  63 

medium  fine-grained  to  crypto-crystalline,  and  the  fracture  from  uneven 
and  hackly  to  conchoidal."^  To  this  I  have  only  to  add  that  a  purplish 
shade,  varying  from  a  brownish-purple  to  a  bright  reddish-purple,  is  a  very 
common  one,  and  that  a  true  conchoidal  fracture  is  characteristic  only  of 
the  kinds  here  set  apart  as  a  separate  group  under  the  name  of  ashbed- 
diabase. 

The  pseud-amygdaloidal  alteration  of  these  rocks,  while  it  has  taken 
place  largely  in  the  true  amygdaloid,  and  has  commonly  affected  the  entire 
thickness  of  a  bed  in  some  degree,  is  especially  characteristic  of  the  middle 
portions  of  the  bed.  The  common  pseud-amygdules  are  chlorite^  quartz, 
prehnite  and  calcite.  They  vary  greatly  in  size,  running  from  a  quarter 
of  an  inch  or  more — rarely  several  inches — in  diameter,  down  to  minute 
particles.  In  the  former  case  the  rock  is  coarsely  blotched  with  the  colors 
of  the  pseud-amygdules,  looking,  as  said,  like  a  true  amygdaloid,  while  in 
the  latter  case  the  only  effect  on  the  external  appearance  is  a  variation  of 
the  general  shade. 

Under  the  microscope,  when  not  too  profoundly  altered,  this  diabase 

"is  seen  to  have  for  primary  constituents  plagioclase,  augite  and  an  opaque 

black  mineral,  which  may  be  either  magnetite  or  titaniferous  iron  ore."^ 

The  last-named  ingredient  never  shows  any  distinct  crystalline  outlines. 

It  appears  generally  to  be  strongly  attracted  by  the  magnet,  while,  judging 

from  the  results  of  the  analyses  cited  by  Pumpelly^  of  the  rocks  of  several 

beds  of  the  Eagle  River  section  of  Keweenaw  Point,  titanic  acid  is  often 

present.     In  these  rocks,  then,  as  in  the  coarser  kinds  previously  described, 

the  iron-oxide  constituent  appears  to  be  a  titaniferous  magnetite. 

The  plagioclase  appears,  from  optical  measurements,  to  belong  near  oligoclase  in 
the  feldspar  series.  It  appears  in  tabular  polysynthetic  crystals,  whose  long,  narrow 
sections  are  scattered  confusedly  through  the  section,  while  the  spaces  between  the 
crystals  are  occupied  by  augite,  the  augite  in  each  space  generally  giving  the  integral 
polarization  which  indicates  a  single  individual.  In  ordinary  light  the  augite  is  dis- 
tinguishable from  the  plagioclase  by  its  very  faint,  delicate,  violet-gray  color,  and  by 
its  anastomosing  cracks.  The  sharpness  with  which  it  fills  the  interstices  between  the 
feldspar  crystals  shows  that  it  crystallized  after  them.  The  magnetite  occurs  in  small 
grains,  rarely  with  an  appearance  of  crystal  outlines.* 

'  TJ.  Pumpelly,  Geology  of  Wisconsin,  Vol.  Ill,  p.  31. 
^E.  Pumpelly,  Geology  of  Wisconsin,  Vol.  Ill,  p.  32. 
'E.  Pumpelly,  Metasomatic  Development,  pp.  285-293,  etc. 
■*£.  Pumpelly,  Metasomatic  Development,  p.  270. 


64        COPPEE-BEARING  EOCKS  OP  LAKE  SUPBEIOR. 

Almost  always,  however,  the  rock  has  undergone  changes  to  a  greater 
or  less  degree.  The  commonly  resulting  alteration-products  are,  as  Pumpelly 
has  shown,  from  the  augite  a  green  and  greenish-brown  chloritic  substance, 
often  with  red-stained  cracks;  and  from  the  feldspar  a  true  chlorite,  ante- 
cedent to  which  has  been  in  many  cases  a  change  to  prehnite.  The  follow- 
ing quotations  are  from  Pumpelly's  account  of  these  changes.-^  The  augite 
has  changed  first,  unless  there  has  been  a  little  residuary  base  present. 

Generally,  in  any  thin  section  of  the  lower  portion  of  a  bed,  a  considerable  pro- 
portion of  the  pyroxene  is  fresh,  either  throughout  whole  individuals  or  iu  parts  of 
these. 

In  thin  sections,  by  ordinary  transmitted  light,  the  pseudomorphous  product  is 
translucent,  faintly  light-green,  with  a  tinge  of  brown.  Between  crossed  nicols,  in  its 
most  characteristic  form,  it  shows  irregular  lamellar  aggregate  polarization.  It  is  very 
soft  under  the  needle,  and  is  traversed  by  red-stained  cracks,  corresponding  to  the 
irregular  fissures  in  the  parent  pyroxene,  and  it  is  by  these,  together  with  the  structure 
as  seen  in  polarized  light,  that  it  is  generally  best  distinguished  from  the  product  after 
residuary  magma-base. 

The  mineral  forming  these  pseudomorphs  is  very  probably  the  result  of  a  process 
which  has  removed  lime  and  some  iron,  magnesia,  and  silica  from  the  pyroxene  and 
brought  in  water,  and  it  is,  probably,  poor  iu  alumina. 

The  plagioclase  is  generally  the  last  constituent  that  has  been  altered  to  any 
great  extent.  The  usual  product  is  chloritic.  It  is  very  usual  to  observe  very  minute 
particles  of  a  green,  apparently  structureless  substance,  suspended  in  the  interior  of 
the  feldspar  in  such  a  manner  as  to  render  the  supposition  quite  possible  that  they  are 
due  to  an  alteration  of  inclosed  particles  of  hyaline  base.  But  an  actual  pseudomor- 
phism of  a  chlorite  after  plagioclase  is  observable  on  a  large  scale.  In  the  first  stages 
small,  tuft-shaped  particles,  consisting  of  laminae,  or  fibers,  radiating  from  a  point, 
occur  scattered  through  the  interior  of  the  feldspars,  and  these  may  wholly  occupy  a 
considerable  portion  of  a  crystal,  while  the  rest  still  shows  twin  striation  in  polarized 
light.  In  the  finished  state  no  trace  of  the  feldspar  is  visible  except  the  outlines.  The 
pseudomorph  then  shows  au  aggregate  polarization,  due  to  a  confusedly-felted  mass  of 
minute  chlorite  tufts.     The  substance  is  poorly  characterized.    *    *    *    * 

There  is  another  occurrence  of  chlorite  in  which  the  progress  of  growth  is  from 
within  outward.  Throughout  the  pseudo-amygdaloid  occur  grains  of  chloritic  sub- 
stance, which,  in  places,  reach  a  diameter  of  one-fourth  to  two-thirds  inch,  with  often 
more  or  less  irregular  outlines,  often  nearly  round  or  oval.  These  consist  of  a  dark- 
green  mineral,  with  H=2.5,  which  fuses  B.  B.  at  3 — 3.5  to  a  black  magnetic  slag.  In 
different  beds  its  texture  under  the  hand-glass  varies  from  amorphous  to  finely 
scaly.  In  thin  sections,  in  polarized  light,  the  substance  often  resembles  closely  that 
in  the  pseudomorphs  after  plagioclase,  except  that  it  shows  evident  growth  from  within 
outward.    There  is  no  defined  wall,  as  of  a  pre-existing  cavity,  but  the  chlorite  often 

'  K.  Pumpelly,  Metasomatic  Development,  pp.  270-272. 


OLIVINE-FREE  DIABASE.  65 

sends  out  long  arms,  wliicli  surround  or  penetrate  the  adjoining  primary  constituents. 
In  this  manner  the  chlorite-like  pseudomorphs  after  plagioclase  and  pyroxene,  etc.,  are 
sometimes  incorporated  into  these  pseudo-amygdules.  Very  often  one  of  these  bodies 
has  a  large  central  area  filled  with  closely -packed  radiating  spheres,  surrounded  by 
fragments  of  a  once  continuous  band  with  cross-fibrous  structure,  which  evidently 
once  formed  the  outer  limit;  outside  of  these  fragments  is  an  outer  chlorite  area, 
resembling  that  in  the  center,  and  generally  bordered  on  its  outer  limits  by  a  narrow 
cross-fibrous  band  which  adapts  itself  closely  to  the  primary  constituents.  The  greater 
number  of  these  bodies  seem  to  have  resulted  from  a  gradual  change  of  the  primary 
minerals  into  chlorite  by  progress  from  molecule  to  molecule.  At  the  first  glance  the 
structure  does  not  seem  to  confirm  this  view;  for  the  narrow  outer  band  inclosing  a 
large  central  filling  seems  to  suggest  either,  1st,  a  pre-existing  cavity,  on  the  walls 
of  which  the  thin  outer  layer  was  deposited,  as  the  older  member,  and  within  this  the 
central  filling  as  the  younger;  or,  2d,  the  replacement  by  chlorite  of  a  former  secondary 
mineral,  which  was  attacked  at  the  same  time  around  its  circumference,  producing  the 
outer  band  (shell),  and  throughout  the  interior. 

Amygdules  resulting  from  both  these  processes  are  abundant  in  the  amygdaloids 
proper;  but  they  betray  their  origin  in  a  marked  manner,  and  differ  essentially  from 
these  pseudo-amygdules. 

Whatever  the  chemical  nature  of  the  process  resulting  in  these  pseudomorphs, 
the  central  area  is  the  oldest  member,  while  the  outer  band  is  the  younger,  and  its 
cross-fibrous  structure  is  only  a  transitional  form  destined  to  be  changed  to  spheres 
with  radiating  structure.  If  we  examine  the  structure  of  the  outer  band  we  find  that 
its  line  of  contact  with  the  primary  minerals,  or  its  axial  line,  is  usually  more  or  less 
serpentine,  and  that  the  cross-fibers,  instead  of  being  parallel  to  each  other,  are  more 
nearly  perpendicular  to  the  axial  line  of  the  band,  and  form  closely -packed  groups,  in 
each  of  which  the  fibers  radiate  from  a  central  point  on  the  axial  line,  forming  minute 
hemispheres,  which  bristle  towards  the  interior  of  the  body.  The  next  stage  of  growth 
finished  the  other  half  of  each  sphere,  and  what  was  a  cross-fibered  band  becomes  now 
indistinguishable  from  the  rest  of  the  central  filling,  a  new  band  having  formed  out- 
side of  the  previous  one.  In  places  we  find  perfectly  straight  bands,  with  actually 
parallel  cross-fibers,  which  can  hardly  be  supposed  to  break  up  into  spheres;  and, 
indeed,  we  find  that  new  parallel  bands  are  formed  outside  of  these  until  the  line  of 
attack  becomes  crooked,  when  the  normal  mode  of  growth  is  re-established.  The 
remnants  of  these  straight  bands  are  then  preserved  in  the  interior  of  the  body. 

The  fine-grained  olivine-free  diabases  are  typically  developed  in  beds 
22,  69  and  87  of  Marvine's  Eagle  River  section,  Keweenaw  Point ;^  at  the 
old  Union  minein  the  Porcupine  Mountain»;  at  the  upper  falls  of  the  Montreal 
River,  where  the  exposures  are  very  large;  and  at  numbers  of  other  points 
on  the  South  Shore.  Similar  rocks,  apparently,  are  largely  developed  on 
Isle  Royale,  and  again  on  Michipicoten,  though  this  has  not  been  proved 
by  microscopic  study.    On  the  Minnesota  coast,  however,  though  occurring, 

'  Geological  Survey  of  Michigan,  Vol.  I,  part  II.,  Chapter  VII. 
5  L  S 


06 


COPPEE-BEARING  ROOKS  OF  LAKE  SUPERIOR. 


the  olivine-free  fine-grained  diabases  are  greatly  subordinate  to  the  oli- 
vinitic  diabases,  ashbed-diabases  and  diabase-porphyrites  among  the  fine- 
grained kinds,  and  to  the  non-orthoclastic  and  orthoclastic  gabbros  among 
the  coarse-grained  kinds. 
Tabulation  of  observations  upon  fine-grained  olivine-free  diabases  of  the  ^^  ordinary  type." 


Angle    between 

1 

a 
g 
g 

to 

Sf 

Place. 

1 

u 

a 

§ 
1 

P. 

<0 
fcD 

1 

Macroscopic  char- 
actors. 

Constituents  as  determined  by 
microscope,  in  order  of  age. 

maximum  ex- 
tinctions of 
adjacenthemi- 
tropic  bands  of 
theplagioclaae 
in  sections  cut 
at  random  in 
the  zone  0:  il. 

Angles  on 
opposite 
s  1  a  e  s  of 
cross-hair. 

®   . 
0.2 

0 

0 

0 

Bed    22,    Eagle 

SW. 

19 

58 

31  "W. 

Bather      fine- 

Labradorite; augite^iny&rtqmte 

34 

Elver     section, 

grained;  indefi- 

fresh, in  part  altered  to  a  green 

34 

Keweenaw 

nitely     mottled 

pseudomorph. 

36 

Point,    MicM- 

dark  green  and 

51 

ganilowerzone.' 

purple.    Sp,  gr., 
2.77. 

60 
62 

Bed    22,    Eagle 

sw. 

19 

58 

31 W. 

Like     preceding, 

Labradorite,  all  more  or  less  al- 

Eiver   section, 

with  numerous 

tered  to  prehnite,  the  change 

Keweenaw 

prehnite  pseud- 

being  at  times  complete ;  au- 

Point,  Michi- 

amyg dulea , 

gite   much   altered;  prehnite 

gan;    psead- 

averaging  il,  to 

also  forms  countless   pseud- 

amygdaloid 

1  inch  apart,  and 

amygdules  from  microscopic 

zone.2 

ahout  -^is  inch  ia 
diameter.     The 
prehnite  is  usu- 
ally of  a  fleah- 
pink  color,  with 
orwithoutvitre- 
ous  luster,  and 
is  -radiated    or 
not.  Sp.gr.,2.72. 

size  up  to  J  inch  in  diameter; 
also  chlorite  paeud-amygdules, 
and  pseudomorphs  after  plag- 
iocL-ise. 

Bed    87,    Eagle 

NE. 

30 

58 

31 -W. 

Rather      fine- 

Residuary base  altered  to  dirty 

22 

River,    Kewee- 

grained ;     dirty 

white  substances;  apatite!; 

23 

naw     Point, 

gray-green; 

oligoclase    much    altered    to 

24 

Michigan; 

powder     yields 

pseudomorphs     and     paeud- 

34 

lower  zone.^ 

some  magnetite. 
Sp.   gr.,  2.6   to 
2.73. 

Composition : 
SiOj  46.32;  AI2O3 

15.95;  FezOs  .86; 

FeO  8.92;  MnO 

amygdules  of  chlorite;   mag- 
netite; auglte  highly  fisaured 

36 

. 

and  more  or  leas  thoroughly 
changed    to     chloritic    sub- 
atauce,  with  which  are  asso- 
ciated red   and   black   iron 

.89;  CuO  10.28; 

stains,  which   are  -probably 

• 

MgO  4.08;  TiOa 

the   source   of  some  of  the 

2.78;  K2O  1.23; 

magnetite. 

Na20  3.56;  H2O 

1 

3.25  =  100.12. 

iPnmpelly,  Metasomatio  Development,  p.  281. 


2I6i(e.,  p.282. 


^Ibid.,  p.  283. 


OLIVINE-FEEE  DIABASE. 


67 


Tabulation  of  observations  upon  fine-grained  olivine-free  diabases  of  the  "  ordinary  type^^ — 

Continued. 


Angle    between 

a 
1 

Place. 

g 

is 

1 
1 

a 

i 
1 

a 

a 

1 

Macroscopic  char- 
acter. 

Constituents  as  determined  by 
microscope,  in  order  of  age. 

maximum  ex- 
tinctions of 
adjacent  homi- 
tropic  bands  of 
the  plagiodase 
in  sections  cut 
at  random  in 
the  zone  0;  ii. 

Angles  on 
opposite 
sides  of 
cross-hair 

Bed    87,    Eagle 

NE. 

30 

5S 

31 W. 

Coarser     grained 

Oligoclase,  very  much  altered; 

o 

o 

o 
26 

Eiver    section, 

than  lower  zone. 

augite,   wholly  changed   to 

29 

Keweenaw 

with  numerous 

green  pseudomorph  with  iron- 

34 

Point,    Michi- 

pink   feldspars 

stained  cracks;  large  pseud- 

35 

gan;     middle 

and      large 

amygdules   with   dark-green 

36 

zone.i 

bunches  of  chlo- 
rite. Sp.gr.,  2.73 
to  2.75. 

chlorite  (delessite). 

- 

Bed    69,    Eagle 

NE. 

30 

58 

31 W. 

Fine-grained; 

Plagiodase,  largely  altered  to 

Eiver  section.* 

dirty-green.  Sp. 
gr.,  2.87  to  2.95. 
Powder  yields  a 
little  magnetite. 

chlorite;    magnetite;    aiigite, 
in  part  fresh,  in  part  changed 
to  greenish  substance. 

1966... 

Bohemian  Bange, 

9 

56 

31 W. 

Fine-grained; 

Altered    magma;       oligoclase ; 

7 

4 

11 

Keweenaw 

darkbrownmot- 

monetae;  augite,  only  rarely 

13 

17 

30 

Point,  Michi. 

tled  with  green ; 

fresh,  mostly  altered  to  green 

gan. 

abundant   chlo- 
rite pseud-amyg- 
dules. 

substance,  with  red  and  black 
chlorite  pseud-amygdules. 

2526... 

Union  mine.  Por- 

NW. 

27 

51 

42  W. 

Fine-grained; 

Altered   magma;    labradarite; 

19 

16 

35 

cupine    Mount- 

dark   purplish- 

augite  wholly  altered  to  green 

18 

21 

39 

ains,  Michigan; 

brown;  abundant 

and    brown    pseudomorphs ; 

19 

20 

39 

l,950north,  1,750 

large      calcite 

calcite    and    chlorite    pseud- 

west. 

pseud-amy  g- 
dules. 

amygdules  abundant. 

2529... 

Rock  underlying 
Union      mine 

SW. 

22 

61 

42  W. 

Altered  tnagma ;  oligoclase;  au- 
gite, in  part  fresh,  but  mostly 

3 
5 

8 

XI 

7 

12 

amygdaloid. 

altered   to   green    substance 

Porcupine 

with   red    bands  f    abundant 

Mountains, 

pseud-amygdaloidal  chlorite. 

Michigan ;     200 

north,  1,750  west. 

22  W. 

Ashland  County, 
Wisconsin. 

34 

46 

IE. 

Pine-grained; 
dark  brown  mot- 
tled with  green- 
ish black;  highly 
weathered.    Sp. 
gr.,2.85. 

Oligoclase;    magnetite;    augite, 
wholly  altered;  pseud-amyg- 
dules  of  stellated  chlorite  and 
calcite;     whole    rock    much 
charged  with /crric  oxide. 

26  W. 

Montreal    Eiver, 

21 

47 

IE. 

Anorthite;  augite;  chlorite  ani 

"Wisconsin,     at 

calcite  pseud-amygdules. 

the  crossing  of 

the     Flambeau 

trail. 

'  E.  Pumpelly,  "Metasomatic  Development,"  p.  284. 


'Ibid.,  p.  288. 


68 


COPPEE-BBARING  EOGKS  OP  LAKE  SDPEBIOE. 


Tabulation  of  observations  upon  fine  grained  olivine-free  diabases  of  the  '^ordinary  type" — 

Continued. 


a 

a 
a 

1 

Place. 

g 
1 

9 

1 

o 
to 
m 

1 

§1 

w  ■ 

Macroscopic  char- 
acters. 

Constituents  as  determined  by 
microscope,  in  order  of  age. 

Angle     between 
maximum    ex- 
tinction b  of 
adjacent  hemi- 
tropic  bands  of 
the  plagioclase 
in  sections  cut 
at  random    in 
the  zone  0 :  ii. 

Angles  on 
opposite 
sides  of 
cross-hair. 

i-H  d 

40  yf. 

63  "W. 

49  "W. 
404  S.. 
8  SW. 
12  SW 

Montreal    Hiver, 
Wisconsin. 

AshLind  County, 
Wisconsin. 

Ashland  County, 
Wisconsin, 
Flambeau  trail. 

Moose     Creek, 
Douglas  County, 
Wisconsin. 

Copper  Creek 
mine,  Douglas 
County,  Wis- 
consin. 

Fond  du  Lac  mine, 
DouglasCounty, 
Wisconsin. 

SB. 

SE. 
cor. 

NE. 

20 
16 

19 
2 

IS 
8 

47 
46 

46 

44 
47 

47 

IE. 
2E. 

2E. 

13  W. 

14  W. 
14  W. 

Sp.gr., 2.81 

Rather      fine- 
grained;  dark- 
greenish.      S  p. 
gr.,  2.90. 

Rather  fiue- 
grained;  green- 
ish-gray to  dark 
green. 

Medium -grained; 
dark  greenish- 
black.  Sp.  gr., 
2.87. 

Fine-grained; 
dark  brownish- 
gray.  Sp.  gr., 
2.84. 

Fine-grained; 
speckled  brown, 
light  and  dark 
green. 

Zabradorite;  augite  largely  al- 
tered to  green  and  brown  sub- 
stance; chlorite  pseud -amyg- 
dules. 

Oligoclase  largely  altered  to  chlo- 
rite; Tnagnetite  in  rod-like 
forms,  apparently  filling 
cracks;  augite  abundant  and 
f ?esh ;  chlorite  pseud  -  amyg- 
dules. 

Oligoclase,  partly  altered  to  chlo- 
rite; magnetite,  as  in  63  "W.; 
augite  quite  fresh;  chlorite 
pseu  d-amy  gdules. 

Oligoclase;  magnetite;  augite, 
largely  altered  to  green  sub- 
stance. 

Plagioclase  in  matrix  and  in 
porphyritic  crystals;  augite, 
wholly  altered  to  green  sub- 
stance. 

Oligoclase;  magnetite  very 
abundant;  augite,  largely 
fresh,  but  partly  altered  to  a 
reddish  -  brown  transparent 
substance ;  pseud-amygdules 
of  epidoie,  chlorite  and  calcite  ; 
TnetaUic  copper. 

0 

° 

0 

Olivinitic  fine-grained  diabases,  including  Pumpelly's  melaphyrs. — To  these 
rocks,  which  grade  through  coarser  and  coarser  kinds  directly  into  the 
coarse-grained  olivinitic  gabbros  already  described,  Pumpelly  has  given  the 
name  of  melaphyr  (following  the  nomenclature  of  Rosenbusch),  because 
of  the  presence,  in  those  examined  by  him  from  Keweenaw  Point,  of  a  small 
amount  of  altered  residuary  magma.  Precisely  the  same  rocks  are,  how- 
ever, found  without  the  residuary  base,  which  is  at  best  but  a  very  unim- 
portant ingredient,  so  that  I  have  preferred  to  call  them  here  all  by  the 


F  LAKE  SUPERIOR   PL 


Srcrm  north  coasi  of  Z<Jcc  Sztperz^or,  S.H.'ij  <yec.f,Tj-i  J^.ii.7f'7^i.7Z7h  OrcL^ariflifAi       O ca2t.*o  cixame(xT3 


>n  finegt' 


sm^^s^ 


UNITED  STATES   GEOLOGICAL  SURVEY 


COPPER-BEARING  ROCKS  OF  LAKE  SUPERIOR   PL,  IX 


OLIVINE  -  DIABASE  OR.  IvIELAPITt'R 


N. 


OLIVINITIC  DIABASE— MELAPHYR.  69 

above  general  title.     The  term  melaphyr  is,  however,  used  in  the  detailed 

descriptions  of  subsequent  chapters  whenever  the  residuary  base  appeared 

to  be  present. 

Although  grading  through  coarser  kinds  into  the  coarse  olivine-gab- 

bros,  the  fine-grained  rocks  here  considered  deserve  a  place  by  themselves. 

The  gradation  into  the  coai'ser  kinds  has  never  been  observed  in  any  one 

bed,  and  they  are  very  stronglj^  marked  by  their  external  characteristics 

both  in  the  flftsh  and  altered  states.     They  have  commonly  undergone  great 

alteration,  the  amount  of  change  lessening  rapidly  as  the  rock  becomes 

coarser.     As  a  type  of  the  fresh  state  of  this  rock  Pumpelly  has  selected 

"The  Greenstone"  of  Keweenaw  Point,  which  is  one  of  the  transition 

phases  toward  the  coarse  gabbros.     The  following  is  his  description  of  these 

rocks  as  developed  on  the  South  Shore.^   It  applies  equally  well  to  the  olivi- 

nitic  kinds  of  the  North  Shore,  where  "luster-mottled"  fine-grained  rocks  are 

very  abundant. 

In  its  fresh  state  it  is  dark  green,  or  greenish  black,  iinely  crystalline,  very  com- 
pact, hard  and  brittle,  and  breaks  with  an  uneven  to  semi-conchoidal  fracture.  The 
powder  of  the  rock  yields  to  the  magnet  a  beard  of  magnetite.  The  specific  gravity  is 
2.90  to  2.95.  It  is  an  important  characteristic  of  this  rock  that  its  freshly  fractured 
surface  is  mainly  occupied  by  spots  one-sixteenth  to  three-fourths  of  an  inch  in  diam- 
eter, each  of  which  reflects  the  light  with  a  satin-like  sheen.  The  reflection  is  not 
carried  to  the  eye  from  all  the  spots  at  once.  It  is  generally  necessary  to  change  the 
position  of  the  specimen  many  times  to  observe  the  different  reflections.  Aside  from 
this  sheen  there  is  nothing,  either  in  difference  of  color  or  texture,  visible  to  the  naked 
eye  to  betray  the  presence  of  these  spots,  which  might  be  called  lusfcer-mottlings.  To 
the  naked  eye,  this  phenomenon  suggests,  at  once,  interrupted  cleavage  of  large  indi- 
viduals of  one  of  the  constituents,  as  the  cause ;  but  under  a  strong  hand  glass  these 
reflecting  surfaces  show  the  same  granular  texture  and  character  as  the  rest  of  the  rock ; 
and  it  is  only  when  examined  under  the  microscope,  with  an  objective  of  low  power 
and  in  polarized  light,  that  the  appearance  to  the  unaided  eye  is  corroborated.  We 
here  find  the  cause  in  the  fact  that  each  spot  is  the  cross-fracture  or  cleavage  of  a 
crystal  of  pyroxene,  which,  in  crystallizing,  has  inclosed  hundreds  of  feldspar  crystals. 
The  weathered  surface  is  rusty  gray,  scarcely  one-fiftieth  of  an  inch  thick;  but  it  is 
covered  with  knobs,  which  are  due  to  the  more  rapid  destruction  of  the  materials  be- 
tween the  pyroxene  individuals.  Examining  thin  sections  under  the  microscope,  we 
find  the  constituents  to  be  plagioclase,  pyroxene,  olivine,  and  the  alteration -product  of 
the  latter,  as  well  as  magnetite,  and  an  unindividualized  substance,  both  fresh  and 
altered,  occupying  interstices.  In  thin  sections  the  plagioclase  is  seen  to  exist  in 
very  sharply  defined,  and  fresh,  thin,  tabular  crystals,  .001  to  .002  inch  thick,  and  .01 

'  Geology  of  Wisconsin,  Vol.  Ill,  p.  33. 


70        COPPEE-BEAEING  EOOKS  OF  LAKE  SUPBEIOE. 

inch  aud  less  long.  It  coutains  scattering  interpositions  of  an  opaque  black  sub- 
stance, and  minute  brown  particles,  whicli  may  be,  or  have  been,  glass.  The  crystals 
of  plagioclase  have  predetermined  the  contours  of  all  the  other  constituents,  except 
the  olivine,  which  crystallized  first.  The  predominating  feldspar  is  near  anorthite,  as 
determined  by  the  angle  between  the  principal  sections.  Occasional  exceptionally 
large  individuals,  evidently  cut  in  the  plane  il,  have  their  principal  sections  at  an 
angle  of  23°,  with  the  edge  0 :  n,  which  would  indicate  albite  or  labradorite.  The 
augite  is  very  fresh  and  transparent,  almost  colorless  in  the  thin  section,  but  with  a 
tendency  to  purple-gray.  An  imperfect  cleavage  is  indicated  by  somewhat  irregular 
parallel  fractures.  It  fills  the  interstices  between  the  closely-packed  individuals  of 
feldspar  in  such  a  manner  that  a  single  pyroxene  crystal  incloses  many  hundreds  of 
these,  while  its  crystalline  integrity  is  shown  by  the  uniform  color  in  polarized  light, 
and  by  the  arrangement  of  the  cleavage  cracks  throughout  the  area  of  the  augite 
individual.  It  is  a  remarkable  fact  that,  while  these  large  individuals  of  pyroxene 
contain  thousands  of  feldspar  crystals,  they  inclose  only  very  few  of  olivine  or  of  mag- 
netite. These  minerals,  together  with  the  unindividualized  substance,  are  crowded 
into  the  spaces  between  the  pyroxenes.  In  this  intermediate  space,  which  surrounds 
the  pyroxene  individuals  with  a  continuous  net-work,  we  find,  also,  a  few  small  py- 
roxenes, just  as  isolated  grains  of  olivine  occur  in  the  pyroxene  areas.  A  careful 
examination  of  this  occurrence  will,  I  think,  convince  the  observer  that,  at  the  time 
the  pyroxene  crystallized,  both  the  olivine  and  feldspar  crystals,  and  apparently  the 
magnetite,  were  already  individualized;  for  where  we  find  any  of  these  in  contact 
with  the  augite  we  find  that  the  latter  has  adapted  itself  to  the  already  defined  con- 
tours of  the  others.  While  the  augite  inclosed  the  feldspar  crystals  with  ease,  it 
crowded  the  other  constituents  almost  wholly  into  the  surrounding  spaces — a  process 
which  was  facilitated  by  the  presence  of  the  then  fluid  unindividualized  substance. 
The  magnetite  is  in  irregular-shaped  bodies,  which  mold  themselves  sharply  around 
the  contours  of  the  feldspar  and  olivine.  The  olivine  is  abundant  in  grains  and 
roughly-outlined  crystals,  but,  as  a  rule,  however  fresh  the  melaphyr  may  otherwise 
be,  the  olivine  is  partly  or  wholly  altered. 

The  following  is  Pumpelly's  more  detailed  account  of  the  internal 
changes  which  these  luster-mottled  rocks  have  at  times  undergone.^  The 
residuary  base  and  the  olivine  are  usually  wholly  altered,  while  the  augite 
and  plagioclase  are  much  more  commonly  fresh  than  in  the  olivine-free 
kinds. 

The  first  and  ever-present  stage  of  alteration  is  caused  by  the  change  of  the 
residuary  magma-base  which  fills  the  interstices  between  the  crystalline  constituents, 
and  in  places  penetrates  into  or  is  inclosed  in  the  interior  of  these.  The  physical  and 
chemical  character  of  this  seems  to  have  predisposed  it  to  an  easy  change.  It  is  now, 
as  a  rule,  when  seen  in  thin  sections,  a  darker  or  lighter  olive-green  substance,  and 
v.ery  soft  under  the  needle  (hardness  not  over  2.5).    In  polarized  light  it  exhibits  a 

'  "Metasomatio  Development  of  tie  Copper-bearing  Rocks  of  Lake  Superior."  Proo.  Am.  Acad. 
Sci.,  Vol.  XIII,  pp.  269-270. 


OLIVmiTIC  DIABASE— MELAPHYE.  71 

fibrous  aggregate  polarization,  and  shows  well  its  structure,  which  is  short,  fibrous, 
converging  towards  the  center.  The  central  portion  shows  sometimes  little  or  no 
double  refraction,  but  more  generally  it  is  filled  with  very  minute  polarizing  spheres 
formed  of  radiating  fibers.  With  one  nicol  this  substance  shows  only  absorption  for 
intensity.  The  contours  are  generally  sharply  defined  by  the  feldspar  and  pyroxene 
crystals,  and  the  result  is  usually  a  more  or  less  wedge-shaped  form. 

The  next  stei>  has  been  the  change  of  the  chrysolite.  In  the  so-called  greenstone 
this  has  been  only  partial ;  but  generally  in  the  chrysolite-bearing  beds  it  is  complete. 
The  result  in  thin  sections  is  a  faiutly  green  substance,  soft  under  the  needle,  and  sur- 
rounded, within  the  original  contours  of  the  crystal,  by  a  more  or  less  opaque  deposit 
of  iron  oxide,  which  also  traversed  it  in  fissures.  The  green  substance  shows  by  a 
well-defined  cleavage  in  one  direction  that  it  is  in  thin  laminae.  Between  crossed  nicols 
these  laminse  have  an  appearance  of  twin  structure,  polarizing  the  light  in  alternate 
lines  of  brilliant  red  and  green.  The  whole  pseudomorph  becomes  dark  when  the 
cleavage  is  i^arallel  to  a  nicol  plane;  and  some  individuals,  probably  cut  parallel  to  the 
cleavage,  remain  darii  through  a  revolution  of  the  stage.  The  substance  is,  therefore, 
very  ijrobably  uniaxial.  It  has  very  appreciable  absorption  for  intensity  and  a  very 
feeble  one  for  color. 


The  green  substances  which  I  have  described  so  unsatisfactorily  as  being  undoubt- 
edly pseudomorphous  after  residuary-base,  pyroxene,  and  plagioclase,  have  in  them- 
selves no  physical  properties,  recognizable  under  the  microscope,  which  are  sufiSciently 
persistent  to  invariably  characterize  them  respectively.  Often  the  best  means  of  dis- 
tinguishing between  them  is  in  the  internal  structure  of  the  aggregate,  since  this  is 
intimately  connected  with  certain  physical  characteristics  of  the  original  mineral  which 
predetermined  the  mode  of  growth  and  gradual  arrangement  of  the  secondary  product. 
In  other  instances,  the  presence  or  absence  of  cracks  stained  with  iron  oxide  are  char- 
acteristic. 

Thus  the  change  of  the  residuary  base  has  resulted  in  a  tendency  to  form  bauds 
parallel  to  the  contour  of  the  area  in  a  manner  that  indicates  a  gradual  progress  along 
concentric  shells  from  the  circumference  inward  or  the  reverse;  while  in  the  feldspar 
the  growth  appears  to  have  begun  without  any  regularity  throughout  the  cleavage  and 
twinning  planes  of  the  crystals.  The  pseudomorphs  after  pyroxene,  are  almost  invari- 
ably defined  by  the  red-stained  cracks  and  slight  mixtures  of  brown  and  green. 

The  "luster-mottling"  described  by  Pumpelly  as  due  to  the  crowding 
of  the  magnetite  and  olivine  into  the  interspaces  of  the  augites  is  so  pro- 
nounced and  constant  a  characteristic  of  these  rocks  that,  even  when  they 
are  too  fine  and  too  much  altered  to  show  it  in  the  hand  specimen,  a  single 
glance  at  the  thin  section  with  the  unaided  eye  is  always  sufficient  to 
reveal  it. 

As  typical  instances  of  these  fine-grained  olivinitic  rocks  may  be  men- 
tioned "The  Greenstone"  of  Keweenaw  Point;  the  rock  of  the  north  shore 


72        COPPEE-BEARmG  EOCKS  OP  LAKE  SUPEEIOE, 

of  Bgte  Grise  Bay,  Keneenaw  Point,  S  W.  I,  Sec.  27,  T.  58,  R.  28  W.;  that 
of  the  S.  W.  ^,  Sec.  17,  T.  51,  E,  42  W.,  Porcupine  Mountains,  Michigan; 
that  of  the  bed  of  Moose  Creek,  Douglas  County,  Wisconsin,  N.  W.  ^,  Sec. 
2,  T.  44,  R.  13  W.;  that  of  the  falls  of  French  River,  Minnesota,  N.  W.^, 
Sec.  17,  T.  51,  R.  12  W.;  that  of  the  coast  of  Lake  Superior  between  the 
mouths  of  French  and  Sucker  Rivers,  Minnesota,  N.  E.  ^,  Sec.  16,  T.  51, 
R.  42  W. ;  that  of  the  south  point  of  Agate  Bay  on  the  Minnesota  coast ; 
that  of  Caribou  or  Black  Point  on  the  same  coast,  and  that  of  the  copper 
mines  on  Michipicoten  Island. 


OLIVESriTIC  DIABASE— MELAPHTE. 


73 


Tabulation  of  the  results  of  a  microscopic  study  of  finegrained  olivinitic  diabases  {includ- 
ing melaphyrs). 


Place. 


O* 


Macroscopic  char- 
acters. 


CoBstitnents  as  determined  by 
microscope,  in  order  of  age. 


Angle  between 
maxim  am  ex- 
tinctions of 
adjacent  heml- 
tropic  bands  of 
the  pla^oclase 
in  sectiona  cut 
at  random  in 
the  zone  O:  it. 


Angles  on 
0  p  p  o  site 
sides  of 
cross-hair. 


'The  Greenstone," 
Eagle  Biver,  Ke- 
weenaw Point, 
MichigaD.i 


SE. 


19 


31  "W. 


Lower  zone  Mar- 
vine's  bed  64;== 
Eagle  Eiver, 
Keweenaw 
Point,  Michi- 
gan.' 


NW. 


19 


58 


31  "W. 


M  e  d  i  nm-grained 
to  fine-grained; 
very  compact ; 
hard  and  brit> 
t]e;  uneven  to 
semi-conchoidal 
fractnre ;  dark- 
green  to  green- 
ish-black; "lus- 
ter-mottlings" 
i  to  I  inch  in  di- 
ameter ;  occa- 
sional pinkish 
porphyritic  feld- 
spars i  inch  in 
length.  Sp.gr., 
2.96. 


Fine-grained  to 
aphanitio;  dark- 
green  to  nearly 
black.  SiOs,  47.74. 
Sp.gr..  2.69. 


Clivine,  in  "integral  and  aggre- 
gated grains,  and,  very  rarely 
in  crystals  with  recognizable 
though  rounded  contours," 
partly  fresh,  partly  altered 
along  fissures  to  a  "very  pale- 
green  substance,  aometimes 
tinged  with  brown";  anorthite^ 
*'  in  very  sharply  defined  and 
fresh,  thin,  tabular  crystals 
.001  to  .002  inch  thick  and  .01 
inch  and  less  long,"  holding 
"scattering  interpositions  of 
an  opaque  black  substance, 
which  may  be,  or  have  been, 
glass";  magnetite,  in  irreg- 
ular areas,  moulded  "  sharply 
around  the  contours  of  the 
feldspar  and  olivine";  augite, 
very  fresh  and  transparent, 
relatively  to  the  other  ingredi- 
ents; in  very  large  individ- 
uals, which  often  contain 
thousands  of  feldspars,  but 
have  crowded  almost  all  of 
the  olivine  and  angite  into 
the  interstices ;  residuary 
magma,  largely  altered  to 
greenish,  feebly  polarizing 
substance,  filling  interstices 
between  the  feldspars  not  oc- 
cupied by  Bugite  and  pene- 
trating the  feldspars  in  cracks. 

Olivine,  wholly  altered  to  a  soft, 
dark-green  mineral;  anorthite, 
in  small  crystals;  augite, 
fresh,  subordinate;  titanifer- 
<ma  magnetite. 


'  K.  Pumpelly,  in  Metasomatio  Development,  p.  261. 

»  Greology  of  Michigan,  Vol.  I,  Part  II. 

*S.  Pompelly,  Metasomatio  Development,  pp.  291-293. 


74 


COPPER-BBAEmG  BOOKS  OP  LAKE  SUPEEIOE. 


Tabulation  of  the  results  of  a  microscopic  study  of  fine-grained  olivinitic  diabases  (include 

ing  melaphyrs) — Continued. 


Angle    between 

i 

g 

a 
•3 

p. 

CD 

Place. 

g 

•43 

s 

1 

d 
& 

p< 

I 

Macroscopic  char- 
acters. 

Constituents  as  determined  by 
microscope,  in  order  of  age. 

maximum    ex- 
tinctions   of 
atyacent  hemi- 
tropic  bands  of 
the  plagioclase 
in  sections  cut 
.at  random    in 
the  zone  0 :  ti. 

Angles  on 
opposite 
sides  of 
cross-hair. 

■ 

0 

0 

0 

1876... 

ITorth  Shore  Bfite 

sw. 

27 

68 

28W. 

Fine-grained, 

Olivine,    very    abundant,    and 

32 

33 

05 

Griae  Bay,  Ke- 

greenish-bLack ; 

wholly  altered  to  a  green  sub- 

30 

31 

61 

■weenaw  Point, 

greasy;    "lus- 

stance with  brown  and  red 

21 

23 

44 

Michigan,  near 

ter-mottled." 

stripes  and  crowded  with  the 

junction     with 

magnetite  into  the  Interspaces 

Eastern     Sand- 

of theangites;  anorthite,  fresh, 

stone. 

tabular,    small ;     magnetite  ; 
aitgite    in  the  characteristic 
areas. 

1884... 

Hillside  above  old 
smelting  works, 
north  side  Lac 
La    Belle,    Ke- 
weenaw Point, 
Michigan. 

irw. 

31 

58 

29  W. 

M  e  dium-grained, 
dark  brownish- 
gray;   much- 
weathered; 
"luster-mot- 
tlings,"  not  pro- 
nounced. 

Olivine,  wholly  altered  to  green 
substance   with    dark-brown 
streaks  and  patches,  crowded 
between  the  augites ;  anorth- 
ite; magnetite,  not  abundant; 
augite,    quite   fresh,    in    the 
usual  large  areas,  including 
many  feldspars. 

1919... 

Eoad  from  Lac  La 
Belle   to    Dela- 
ware Mine,  Ke- 
weenaw  Point, 
Michigan. 

sw. 

30 

58 

29  W. 

Black     to     dark 
brownish-gray; 
fine-grained; 
"luster-mot- 
tled." 

Olivine,  in  rounded  grains,  very 
abundant,  and   crowded  be- 
tween   the    angites,    whollj- 
altered    to    a    greenish   and 
brownish  substance;  the  large 
amount      of      ochre-staining 
seeming  to  indicate  an  olivine 
rich  in  iron;  anorthite;  mag- 
netite; augite,  very  fresh,  in 
the  usual  large  areas. 

22 

26 

48 

2597... 

Porcupine  Mount- 

sw. 

17 

51 

42  W. 

Fine-grained, 

Olivine,  wholly  altered  to  green- 

34 

32 

66 

ains,  Michigan. 

brownish-gray. 

ish  substance  with  red  bands ; 
anorthite;  tnagnetite;  augite, 
predominant,    mostly    fresh, 
but  partly  altered  to  viridite, 
in    the    usual    large    areas ; 
chlorite  pseud-.amygdules. 

32 
32 

31 

29 

63 
61 

81  W.. 

Potato  Eiver, 
Ashland  Coun- 
ty, Wisconsin. 

17 

4S 

IW. 

Ver.y  fine-grained, 
luster-mottlings 
of  minute  green- 
ish   spots   one- 
twentieth   inch 
in   diameter, 
surrounded    by 
brown  altera- 
tion-product. 

Olivine,  wholly  altered:  plagio- 
clase,  fresh ;  augite,  fresh. 

•B.  PnmpeUy,  Geology  of  Wisconsin,  Vol.  lH,  pp.  38,  41,  42. 


OLIVINITIC  DIABASE— MELAPHYR. 


75 


Tabulation  of  the  results  of  a  microscopic  study  of  fine-grained  olivinitio  diabases  {in- 
cluding melaphyrs) — Continued. 


a 

I 

Place. 

1 

d 

.S 

1 

03 

o 
H 

ID 
bo 

g 

Macroscopic  char- 
acters. 

Constituents  as  determined  by 
microscope,  in  order  of  age. 

Angle    between 
maximum    ex- 
tinctions of 
adjacent  hemi- 
tropic  bands  of 
the  plagioclase 
in  sections  cut 
at    random  in 
the  zone  0;  ii. 

Angles  on 
opposite 
sides  of 
cross-hair. 

rB  p 

0 

0 

O 

365  S... 

Totogatig  district, 
Douglas  Coonty, 
Wisconsin. 

12 

44 

9W. 

Fine-grained, 
compact,    red- 
dish-brown, 
small    .-ind    in- 
conspicuous 
"luster-mot- 
Uings." 

Olivine^  wholly  altered  to  red 
product ;    labradorite  or    an- 
orthite;  very  little  ntasneiite; 
(mgite,   in   large   individuals 
enveloping  the  feldspar  crys- 
tals. 

31 
35 

36 
31 

67 
66 

400  S... 

Moose  Greek, 
DonglasCoimty, 
"Wisconsin. 

irw. 

2 

44 

13  "W. 

Olivine,     abundant,    altered 
wholly  to  a  soft  green  sub- 
stance, crowded  as  usual  into 

37 

37 

74 

22 

23 

45 

the  interspaces  of  the  angites ; 
labradoHUOT:  anorthite ;  mag- 

netite, chiefly  in  the  augite  in- 

terspaces ;  augite,  in  large  in- 
dividuals, including  countless 
small   crystals   of    feldspar; 
pseud-.amygdules  of  chloritic 
substance.      Eesembles     ex- 
actly the  rock  of  "The  Green- 
stone "  of  Keweenaw  Point. 

427 

Hear  the    upper 

SW. 

15 

43 

14  W. 

Fine-grained, 

OliviTie,  altered  to  a  green  and 

30 

37 

67 

Saint  Croix 
Eiver,  Douglas 
County,  'Wis- 

bright  red,  al- 
tered.   Sp.  gr., 
2.30. 

red    product;    plagioclase, 
fresh ;  augite,  fresh  ;  chlorite 
pseud-amygdules. 

34 
35 

32 
39 

66 

74 

consin. 

31 

Bed    of    Lester 
Eiver,     Minne- 
sota,  north 
shore  Lake  Su- 
perior. 

NW. 
cor. 

4 

50 

13  TV. 

Medium  -  grained, 
crystalline, 
bright  red,  mot- 
tled with  green 
and  black. 

Olivvne,   abundant,    wholly   al- 
tered to  dark  green  substance, 
crossed  by  deep  brown  and 
red,  to  which  alteration  the 
color   of    the   rock   is    due ; 
plagioclase  partly  fresh,  part- 
ly clouded;  augite,  fresh,  in 
large   areas  including  many 
feldspars ;     pseud-amygdules 
of  chlorite. 

563 

Korth  shore  Lake 

NW. 

17 

61 

12W. 

Fine-grained, 

Olivine,  altered  to  green  sub- 

25 

25 

50 

Superior,     falls 

rough  fracture, 

stance,  and  crowded  into  the 

29 

32 

61 

of  French  River, 
Minnesota. 

dark    purplish- 
brown,  "Instor- 
mottlings"   not 
pronounced. 

augite  interspaces  with  the 
magnetite ;  labradorite ,-  mag- 
netite;   augite,  in  the  usual 
large  areas. 

28 

28 

66 

76 


COPPEE-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 


Tabulation  of  the  results  of  a  microscopie  stiicly  of  fine-grained  olivinitic  diabases  (includ- 
ing melaphyrs) — Continued. 


a 

Place. 

a 

.2 

p. 
la 

o 

Macroscopic  char* 
acters. 

Constituents  as  determined  by 
microscope,  in  order  of  age. 

Angle    between 
maximum    ex- 
tinctions  of 
adjacent  hemi- 
tropic  bands  of 
the  plagioclase 
in  sections  cut 
at  random   in 
tile  zone  0;  it. 

g 

a 

TO 

1 

a. 
.2 

1 

Angles  on 
opposite 
sides  of 
cross-hair. 

•Si 

o 

o 

o 

568.... 

North  shore  Lake 
Superior,    be. 
t -we  en       the 
mouths  of  French 
and    Sucker 
Kivera,  Minne- 
sota. 

NE. 

16 

51 

12W. 

Kne-gradned,  dark 
gray,  very  pro- 
nounced lumpy 
fracture  result- 
ing    from     the 
deeper  weather- 
ing of  the  augite 
interspaces. 

Olivme,  wholly  altered  to  green 
and  brown  substance,  crowd- 
ed into  the  interspaces  with 
the   magnetite;    plagioclase; 
magnetite;     augite,    fresh, 
greatly    predominating    in 
large  areas. 

605.... 

North  shore  Lake 

NE. 

29 

51 

IIW. 

Greenish-grayflne 

Olivine,  wholly  altered  to  green 

35 

31 

66 

Superior,  one 

grained,  minute- 

substance,   abundant;    anor- 

32 

37 

69 

mile      below 

ly  "luster  moi- 

thite ;  magnetite ;  augite,  very 

month  of  Knife 

tled." 

fresh,  in  large  areas,  includ- 

Kiver,    Minne- 

ing great  numbers  of  the  tab- 

sota. 

' 

ular  feldspars.    A  very  typi- 
cally developed  "luster-mot- 
tled" rock. 

628.... 

North  shore  Lake 
Superior,  south 
point  of  Agate 
Bay,  Minnesota. 

NE. 

11 

52 

11  "W. 

Fine-grained;  dark 
pnrplish-gray; 
"lustre  -  mot- 
tling"  not  pro- 
nounced. 

Olivine,  whoUy  altered  to  green 
substance    with   red   bands; 
plagioclase  too  much  altered 
for  measurement ;  magnetite  ; 
augite,    fresh,    predominant, 
in  the  usual  large  areas ;  chlo- 
rite psend-amygdules. 

660.... 

North  shore  Lake 

N"W. 

22 

S3 

low. 

Very  flne-grauied. 

Olivine,  wholly  altered  to  brown 

34 

36 

70 

Superior,-  from  5 

greenish -black. 

substance;  anorthite;  magne- 

33 

32 

65 

feet  dike,  below 

semi-conchoidal 

tite;  augite,  in  the  usual  large 

32 

32 

64 

mouth  of  Silver 

fracture. 

areas.     An  excessively  fine- 

31 

31 

62 

Creek,    Minne- 

grained rock. 

sota. 

688.... 

North  shore  Lake 

NE. 

27 

64 

9W. 

Very  fine-grained ; 

Olivine,  wholly  altered  to  red 

27 

31 

58 

Superior,  J  of  a 

rough  fracture; 

substance,  as   usual  in    the 

27 

29 

56 

mUe  above   ^  le 

light  brown. 

interspaces  with  the  magne- 

26 

33 

59 

mouth  of  Groose- 

tite;  labradorite:  magnetite.- 

berry   River, 

amgite,  very  fresh,  and  in  the 

Minnesota. 

usual  relatively  large  areas, 
including    many     feldspars ; 
but  in  this  rock  the  "luster- 
mottling"    is   wholly  micro- 
scopic. 

696.... 

North  shore  Lake 

NE. 

23 

51 

9  TV. 

Fine. grained; 

Olivine,  in  very  abundant  and 

31 

37 

68 

Superior,   2i 

altered ;  mottled 

unusually   minute   particles. 

32 

35 

67 

miles  above  the 

red   and  light 

largely  altered  to  green  and 

34 

27 

61 

mouth  of  Split 

gray. 

red   substances;    magnetite; 

33 

32 

65 

Rock  River, 

cmorthite;  augite,  very  fresh 

29 

33 

62 

1 

Minnesota. 

in  the  usual  areas. 

31 

33 

64 

UNITED  STATES  GEOLOGICAL  SURVEY 

COPPER-BEARING 

ROCKS 

OF   LAKE   SUPERIOR    PL^  X 

J^^^m^^ff\^ 

H 

1 

mm 

■ 

\ 

HB^^  ^^S^flR^Ll:^^^ 

fi  j! 

i 

1 

'''^Inflrill 

Jr. 

^ 

tcW^ 

^^x^ 

i[||ifc... 

^ 

V 

1 

k^l 

^^iB 

E^ 

1 

1 

■    "           \     ■ 

\  ■ 

^^^^1 
^    ^^^^^^^1 

:L., ■..:,?i 

'Mm 

i 

DMBASE  PORPHYRITES  AND  ASHBED  -DIABASE. 


A.ll..,-ii*>',.  vi'li-l 


^.    . y^JJ^O^^CH  {i ».', '  «U       f^    U 


;n.o;i-paiBT)t,  j-inrl  n.npftn.ra  at-tiuaes  to  be 


iivkUjj^^lizedshbstattce^^l'hese  rockn 
ueeriaw  Serias,  and  3^6  tilway^  very 

>t'  thestV  rocks,  as  already  stat«!t 
shbed-cllalbase,  t^lie^: 


,base  OFli  flow,  wli 
'  ashWl  of  K 
if  rocks,  Pump 
^^  gesture,  £oSd  a  {^pnc 

characters,  (tho  subordiiWite    > 
■ence  inp'ounded  arai 


en^T-an'jt^,    tA»ir    revTC'j'ejita.-^zon,    it-ma    icyoncZ    tAe,  eyi.or-a.ye.7-s  a.r6  k 


Jf'i'o.  y    asTiied-dxa-iaJe-  Jroni-  Taiajfo-tiy  /iiyer,  /fit, 
Sca2e  io  cltantt^TS       Tilarc:^^  iiy7d        See-Jiayet  /it  ^  f3 
JPie-yt.oc.7afe^J;atc^i'iSe^i7t-yyAz.nS(.zyy  7na^n-e,ttde.(^J ■ 


an       tn-a^SQ^rM  it..  j'tx2^^a-nc.e- 


DIABASB-PORPHYRITE. 


77 


Tabulation  of  the  results  of  a  microscopic  study  of  fine-grained  oUvinitie  diabases  {includ- 
ing melaphyrs) — Continued. 


Place. 

0) 
CO 

1 

1 

p< 
g 

to 

Macroscopic  char- 

acters. 

Constitutents  as  determined  hy 
microscope,  in  order  of  age. 

Angle    between 
maximum  ex- 
tinctions of 
adjacent  hemi- 
tropic  bands  of 
theplagioclase 
in  sections  cut 
at  random  in 
the  zone  0;  ii. 

Angles  on 
opposite 
sides  of 
cross-hair. 

958.... 

North  shore  Lake 
Superior,    Cari- 
bou   or    Black 
Point,     Minne- 
sota, from   a 
layer  overlying 
red  sandy  shale. 

s-w. 

11 

60 

2'W. 

Fine-grained; 
dark   reddish- 
brown. 

Olivine,  abundant,  in  small 
grains,  wholly  altered  to  a  red 
substance  and  crowded  into 
th.e  augite  interspaces ;  anorth- 
ite;  Tnagnetiie;  augite^  in  the 
usual  fresh  areas;  an  exces- 
sively fine-grained  rock. 

0 

34 
32 
25 
35 

0 

32 
35 
30 

34 

o 
66 
67 
55 
69 

Lialase-porphyrite  and  ashbed-diabase. — The  olivine-free,  fine-grained 
diabases  of  the  ordinary  type  pass  into  still  finer-grained  kinds,  in  which 
there  is  a  black  or  brown  color  and  a  more  or  less  perfectly  developed  con- 
choidal  fracture.  The  finest  of  these  rocks  are  completely  aphanitic,  and 
all  kinds  tend  to  a  porphyritic  development,  carrying,  as  porphyritic  ingre- 
dients, oligoclase  and  orthoclase,  and,  more  rarely,  labradorite  and  augite. 
The  augite  is  always  a  subordinate  ingredient,  and  appears  at  times  to  be 
almost  wholly  absent,  at  least  as  an  individualized  substance.  These  rocks 
play  a  very  prominent  role  in  the  Keweenaw  Series,  and  are  always  very 
strongly  characterized  in  the  field. 

Some  of  the  more  distinctly  crystalUne  of  these  rocks,  as  already  stated, 
Pumpelly  has  described  under  the  name  of  ashbed-diabase,  the  name  being 
given  from  the  fact  that  such  a  diabase  forms  the  base  of  a  flow,  whose  upper 
vesicular  portion  is  the  well-known  and  so-called  ashbed  of  Keweenaw 
Point.  As  the  external  characteristics  of  this  class  of  rocks,  Pumpelly  gives 
a  light  or  dark  gray  or  black  color,  a  very  compact  texture,  and  a  conchoidal 
fracture,  and,  as  the  accompanying  microscopic  characters,  the  subordinate 
position  of  the  augite,  and  more  especially  its  occurrence  in  rounded  grains, 
whose  contours  are  not  determined  by  the  feldspars.^ 

'Geology  of  Wisconsin,  Vol.  Ill,  p.  32. 


78        COPPEE-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 

My  own  study  has  shown  that  the  typical  ashbed-diabases  of  Pum- 
pelly  are  but  phases  of  a  large  class  of  rocks ;  that  between  these  typical 
kinds  and  the  fine-grained,  olivine-free  diabase  of  the  ordinary  type  there 
are  various  gradation-forms,  in  which  the  rounded  grains  of  augite  are 
mingled  with  more  and  more  of  the  augite  whose  contours  are  determined 
by  the  feldspars ;  and  also  that,  in  the  other  direction,  there  are  gradation- 
forms  into  aphanitic  kinds,  in  which  there  is  much  non-polarizing,  unindi- 
vidualized  material.  In  extreme  cases,  as  in  a  rock  from  Michipicoten,  fig- 
ured at  Plate  IX.,  Figs.  1  and  2,  the  unindividualized  base  makes  up  the 
greater  part  of  the  rock.  The  presence  of  unindividualized  base  and  the 
absence  of  recognizable  olivine  place  these  rocks  among  the  diabase-por- 
phyrites,  according  to  Rosenbusch's  nomenclature.  But  the  ashbed- 
diabases  are  so  plainly  linked  with  these,  both  through  intermediate  kinds 
and  through  similarity  of  occurrence  in  the  field,  that  all  are  considered 
here  together.  They  all  indicate  rapidity  of  solidification,  not  only  by  the 
presence  of  unindividualized  matter,  but  by  the  mode  of  occurrence  of  the 
augite. 

The  groundmass  of  these  rocks  externally  varies  from  light-gray  to 
dark-gray  in  color,  in  more  distinctly  crystalline  kinds ;  and  is  from  light- 
gray  to  jet-black,  and  in  one  phase  deep  reddish-brown,  in  the  less  crystal- 
line kinds.  With  the  former  kinds  the  fracture  is  sub-conchoidal,  with  the 
latter  very  highly  conchoidal  and  even  glass-like,  as  in  the  case  of  some 
black  semi-vitreous  rocks  that  have  a  large  development  on  Michipicoten 
Island. 

Under  the  microscope  the  tabular  oligoclases  usually  make  up  most  of 
the  section  in  the  more  distinctly  crystalline  kinds,  the  more  acid  phases 
containing  also  orthoclase  with  the  oligoclase.  The  augite  is  in  irregularly- 
outlined  particles,  commonly  very  subordinate  in  quantity,  though  occa- 
sionally, as  in  some  dense  black  rocks  from  Portage  Bay  Island,  at  the 
east  end  of  the  Minnesota  coast,  it  makes  up  most  of  the  section.  The 
augite  particles  lie  in  the  little  spaces  between  the  feldspars,  which,  how- 
ever, no  individual  particle  ever  completely  fills.  Several  augite  particles 
will  occur  together  in  such  a  space,  or  fill  it  along  with  the  magnetite,  or — 
and  on  the  whole  this  is  much  more  common — there  is  present  more  or  less 


DIABASE-POBPHTEITE.  79 

of  a  non-polarizing,  cloudy,  grayish,  or  red-stained  substance,  whicli  repre- 
sents the  original  magma.  This  substance,  when  present  in  small  quan- 
tities, fills  the  sharper  angles  between  the  feldspars.  From  these  smaller 
quantities  it  increases  in  amount  until  it  becomes  a  preponderating  ingre- 
dient in  the  denser  and  more  highly  conchoidally  fracturing  kinds,  when 
the  feldspars  are  seen  floating  about  in  it  in  wholly  separate  particles.  The 
red  ferrite,  which  is  an  important  ingredient  in  all  of  the  browner  kinds, 
has  evidently  come  from  the  alteration  both  of  the  original  magma  and  the 
augite. 

Among  the  porphyritic  ingredients  of  these  rocks  the  feldspars  are 
much  the  most  prominent.  Commonly  they  are  red,  though  occasionally 
white  or  colorless.  The  size  usually  is  one-eighth  to  one-sixteenth  inch  in 
length  or  less.  Orthoclase  occurs  among  these  feldspars,  but  they  are  more 
commonly  oligoclase  and  rarely  labradorite.  Augite  occurs  as  a  porphy- 
ritic ingredient,  but  much  more  rarely  than  the  feldspars.  It  is  commonly 
much  altered  to  chlorite.  As  adventitious  ingredients  may  be  mentioned 
epidote,  quartz,  calcite  in  pseud-amygdules  and  true  amygdules,  and  apa- 
tite in  the  usual  crystals. 

These  diabase-porphyrites  frequently  assume  an  amygdaloidal  char- 
acter in  the  upper  portions  of  the  flows,  when  they  are  commonly  extraor- 
dinarily vesicular,  very  often  with  the  vesicles  elongated  in  a  common 
direction.  Frequently  these  extraordinarily  vesicular  amygdaloids  have 
mingled  with  them,  and  often  filling  the  vesicles,  a  red,  shaly  matter. 

The  rocks  here  included  vary  considerably  in  silica  content,  ranging 
from  48  to  60  per  cent.  It  is  possible  that  some  of  the  more  basic,  blackish 
kinds  may  represent  the  half-glassy  forms  of  the  olivinitic  diabases,  but 
this  has  not  been  proved  by  analysis  or  recognition  of  olivine  as  an  ingre- 
dient. On  the  other  hand,  there  is  evidently  in  some  kinds,  especially 
in  some  of  the  redder  varieties  nearly  free  from  augite,  a  greater  amount 
than  usual  of  orthoclase  material,  and  with  this  often  is  a  little  secondary 
quartz.  These  kinds  make  up  much  of  the  so-called  quartzless  por- 
phyries, and  are  plainly  the  half-glassy  form  of  the  more  acid  orthoclase- 
gabbros.     These  kinds  have  about  55  to  60  per  cent,  of  silica,  and  stand 


80        COPPEE-BEAEDfG  BOOKS  OF  LAKE  SUPEEIOE. 

between  the  acidic  and  basic  lialf-glassy  rocks,  just  as  tbe  orthoclase  gab- 
bros  do  between  the  basic  and  acid  granular  rocks. 

As  typical  localities  for  the  rocks  of  this  class  may  be  mentioned,  for 
the  more  distinctly  crystalline  kinds,  Beds  45,  65  and  Q6  of  Marvine's  Eagle 
River  section,  Keweenaw  Point  ;^  for  the  kinds  highly  porphyritic  with  red 
feldspars,  but  without  much  non-polarizing  matter  in  the  base,  the  porphyry 
at  the  Duluth  elevator,  Duluth,  Minn.;  for  the  black  and  dark-gray  kinds, 
with  much  non-polarizing  matrix,  the  south  shore  of  Michipicoten  Island; 
and  for  the  red  aphanitic  and  brown  aphanitic  kinds,  the  north  shore  of 
Lake  Superior,  one  mile  below  the  mouth  of  Silver  creek,  N.  E.  \,  Sec.  22, 
T.  53,  R.  10  W.,  and  the  bay  above  the  Grreat  Palisades,  on  the  same  coast. 

'  Geological  Survey  of  Michigan,  Vol.  I,  Part  II,  Chapter  VIII. 


DIABASE-PORPHYEITE. 


81 


Tabulation  of  the  results  of  a  mierosco;pic  study  of  diahase-por^hyrite  and  aslibed-diabase. 


1963. 


Place. 


Bohemian  Kange, 
Keweenaw 
Point,  Mioliigan. 

Old  Suffolk  min- 
ing   location, 

Praysville,  Ke- 
weenaw Point, 
Michigan. 


STV. 
cor. 


58 


10      57 


29  W. 


31  "W. 


Macroscopic  char- 
acters. 


Aphanitic ;  dark 
brownish-gray ; 
of  a  conchoidal 
fracture. 

Matrix  aphanitic; 
mottled  dark 
reddish  -  hrown 
andhlack.  Very 
abundant  red 
porphyritxc  feld- 
spars, which  at 
times  carry  na- 
tive copper  and 
native  silver  as 
replacements. 
Si02,  59.52  per 
cent. 


Composition  as  determined  hy 
the  microscope. 


Angle  between 
maximum  ex- 
tinctions of 
adjacent  hemi- 
tropic  bands  of 
the  plagioclafle 
in  sections  cut 
at  random  in 
the  zone  0 :  ii. 


Ajigles  on 
opposite 
Bides  of 
cross-hair. 


Oligoclase  predominates ;  augite, 
in  minute  grains ;  Tnagnetite  ; 
chlorite  pseud  -  amygdulea. 

This  rock  presents  a  deeply- 
stained  groundmass,  in  which 
are  scattered  very  large  and 
abundant  porphyritic  oligo- 
clases  and  orthoclases,  the 
former  much  the  more  abun- 
dant of  the  two.  In  some  sec- 
tions the  groundmass  pre- 
sents much  the  appearance 
seen  in  1799  (infra),  except 
that  the  minute  tabular  feld- 
spars are  more  often  recog- 
nizable. In  these  sections 
also  are  found  the  same  al- 
tered porphyritic  angites  that 
are  characteristic  of  1799.  In 
other  sections  the  ground- 
mass  presents  a  peculiar  in- 
tertwisting of  colorless  and 
deeply  brown-stained  por- 
tions, often  so  arranged  as  to 
indicate  flowage,  and  in  some 
of  these  sections  there  are 
abundant,  minute,  elongated 
gas  vesicles,  now  filled  with 
quartz  or  a  greenish  chlorite. 
These  vesicular  sections  are  of 
specimens  taken  from  nearer 
the  upper  surface  of  the  bed. 
The  proportion  of  unindivid- 
ualized  matter  is  large  in  the 
more  vesicular  sections  and  in 
those  indicating  flowage;  the 
deep-brown  portions  represent- 
ing the  uuindividualizedmate- 
rial,  the  ligbter  portions  being 
made  up  of  completely  indi- 
vidualized quartz  and  feld- 
spar. The  rock  verges  close 
on  the  limit  between  the  more 
acid  diabase-porphyrites  and 
the  quartzless  porphyries. 


5 

3 

8 

12 

12 

24 

13 

14 

27 

7 

11 

18 

5 

6 

11 

6 

7 

13 

0 

6 

a 

6  L  S 


82 


COPPER-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 


Tdbitlation  of  the  results  of  a  microseopiG  study  of  diabase-porphyrite  and  ashhed-diahase 

— Continued, 


Place. 


MacroBCopic  char* 
acters. 


Composition  as  determined  by 
the  microscope. 


Angle  between 
maximum  ex- 
tinctions  of 

adjacent  hemi- 
tropic  bands  of 
the  plagioclase 
in  sections  cut 
at  random  in 
the  zone  O:  ii. 


Anglea  on 
opposite 
sides  of 
cross-hair. 


2560. 


Porcupine  Mount- 
ains, Michigan. 


KW. 


23 


51 


43  W. 


1245. 


Falls,  Little  Carp 
Eiver,  Porcu- 
pine Mount- 
ains, Michigan. 


SE. 


17 


50 


44  "W. 


2568. 


2562. 


Porcupine  Mount- 
ains, Michigan. 


.do. 


ITE. 


NW. 


23 


24 


43  W. 


51 


43  W. 


Aphanitic;  dark 
p  arplish-brown ; 
conchoidal  frac- 
ture; carries 
minute  porphy- 
ritic  feldspars. 
Si02,  59.38  per 
cent. 


Nearly  aphanitic ; 
dark-  brown; 
conchoidal  frac- 
ture; numerous 
porphyritic  feld- 
spars. 


Aphanitic ;  dark 
chocolate- 
brown;  holds 
minute  porphy- 
ritic feldsjiars. 

Aphanitic;  dark- 
brownish    to 

black. 


Groundmasa  x—plagioclasey  in 
very  minute  tabular  crystals; 
unindividualized  svbsta/nce  ; 
■magnetite;  much  red  and 
brown  ferrite ;  rare  and  bril- 
liantly polarizing  particles  be- 
longing to  augite;  secondary 
qtiartz.  Porphyritic  ingredi- 
ents:— ^rather  rare  orthoclases 
and  oligoelases ;  much  nirer 
augites;  pseud-amygdules  of 
calcite. 

G-roundmasa : — ^inordinary  light, 
much  stained  with  ferrite; 
consists  of  unindivid/uaMzed 
substance,  oligoclase,  magnet- 
ite, and  f&rrite  recognizable; 
augite  particles  very  rare. 
Porphyritic  ingredients: — or- 
tkoclase  and  oligoclase  very 
abundant;  augites  rarer,  of 
good  size,  almost  wholly  al- 
tered to  ferrite  and  green  sub- 
stance. 

Groundmassasin2o60.  Porphy- 
ritic feldspars  larger  and  more 
abundant;  the  augite  com- 
monly much  altered  to  green- 
ish sabstance  with  bands  of 
red  and  brown  ferrite. 

Groundmasa  %-~plagioclase ;  un- 
individualized substance;  mag- 
netite ;  ferrite ;  augite,  in  mi- 
nute, not  abundant  grains, 
often  altered  to  a  green  sub- 
stance; secondary  quartz. 
Porphyritic  ingredients: — not 
very  abundant,  good-sized  or' 
thoclases  and  oligoelases,  much 
altered  to  clilorite;  augite^ 
much  altered  to  green  sub- 
stance with  bands  ot  ferrite* 


DIABASE-POEPHTEITE. 


83 


Tabulation  of  the  results  of  a  microscopic  study  of  diabase-porphyrite  and  ashbed-diabase — 

Continued. 


a 
g 

a 
1 

Place. 

1 
1 

a 

1 

P4 

1 

Macroscopic  char- 
acters. 

Composition  as  determined  by 
the  microscope. 

Angle    between 
maximum    ex- 
tinctions of 
a<^jacent  hemi- 
tropic  bands  of 
the  plagioclase 
in  sections  cut 
at  random   in 
the  zone  0:  ii. 

Angles  on 
opposite 
sides  of 
crosshair. 

o 

o 

o 

2W1.. 

Potato  Eiver,  Ash- 
land      County, 
"Wisconam. 

24? 

46 

i-w. 

Nearly  aphanitic; 
compact;  black; 
conchoidal  frac- 
ture. 

Labradorite,  in    smaU    strips; 
a/ugite^  in  small  grains;  green 
substance  in  clouds  of  granu- 
lar masses. 

9"W>.. 

Old  Ironton  trail, 
Ashland  County, 
■Wisconsin. 

34 

46 

IW. 

Fine-grained ;  com- 
pact; black;  sub- 
conohoidal  frac- 
ture. Sp.gr.,  2.98. 

Oligoclase  ;  augite,  in  aggregated 
grains;  magnetite  or  tit(m\fer- 
ous  iron. 

386  S>. 

Clam  Falls  district. 

NW. 

24 

37 

17  "W. 

Aphanitic;  black; 

Oligoclase;   augite,  in  rounded 

■^$® 

10 

18 

Polk      County, 

conchoidal  frac- 

grains; magnetite;  cWorite  and 

IJo 

4 

4 

"Wisconsin. 

ture. 

quartz  pseudamygdnles. 

3'- 

393  S>. 

Totogatig  district, 
.Douglas  County, 
"Wisconsin. 

SW. 

28 

42 

11  "W. 

Aphanitic;  black. 

Ptoffiociase,  predominant ;  augite, 
ingrains;  magnetite. 

426  S'. 

Tipper  Saint  Croix 
district,  Douglas 
County,  Wis. 

SB. 

6 

43 

14  W. 

Fine-grained; 
black;  conchoi- 
dal fracture. 

Oligoclase;    augite,    in  grains; 
magnetite. 

1 

Month  of  Brewery 
Creek,    Duluth, 
Minn.    75  north, 
1,850  west. 

S"W. 

23 

60 

14  W. 

Aphanitic;  choco- 
late     brown; 
highly    conchoi- 
dal  fracture; 
carries  very  mi- 
nute porphyrltic 
feldspars. 

Groundmass:  —  much   stained 
with  red  ferrite;  consists  of 
plagioclase  in  muiute  tabular 
CTjBta,]a;unindividualizedsub- 
stance;    magnetite;    ferrite; 
augite,  in  very  minnte  grains. 
Porphyriticingredients;— rare 
feldspars;  alteTed  augites. 

13 

Bed   of   Brewery 
Creek,    Doluth, 
Minn. 

SE. 

22 

SO 

14  "W. 

Matrixblack;  aph- 
anitic ;     thickly 
crowded       with 
smaU   red    feld- 
spars      one-six- 

o 

i 

•a 

'8 

4 

12 

teenth    to    one- 

fourth  inch  long; 

1 

alsopseud-amyg- 

!^ 

4 

5 

9 

dules  of  epidote 

is' 

4 

9 

and  calcite. 

il 

6 

9 

45 

Bed    of    French 

H"W". 

6 

61 

12  W. 

Very  fine  grained; 

Oligoclase,  both  in  the  ground- 

Kiver,      Minne- 

dark-gray;    con- 

mass and  in  rare  porphyrltic 

sota. 

choidal  fracture. 

crystals;  augite,  abundant  in 
grains;  unindividualized  sub- 
stance;   magnetite;    ferrite; 
chlorite  pseud-amygdules. 

82 

Bed    of    Beaver 

N. 

2 

66 

8"W. 

Nearly  aphanitic; 

Oligoclase,  predominates ;  augite, 

2 

7 

9 

Eiver,      Minne- 

line. 

black;    conchoi- 

in grains;  magnetite. 

13 

17 

30 

sota. 

dal  fracture. 

8 

8 

IS 

'E.  Pumpelly,  Geology  of  "Wisconsin,  Vol  HT,  pp.  37-38. 


84 


COPPEE-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 


TahtilaUon  of  the  results  of  a  microsco])ic  study  of  diahase-:por;pliyrit€  and  ashhed-didbase— 

Continued. 


547. 


589.. 


655. 


713. 


Place. 


C? 


Near  mouth,  of 
Lester  Hiver, 
Minnesota. 


Nortli  shore  of 
Late  Superior, 
two  miles  helow 
the  mouth  of 
Lester  River, 
Minnesota. 
INorth  shore  of 
Lake  Superior, 
one  mile  above 
the  mouth  of  the 
Knife  Eiver, 
Minnesota. 
North  shore  of 
Lake  Superior, 
one  mile  below 
the  mouth  of 
Silver  Creek, 
foot  of  Encamp- 
ment Bluff,  Min- 
nesota. 
North  shore  of 
Lake  Superior, 
one  -  half  mile 
north  of  Split 
Rock  River, 
Minnesota. 

North  shore  of 
Lake  Superior, 
Beaver  Bay, 
Minnesota. 

North  shore  of 
Lake  Superior, 
bay  above  Great 
Palisades,  Min- 
nesota, under- 
lies the  palisade 
rock. 


SW. 


S"W. 


sw. 


NE. 


50 


51 


22 


13  W. 


Macroscopic  char- 
acters. 


13  W. 


IIW, 


West 
side. 


S"W. 


sw. 


22 


54 


low. 


Constituents  as  determined  by 
the  microscope. 


8W. 


55    SW. 


56 


7W. 


Very  fine  grained, 
but  plainly  crys- 
talline;  dark 
brownish  gray ; 
conchoidal  frac- 
ture. 

Aphanitic ;  very 
dense;  dark 
brown;  rare  mi- 
nute porphyritic 
feldspars. 

Very  fine-grained; 
light  gray ;  euh- 
conchoidal  frac- 
ture. 


Aphanitic ;  red- 
dish brown; 
highly  conchoid- 
al fracture;  car- 
ries elongated 
amygdule  s  of 
calcite.  Si02, 
60.03  per  cent. 

Nearly  aphanitic; 
indefinitely  mot- 
tled red  and 
black,  weathers 
bright  red;  very 
highly  conchoid- 
al fracture. 

Very  fine-grained; 
nearly  black ; 
conchoidal  frac- 
ture. 

Nearly  aphanitic ; 
chocolatebrown, 
conchoidal  frac- 
ture. 


Angle  between 
maximum  ex- 
tinctions of 
adjacent  hemi- 
tropic  bands  of 
the  plagioclase 
in  sections  cut 
at  random  in 
the  zone  O:  ii. 


Angles  on 
opposite 
sides  of 
cross-hair. 


Oligoclase.  predominates ;  augite^ 
grains  not  abundant;  magne- 
tite; ferrite^  abundant. 


Plagioclase ;  unindividualized 
substance;  augite,  in  grains, 
not  abundant ;  very  abundant 
red  and  brown /ernie. 


Labradorite  predominant, 
longer  axes  of  crystals  often 
incommondirections;  Ttiagne- 
tlie;  augite  in  grains,  abund- 
ant; afresh  rock. 

Groundmass : — oligoclase;  much 
unindividualized  substance, 
saturated  with  red  ferrite; 
magnetite.  Porphyritic  in- 
gredients : — oligoclase  ;  much 
altered  augite. 


Oligoclase;  augite,  abundant  in 
grains ;  magnetite ;  ochre- 
stained,  unindividualized  sub- 
stance abundant,  filling  sharp- 
ly the  spaces  between  the 
feldspars. 

Oligoclase;  augite  in  rounded 
particles  very  abundant,  un- 
usually coarse  for  this  class  of 
rocks ;  magnetite. 

Plagioclase,  predominant ;  unin^ 
dividualized  substance ;  rare 
minute  gvoXns  of  augite ;  ■mag- 
netite ;  red  ferrite  completely 
saturating  the  entire  section. 


26 
34 
33 
31 


61 


15 
9 
25 


DIABASB-PORPHTR ITE. 


85 


Tabulation  of  the  results  of  a  microscopic  study  of  diabase-porphyrite  and  ashbed-diabase 

— Continued, 


1 

p 

1 

o 
o 

Pi 

Place. 

a 

1 
? 

a 
& 

a 

o 

f 

o 
H 

i 

Macroscopic  char, 
acters. 

Composition  as  determined  by 
the  microscope. 

Angle    between 
maximum    ex- 
tinctions  of 
adjacent  hemi- 
tropic  bands  of 
the  plagioelase 
in  sections  cut 
at   random   in 
the  zone  0 :  ii. 

Angles  on 
opposite 
sides  of 
cross-hair. 

o 

0 

o 

907.... 

North    shore     of 
Lake  Superior,  2 
miles  below  the 
mouth  of  Bap- 
tism Eiver,  Min- 
nesota. 

SE. 

11 

56 

IW. 

Minutely  crystal- 
line;    dark- 
brown;  semi-con- 
choidal  fracture. 

Plagioelase;  magnetite;  augite 
in   grains ;    unindividualized 
substance;  very  abundant  red 
ferrite. 

1051... 

Bed   of    Cascade 
Eiver,     Minne- 
sota. 

23 

62 

2W. 

Aphanitic ;    dark 
brown ;    conch, 
oidal    fracture ; 
an    excessively 
dense  rock. 

Vnindividualized  substance; 
plagioelase  needles ;  magnetite 
patches. 

1551... 

Korth    shore    of 
Lake  Superior, 
below  Bed  Eock 
Bay,    Indian 
reservation, 
Minnesota  (not 

63 

5E. 

Aphanitic;   dark- 
reddish,  brown; 
highly      conch- 
oidal  fracture; 
comes    out    in 
thin  slabs. 

Plagioelase;     unindividualized 
substance;  magnetite;  augite, 
particles  rare ;  much  red  fer- 
rite. 

1568... 

surveyed). 
Portage   Bay  Isl- 

Black;  aphanitic ; 

Plagioelase,  in   matrix   and  in 

if' 

6 

10 

and,  north  shore 

rough  fracture. 

porphyritic  crystals;  magne- 

? " 

10 

21 

of  Lake  Superior, 

tite;  green  and  brown  altera- 

(^ (2i 

26 

SO 

Minnesota. 

tion-products. 

{') 

South    side   of 
Michipicoten 
Island. 

Nearly  aphanitic ; 
dark  gray;  con- 
choidal fracture; 
no    porphyritic 
ingi-edients. 

Minute  tabular  oligoclases  pre- 
dominant; "magnetite;   augite 
in   grains ;     some   residuary 
magma. 

« 

Southwest  side  of 
Michipicoten 
Island. 

Completely  apha- 
nitic;very  dark- 
gray;    highly 
conchoidal  frac- 
ture;    no   por- 
phyritioingredi- 
ents. 

* 

With  a  medium  power  this  rock 
presents   merely   a  pinkish- 
tinted  background,  minutely 
dotted  with  gray,  and  no  re- 
cognizable ingredients ;  with 
a  higher  power,  in  polarized 
light,  there  are  recognized  ex- 
cessively minute  tabular^Zaj- 
ioclases ;  larger  but  still  mi- 
nute  magnetite  particles; 
rare  and  excessively  minute 
brightly  polarizing  particles 
which  may  belong  to  augite ; 
residuary  magma  predominat- 
ing, producing  no  effect  what- 

1 

ever  bet  ween  the  crossed  nicols. 

1  Macfarlane's  Michipicoten  Collection,  No. 
^Macfarlane's  Michipicoten  Collection,  No. 


2,  "Compact  Melaphyr.'" 
4,  "Compact  Melaphyr." 


86 


COPPEE-BEAEING  BOOKS  OP  LAKE  SUPEEIOE. 


Tabulation  of  the  results  of  a  microscopic  study  of  diabase-porphyrite  and  ashbed-diabase 

— Continued. 


i 

Angle     between 
maximum    ex- 
tinctions  of 
adjacent  hemi- 
tropic  bands  of 
the  plagioclase 

1 

Place. 

i 

<D 

t 

a 
1 

i 

Macroscopic  char- 
acters. 

Composition  as  determined  by 
the  microscope. 

in  sections  cut 
at   random   in 
the  zone  0 ;  ii. 

Angles   on 
opposite 
Bides  of 
cross-hair. 

It 

o 

o 

0 

(•) 

South    side  of 
Michipicoten 
Island. 

Aphanitic ;     dark 
chocolate-brown ; 
conchoidal    frac- 
tnre;     no    por- 
phyritic ingredi- 
ents. 

Tabular  plagioclase  for  the  most 
part,  with  the  longer  axes  in 
a  common  direction,  the  crys- 
tals minute,  but  still  several 
times  larger  than  in  No.  4,  pre- 
dominating ;  magnetite  in  not 
abundant     small     particles ; 
aw^fite  in  minute  grains;  some 
residuary  magma. 

(') 

Copper   mine, 
Michipicoten 

Like  No.  4  of  Mich- 
ipicoten   collec- 

The base  of  this  rock  is  much 
like  that  of  No.  4  of  this  col- 

Island. 

tion. 

lection.     In  it  are  rare  but 
quite  large  (^  to  J  inch  across) 
crystals    of    augite,    largely 
changed   to   a   gi'eenish  ma- 
terial, but  with  cores  of  fresh 
augite  remaining. 

w 

Sontheast   comer 
Miciiipiooten 
Island. 

Matrix  aphanitic ; 
greenish  -  gray ; 
very    abundant 
porphyritic 
white  feldspars, 
^  to  J  inch  long; 
also,  rarer  and 
much  more  mi- 

Base:—light  and  dark  brown, 
unindividualized   material, 
producing  little  or  no  eflTect 
between  the  crossed  nicols; 
this  is  thickly  studded  with 
minute  plagioclases  and  holds 
rarer  and  more  minute  black 
particles  of  magnetite.     Por- 

10 

|-25 

&    ^^ 
o     29 

1 21 

14 
24 

24 
49 

nute  black  por- 

phyritic    ingredients :— very 

27 

59 

phyritic    parti- 

abundant,    large-sized,     and 

30 

59 

cles. 

very  fresh  labradorite,  several 

21 

42 

crystals  often  clustered  in  an 

interesting  way ;  also  augite. 

much  smaller  and  rarer,  com- 

monly much  altered  to  green- 

ish material   and   associated 

with  magnetite. 

m 

do 

Matrix  aphanitic; 

Close    to    preceding,   differing 

■1(16 

20 

36 

dark  brownish- 

only  in  having  much  red  mar 

Ills 

19 

35 

gray;     very 

terial  in  the  base  and  the  feld- 

fM ^ 

abundant   pink 

spars  somewhat  decomposed 

porphyritic 

'     and  dulled.    Figs.  1  and  2  of 

feldspars,  aver- 

Plate IX.  represent  this  sec- 

aging ^  inch  in 

tion. 

length.    Si02, 

60.89  per  cent. 

*  Maofarlan6*8  Michipicoten  Collection,  No.  5,  "Melaphyr." 
^Macfarlane's  Michipicoten  Collection,  No.  9.  "Melaphyr." 


^Macfarlane's  Michipicoten  Collection,  No.  16,  "Porphyrite. " 
■*  Macfarlane's  Michipicoten  Collection,  No.  17,  "Porphyrite." 


UNITED  -STATES  GEOLOGICAL  SURVEY 


COPPER-BEARING   ROCKS   OF  LAKE  SUPERIOR    PL.  XI 


i.JJoonS:  Co.  LlUi.Boltim 


AMYGDALOID 


<1 


^        ^  .iloida  than  an-  theoHv;  A       •  ^^ 

'■a  the  melaphyrs  bu>  _  ?,'\»J»»N»wi»'j  *S      ^^JSoZa 
••uaw  Point.     When  Uiy  appear  to 

' .  vine-free  kinds. 


Jlmij(jdaloid      from,     Gtea.t    Palisades      Minnesota,    coast.      Ordinary   licthf. 
Scale     24   diameters .. 


.^MlCli 


DIABASE-POEPHTEITE— AMYGDALOIDS. 


87 


Tabulation  of  the  results  of  a  microscopic  study  of  diabase-porphyrite  and  ashbed-diabase 

— Continued. 


i 

p 
g 

1 

Place. 

1 

1 

a 

1 

p. 
i 

i 

Macroscopic  char- 
acters. 

Composition  as  determined  by 
the  microscope. 

Angle    between 
maximum    ex- 
tinctions  of 
adjacent  herai- 
tropic  bands  of 
the  pla^ioclase 
in  sections  cut 
at   random   in 
the  zone  0 :  il. 

Angles   on 
opposite 
sides    of 
crosa-halr. 

(D 

South   side   of 
Michipicoten 
Island. 

do 

Aphanitic;  nearly 
black;  highly 
conchoidal  firac- 
ture ;  no  por- 
phyritic  ingre- 
dients. 

Completely  apha- 
nitic; jet-black; 
greasy- vitreous 
luster;  glass- 
like fracture. 
Si02,  57.92  per 
cent. 

An  excessively  dense  rock,  in 
■which  are  recognizable,  with 
a  high  power  in  the  polarized 
light,  very  numerous  minute 
augile  grains  embedded  in  a 
non- polarizing  matrix,  along 
with  much  rarer  plagioclaae 
and  magnetite.  The  rock  is 
remarkable  for  its  relatively 
large  content  of  augite. 

Has  a  base  which  in  ordinary 
light  looks  much  like  that  of 
Ko.  4  of  this  series;  but  there 
is  more  non-polarizing  mate- 
rial, large  areas  remaining 
completely  dark  between  the 
croased  nicols,  and  the  pla- 
gioclaaes  are  still  more  mi- 
nute. Occasional  minute  bril- 
liant points  belonging  to  au- 
gite are  seen,  as  also  some 
magnetite  particles. 

o 
10 

0 

11 

o 
21 

I  Macfarlane's  MicMpicoten  Collection,  Xo.  18,  "Basalticmelaptiyr." 
^Macfarlane'a  Micliipicoten  Collection,  l^o.  19,  "Pitohstone," 

Amygdaloids. — The  flows  of  the  finer-grained  rocks  are  all  commonly 
provided  with  upper  vesicular  portions,  \>j  the  subsequent  filling  of  whose 
vesicles,  and  the  various  degrees  of  alteration  of  whose  matrices  have  been 
produced  the  manifold  types  of  amygdaloid  known  in  the  Lake  Superior 
region.^  The  coarse  rocks — olivinitic  and  orthoclastic  gabbros — are  not 
furnished  with  amygdaloids  save  when  tending  to  a  distinctly  finer  grain 
than  usual. 

Externally,  the  matrix  of  the  amygdaloid  is  commonly  quite  different 

'  Pumpelly  has  spoken  of  the  olivinitic  fine-grained  kinds,  his  melaphyrs,  as  less  commonly  pro- 
vided with  amygdaloids  than  are  the  olivine-free  diabases  of  the  ordinary  type,  hut  in  my  observations 
this  is  only  true  vrhen  the  melaphyrs  have  a  distinct  tendency  to  become  coarse-grained,  as  in  "The 
Greenstone  "  of  Keweenaw  Point.  When  they  are  fine-grained  they  appear  to  have  amygdaloids  qnite 
as  frequently  as  the  olivine-free  kinds. 


88        COPPEE-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 

from  that  of  the  rest  of  the  bed.  This  difference  consists  principally  in 
greater  denseness  of  grain,  from  solidification  while  much  of  the  matter 
was  not  developed  into  distinct  minerals.  The  difference  is  least,  then,  in 
the  case  of  those  beds  whose  lower  portions  are  composed  of  some  phase 
of  the  diabase-porphyrites,  in  which  there  is  also  a  greater  or  less  proportion 
of  unindividuaHzed  matter.  In  some  of  these  beds,  especially  when  the 
rock  is  of  the  dense  brownish  kind  with  highly  conchoidal  fracture,  above 
described,  there  is  no  perceptible  difference  between  the  matrices  of  the 
vesicular  and  non- vesicular  portions  of  the  bed;  but  more  usually  there  is 
a  great  difference  in  this  respect  between  the  lower  and  vesicular  portions 
of  a  flow.  The  internal  changes  to  which  such  an  open  vesicular  substance, 
composed  largely  of  a  molecularly  unstable  material  like  glass,  must  always 
be  liable,  have  greatly  increased  the  difference,  and  have  given  rise,  by  the 
variation  in  the  decomposition-products,  to  a  great  variety  of  amygdaloids, 
which  it  would  seem  at  first  sight  hard  to  place  together. 

Under  the  microscope  the  matrix  of  the  unaltered,  or  relatively  little 
altered,  amygdaloid  shows  nearly  always  much  non-polarizing  matter, 
commonly  deeply  stained  with  red  ferrite.  In  this  are  developed  needles 
of  plagioclase  to  a  greater  or  less  extent,  and  often  these  needles  seem  to 
be  but  microliths  arrested  in  the  process  of  aggregation  into  crystals.^ 
Augite  particles  occur,  but  are  usually  relatively  sparse,  and  frequently 
fail  entirely.  Very  often  there  is  a  fluidal  structure  brought  out  in  the 
arrangement  of  the  plagioclase  microliths  and  other  particles,  and  in  many 
cases  the  flowage  direction  is  found  to  coincide  with  the  longer  axes  of  the 
elongated  vesicles.  The  vesicles  themselves,  filled  or  empty,  as  the  case 
may  be,  are  always  sharply  outHned  in  the  thin  section,  and  there  is  im- 
mediately about  them  a  crowding  of  the  plagioclases  and  ferrite  particles, 
as  if  by  pressure  in  the  cavity.  Moreover,  the  individualized  minerals,  as 
Pumpelly  has  shown,^  are  often  more  minute  in  the  vicinity  of  the  vesicles 
than  away  from  them.  Porphyritic  feldspars,  macroscopically  visible,  are 
frequently  developed  in  the  matrix  of  the  amygdaloids — so  far  as  my 
observation  has  gone  they  are  at  least  as  often  present  as  not — and  in  this 

'  E.  Pumpelly,  "Metasomatic  Development,"  p.  282. 
'Ibid.,  p.  283. 


AMTaDALOIDS.  89 

respect  we  have  yet  another  affinity  between  the  amygdaloids  and  the  non- 
vesicular diabase-porphyrites. 

Macroscopically,  the  vesicles  are  seen  to  be  commonly  filled  with 
secondary  minerals— one  or  more  of  "calcite,  chlorite,  epidote,  quartz, 
prehnite,  laumontite,  copper,  orthoclase,  or  their  products  of  alteration."^ 
Often,  however,  I  have  observed  the  vesicles  empty,  either  from  the  removal 
of  the  amygdules  or  from  their  having  always  remained  empty.  The  walls 
of  these  empty  cavities  are  commonly  found  to  be  smooth  and  dense, 
apparently  from  the  pressure  of  the  confined  vapor. 

Although  a  large  number  of  sections  of  amygdaloids  were  cut  with 
this  object  in  view,  I  have  not  been  able  to  find  the  time  to  extend  the 
studies,  so  ably  begun  by  Professor  Pumpelly,  of  the  changes  which  have 
brought  about  the  fillings  of  the  vesicles  and  the  various  stages  of  altera- 
tion of  the  matrix.  He  sums  up  the  results  of  his  studies  on  the  alterations 
of  both  pseud-amygdaloids  and  the  true  vesicular  amygdaloids  in  the  fol- 
lowing table,  which  is  designed  to  show  the  course  and  final  results  of  the 
most  common  process  of  alteration: 

C      I.  Hydration  of  chrysolite,  when  present. 

II.  Change  of  augite,  loss  of  lime,  and  partial  loss  of 
iron  and  magnesia. 

III.  Change  of  feldspar  to  prehnite,  and   formation  of 

prehnite  pseud-amygdules. 
rV.  Change  of  prehnite  to  chlorite. 
lYa.  Change  of  prehnite  to  orthoclase. 

r      I.  Filling  gas  vesicles  with  prehnite,  or  other  minerals. 
Change  of  matrix  to  ferruginous  prehnite. 
II.  Change  of  the  prehnite,  in  places,  to  chlorite;   in 
others,  to  calcite  and  green-earth;    in  others,  to 
epidote  and  calcite. 

III.  Entrance  of  quartz,  flUing  all  the  interstices,  and  re. 
placing  the  calcite. 

The  following  are  Pumpelly's  comments  on  this  table : 

This  is  the  broader  history.    Orthoclase  is  here,  as  in  the  pseudo-amygdaloid,  of 

sporadic  occurrence,  and  a  product  of  the  prehnite. 

The  changes  under  II.  may  affect  only  the  amygdules,  or,  if  the  matrix  was  preh- 

nitized,  it  applies  to  the  whole  mass  of  the  amygdaloid.    It  does  this  in  such  a  manner 

>  R.  Pumpelly,  Geology  of  Wisconsin,  Vol.  Ill,  p.  31. 


Pseud-amygdaloid  stage .  - 


Amygdaloid  stage. 


90        COPPEE-BEAEING  EOOKS  OF  LAKE  SUPEEIOE. 

that,  where  carried  to  its  extremes,  considerable  portions  of  the  bed  have  lost  every 
semblance  of  an  amygdaloid,  and  consist  now  of  chlorite,  epidote,  calcite,  and  quartz, 
more  or  less  intimately  associated,  or  forming  larger  masses,  of  the  most  indefinite 
shapes,  and  merging  into  each  other.  Sometimes  portions  of  partially  altered  preh- 
nite  occur.  In  places,  considerable  masses  of  rich  brown,  and  green  fresh  prehnite 
filled  with  copper  occur ;  but,  as  a  rule,  this  mineral  has  given  way  to  its  products. 

To  this  process,  the  copper-bearing  beds  of  Portage  Lake — wrongly  called  lodes — 
owe  their  origin.  Considerable  portions  of  these  beds  are  but  partially  altered  amyg- 
daloids,  containing  amygdules  of  prehnite,  chlorite,  calcite,  or  quartz,  with  more  or 
less  copper ;  other  portions  are  in  the  condition  described  above. 

This,  too,  (II.  and  III.),  appears  to  have  been  the  principal  period  of  concentra- 
tion of  the  copper.  In  the  still  amygdaloidal  portions,  this  metal  was  deposited  in 
the  cavities  and  in  cleavage-planes  of  some  minerals,  and  replaced  calcite  amygdules, 
etc.  But  in  the  confused  and  highly  altered  parts  of  the  bed  it  crystallized  free,  where 
it  had  a  chance :  more  generally  it  replaced  other  minerals  on  a  considerable  scale.  It 
formed,  in  calcite  bodies,  those  irregular,  solid,  branching  forms,  that  are  locally 
known  as  horn-copper,  often  many  hundred  pounds  in  weight ;  in  the  epidote,  quartz, 
and  prehnite  bodies,  it  occurs  as  thread  and  flake-like  impregnations ;  in  the  foliaceous 
lenticular  chloritic  bodies,  it  formed  flakes  between  the  cleavage-planes  and  oblique 
joints,  or  in  places — and  this  is  more  particularly  true  of  the  fissure- veins,  which  we 
are  not  now  considering — it  replaces  the  chloritic,  selvage-like  substance  till  it  forms 
literally  pseudomorphs,  sometimes  several  hundred  tons  in  weight. 

When  the  amygdaloid  has  arrived  at  the  condition  we  have  been  describing,  it 
assumes  some  of  the  characters  of  a  vein,  in  that,  although  it  presents  no  open  fissure, 
it  contains  greater  or  smaller  masses  of  calcite  and  other  minerals  that  are  easily  re- 
placed by  an  intruder.  To  this  period,  probably,  belongs  the  replacement  of  calcite 
by  datolite ;  and  here,  also,  the  rather  rare  occurrence  of  analcite  crystals,  and  the 
pseudomorphs  of  orthoclase  after  these. 

As  I  have  already  remarked,  the  pseudo-amygdaloids  are  merely  altered  forms 
of  the  same  rock  as  the  lower  zone.  There  seems  to  be  a  definite  limit  at  which  this 
progressive  change  stops,  and  that  is  when  all  augite  is  changed  to  its  green  pseudo- 
morph,  and  a  large  percentage  of  the  rest  of  the  rock  consists  of  pseudo-amygdules  of 
delessite,  and  partial  pseudomorphs  of  this  after  plagioclase.  The  occurrence  of  epi- 
dote and  quartz  is  not  general,  and  is  then  confined  to  scattering  pseudo-amygdules, 
in  which  these  minerals  have  succeeded  prehnite,  perhaps  in  the  local  absence  of  the 
conditions  necessary  to  produce  the  usual  delessite. 

Thus  I  conceive  that  the  extent  of  the  change  to  the  pseudo-amygdaloidal  form 
is  conditioned  essentially  by  the  amount  of  augite  present,  to  supply  first  the  lime 
necessary  to  aid  in  changing  the  plagioclase  to  prehnite,  and  next  the  iron  and  mag- 
nesia to  form  the  delessite,  whether  by  acting  directly  on  the  feldspar  substance  or  on 
the  prehnite. 

The  amygdaloids  proper  were,  probably,  both  structurally  and  chemically,  some- 
what different  from  the  lower  zone,  in  that  it  is  reasonable  to  suppose  that,  in  addition 
to  being  more  or  less  porous,  they  contained  a  greater  or  less  amount  of  amorphous 
base,  which  is  more  easily  altered  than  a  crystalline  aggregate.  But,  from  whatever 
cause,  the  amygdaloids  have,  as  we  have  seen,  been  capable  of  much  greater  changes 


ACID  OEIGINAL  EOCKS.  91 

than  the  lower  zone :  in  them  the  tendency  is  nndoubtedly  towards  the  formation  of 
quartz,  chlorite,  and  epidote  rocks  as  a  more  stable  limit,  through  the  mediation  of 
prehnite  and  calcite. 

There  are  other  forms  of  alteration  which  Pumpelly's  investigation 

does  not  cover,  but  none  of  so  great  importance  as  those  above  described. 


Section  II.— ACID  OEIGINAL  BOOKS. 

'  As  indicated  in  a  previous  chapter,  I  have  been  able  to  show  that  the 
several  kinds  of  felsite  and  acid  porphyry,  which  make  the  pebbles  of  the 
conglomerates  and  the  material  of  most  of  the  sandstones  of  the  Keweenaw 
Series,  exist  in  the  same  series  in  the  original  condition;  and  that  while  sub- 
ordinated to  the  basic  rocks  in  total  amount  they  yet  form  a  very  important 
element  in  the  make-up  of  the  series,  throughout  its  entire  circuit  about 
the  Lake  Superior  Basin.  These  acid  rocks  may  be  conveniently  described 
under  the  following  heads: 

1.  Quartzless  porphyry. 

2.  Quartziferous  porphyry  and  felsite. 

3.  Augite-syenite,  and  granitell  or  granitic  porphyry. 

4.  Granite. 

Quartzless  porphyry. — There  are  several  phases  of  porphyritic  rocks  in 
the  Lake  Superior  region,  occurring  both  as  pebbles  in  the  conglomerates  of 
the  Keweenaw  Series  and  as  flows  in  the  same  series,  which  would  form- 
erly have  been  classed  together  as  "quartzless  porphyries,"  that  name 
applying  to  felsitic  rocks  in  which  quartz  is  present  neither  in  the  base  nor 
as  a  porphyritic  ingredient.^  These  several  phases  have  in  common  an 
aphanitic,  dark-brown  base,  frequent  abundance  of  porphyritic  feldspars — 
although  kinds  occur  in  which  the  feldspars  sink  out  of  sight — and  freedom 
from  visible  porphyritic  quartz.  They  are  also  distinctly  softer  than  the 
true  acid  quartziferous  porphyries.  A  study  of  the  thin  sections,  however, 
aided  by  silica  determinations,  has  shown  that  in  such  a  grouping  we 
should  really  be  placing  together  kinds  which  are  but  the  half  crystalline 
or  cryptocrystalline  phase  of  the  less  basic  diabases,  others  which  verge  on 

•Conf.  E.  Pumpelly  in  Geological  Survey  of  Michigan,  Vol.  I,  Part  11,  p.  16. 


92        COPPEE-BEAEING  EOGKS  OF  LAKE  SUPEEIOE. 

the  true  acid  felsites  in  acidity  (70  per  cent,  silica  or  over),  and  kinds 
again  which  are  intermediate  between  these.  There  exist  in  the  Lake 
Superior  region,  in  fact,  porphyritic  rocks  which  range  from  the  true  basic 
kinds,  with  less  than  50  per  cent,  of  silica,  to  the  very  acid  felsites  and 
quartziferous  porphyries  with  over  70  per  cent.,  thus  forming  a  continu- 
ous series.  It  thus  becomes  necessary  to  adopt  some  rather  arbitrary 
divisions  between  the  different  phases.  The  kinds  with  from  50  to  60 
per  cent,  of  silica  have  already  been  considered  under  the  heads  of 
ashbed-diabase  and  diabase-porphyrite,  while  those  reaching  70  per 
cent,  are  taken  up  below  with  the  true  felsites.  There  yet  remain  the 
kinds  intermediate  between  these,  both  as  to  silica  content  (60  to  70  per 
cent.)  and  as  to  their  microscopic  characters.  These  are  the  kinds  which 
are  here  considered  under  the  head  of  quartzless  porphyry.  They  are,  in 
fact,  the  semi-crystalline  phases  which  correspond  to  the  completely  crys- 
talline augite-syenites  described  below. 

Macroscopically  these  rocks  show  an  aphanitic  matrix  of  a  dark  red- 
dish-brown or  brown  color,  and  more  or  less  strongly  developed  conchoidal 
fracture.  The  porphyritic  feldspars  vary  considerably  in  size  and  abund- 
ance, but  are  usually  minute.  They  show  habitually  a  red  color,  and  in- 
clude often  striated  as  well  as  unstriated  kinds.  The  thin  sections  generally 
show  a  reddish  background,  with  abundant  brown  ferrite  particles  and 
needles  scattered  through  it.  In  most  sections  more  or  less  of  this  base  is 
isotropic,  being  either  cryptocrystalline  or  truly  glassy  in  its  nature.  There 
are  in  some  slices  darker  and  lighter  bands,  plainly  due  to  flowage.  The 
darker  bands  are  always  the  least  crystalline.  Minute  tabular  feldspars 
are  nearly  always  rather  abundant ;  far  less  so,  however,  than  in  the  sec- 
tions of  the  diabase-porphyrites ;  but  always  much  more  so  than  in  sec- 
tions of  the  acid  felsites,  from  which  they  are  often  completely  absent. 
The  individualized  particles  are,  however,  for  the  most  part  irregular  in 
outline,  and  appear  to  belong  to  both  orthoclase  and  quartz,  which  latter 
mineral  seems  to  be  nearly  always  of  a  secondary  origin,  since  it  com- 
monly occurs  in  the  characteristic  ramifying  forms.  It  is,  however,  far  less 
abundant  in  the  rocks  here  included  than  in  the  felsites  and  the  bases  of 
the  quartziferous  porphyries.     In  a  few  places  these  quartzless  porphyries 


QUAETZLESS  POEPHYEY.  93 

were  found  with  a  tendency  to  a  vesicular  structure,  and  then  the  thin  sec- 
tion shows  flowage  lines  and  isotropic  material  at  the  maximum. 

The  porphyritic  feldspars  are  orthoclase  and  oligoclase,  the  latter  the 
prevailing  one.  Both  are  always  reddened  and  clouded  by  alteration. 
Many  sections  show  also  porphyritic  augites,  with  crystalline  outlines  often 
remaining,  and  always  with  the  peculiar  red  and  brown,  opaque,  ferritic 
alteration-product,  which  characterizes  the  augites  of  all  of  the  acid  rocks 
here  described. 

As  instances  of  the  occurrence  of  these  porphyries — which  are  less  com- 
mon than  any  of  the  other  original  rocks  of  the  Keweenaw  Series — may  be 
mentioned  the  rock  of  the  Stannard's  Rock  reef;  the  prevailing  pebbles  of  the 
Eagle  River  conglomerate,  Keweenaw  Point;  the  massive  to  vesicular  rock 
of  the  old  Suffolk  mining  location  (Praysville),  Keweenaw  Point;  many  of 
the  pebbles  of  the  Portage  Lake  conglomerates ;  the  rock  exposed  in  the  bed 
of  the  Gogogashugun  River  in  the  northern  part  of  Sec.  8,  T.  46,  R.  2  E., 
Wisconsin ;  the  rock  at  the  falls  of  the  Brunschweiler  River,  NW.  |,  Sec. 
22,  T.  45,  R.  4  W.,  Wisconsin;  and  a  rock  showing  on  the  southwest  shore 
of  Michipicoten  Island.^ 

'Called  by  T.  Macfarlane  "porphyritic  melaphyr,"  p.  138,  Eeport  of  Progress  of  the  Geological 
Survey  of  Canada  for  1863  to  1866. 


94 


COPPEK-BEAEmG  EOCKS  OF  LAKE  SUPEEIOE. 


Tabulation  of  the  results  of  a  microscopic  study  of  the  quartzless  porphyries  of  the 

Keweenaw  Series, 


Place. 


Macroscopic  charac* 

ters. 


Microscopio  characters. 


Stannard's      Eock, 
Lake  Superior. 


1781 A 


2616.. 


Pebble  from  Albany 
and  Boston  Con- 
glomerate, Kewee- 
naw Point,  Michi- 
gan. 


Pebble  from  the  con- 
glomerate at  the 
National  Mine, 
HocMand,  Mich. 


NW.., 


8    55 


33  W 


Aphanitic;  dark  red- 
dish-brown; shows 
no  visible  porphy- 
ritio  ingredients. 
Si02,  65.81  per  cent. 


Center 


16    50 


saw. 


Aphanitio;    dark 

chocolate-brown; 
conchoidal  fracture ; 
holds  abundant  mi- 
nute, pinkish,  por- 
phyritic  feldspars. 
SiOa,  65.35  per  cent. 

Matrix  aphanitic;  dark 
reddish-brown;  car- 
ries minute  porphy- 
ritio  tabular  feld- 
spars. 


In  the  ordinary  light,  the  matrix  of  this 
rock  presents  a  general  red  background 
thickly  studded  with  opaque  brown 
ferrites.  The  only  porphyritic  ingredi- 
ents are  augites,  which  are  now  repre- 
sented by  patches  of  red  translucent,  or 
opaque  black  ferrite,  within  which  are 
little  remnants  of  unaltered  augite. 
These  ferrite  patches  often  show  the  crys- 
talline outlines  of  the  original  augites. 
Elongated  holes,  worn  in  the  section  by 
grinding,  were  probably  occupied  by  mi- 
nute decomposed  porphyritic  feldspars. 
In  the  polarized  light,  the  gronndmass 
presents  a  considerable  proportion  of  iso- 
trope  matter,  in  which  are  occasionally 
recognizable  minute  tabular  feldspars. 
Brightly -polarizing,  irregularly-outlined 
particles  and  clusters  of  particles  in  the 
ground-mass  appear  to  belong  to  quartz, 
possibly  also  to  orthoclase.  Some  of 
these  clusters  plainly  belong  to  second- 
ary quartz.  The  low  percentage  of  silica 
(65.81),  the  abundance  of  altered  augite 
and  of  ferrite  particles,  and  the  scarcity 
of  secondary  quartz,  all  serve  to  separate 
this  rock  from  the  more  acid  felsites; 
while  it  is  separated  from  the  diabase- 
porphyrites  by  its  higher  percentage  of 
silica  and  rarity  of  tabular  feldspars. 
Both  in  the  ordinary  and  polarized  lights  the 
fiection  of  this  pebble  resembles  closely 
that  of  the  rock  from  Stannard's  Kock; 
the  large  ferrite  patches,  representing 
altered  augite,  are,  however,  much  less 
abundant  in  this  rock,  which  also  holds 
not  unfrequent  small  porphyritic  oligo- 
clases. 

The  groundmass  of  this  rock  is  much  like 
that  of  1781  A,  containing,  however, 
rather  more  secondary  quartz.  The  por- 
phyritic feldspars  are  oligoclase. 


UNITED  STATES   GEOLOGICAL  SURVEY 


COPPER-BEARING   ROCKS  OF  LAKE  SUPERIOR   PL.  XII 


QUAF?TZ  -PORPHYRIES 


'HYRY. 


.■Uxl 


-.'.""i-^s. 


5^^ 


rle«  minute  rod  por 

phyritio  feldspars.  trope^ 

tion  of  tliatmcrT 

most  of  V " 
1     olasennd'i  ■■ —    , 

^:i!i  i.-kl'.,  .-.    .i'.j  in  iV'l'^alSiSsasW'^iB    -^VS."^ t.3sV^or5«>^ 
iioclase. 
Mintitplv  (rryitfl'  >   contains  little,   if  any,  too- 

Hopic  substance,  tat  ia  made  up  almost 


lip  to  two- 


eutiroly  of  relatiToly  coarso  particles  of  | 
ortUoL'laso  and  quartz,  -with  triclimo  feld- 
spar and  angilo.    Some,  at  least,  of  the 

also 


\" 


-saissS^'^O 


J^tisiitc^  ni-a:{rix  Uj  7rzt?t.  jloTraye,    ■st-rnc^ure.j   corroc^ciZ 


^cZsitic  -nxairixh/  ■<s7ro»iito'  flo-na^e^j    quarts,  (3,J^  oriho- 


J'ty'S.  S  liytd  lA.    Sa-nc?ec2    0-uaTi:z-jaorjahyry     •f'foni     Great  Pplz'iaiies,   .yKiJi-rtesoia.     coait. 
~^a^7-z\xl>;  sViOTyzny  y^oTraye   sirucizir^  ;   ^warizf^y,    cr^Aociase  l3/. 


Orclzjiar^ 


y 


FELSITB  AND  QUAETZ-POEPHYET. 


95 


Tabulation  of  results  of  a  microscopic  study  of  quartzless  porphyries— GontumeA. 


851. 


3057.. 


Place. 


Brunscliweiler's 
Kivor,  AshlaDd 
Connty,   Wiacon- 


DnlTith,  Minn. 


S 

a 


NW... 


22    45 


irw... 


1729.. 


East  end  of  Bead  Isl- 
and, moutli  of  Ni- 
pigon  Straits,  On- 
tario, Canada. 


27    50 


4W 


14  W. 


Macroscopic  charac- 
ters. 


Aphanitio ;  brown ; 
carries  porphyritio 
feldspars  in  long  red 
crystals. 


Microscopic  characters. 


Minutely  crystalline 
to  aphanitic;  dark 
reddish-hrown;  car- 
ries minute  red  por- 
phyritio feldspars. 


Minutely  crystalline ; 
light-brownish;  car- 
ries very  abundant 
porphyritio  red 
feldspars,  up  to  two- 
tenths  of  an  inch  in 
length ;  also  rarer 
and  much  larger 
greenish-grey  feld- 
spars, reaching  five- 
tenths  of  an  inch  in 
length.  Comes  out 
in  thin  tabular  frag- 
ments. SiOa,  64.73 
per  cent. 


Gronndmass  close  to  that  of  1799,  1781  A, 
etc. ,  showing  rather  more  tabular  feld- 
spars and  little  or  no  secondary  quartz. 
There  is  a  considerable  proportion  of 
isotrope  matter,  while  minute  points  of 
augite,  scattered  in  the  matrix,  suggest 
that  the  abundant  minute  ferrites  are 
alteration-products  from  it,  as  the  larger 
ones   evidently   are.     The   porphyritio 
feldspars  are  aIIoligoclases,andar6 much 
altered  to  a  greenish  chlorite. 
In  the  ordinary  light  this  section  shows  a 
groundmass  much  like  that  of  the  last- 
described  rock,  but  in  the  polarized  light 
it  is  seen  to  differ  in  containing  less  iso- 
trope matter,  there  being  a  large  propor- 
tion of  distinctly  individualized  particles, 
most  of  which  probably  belong  to  ortho- 
clase  and  quartz.    The  latter  mineral  oc- 
curs frequently  in  good-sized  clusters  of 
particles.    Augite  particles  are  not  un- 
common through  the  section.     Porphy- 
ritio feldspars  are  in  part  oligoolase  and 
in  part  orthoclase. 
Groundmass   contains  little,   if  any,  iso- 
tropic substance,  but  is  made  up  almost 
entirely  of  relatively  coarse  particles  of 
orthoclase  and  quartz,  with  triclimc  feld- 
spar and  augite.    Some,  at  least,  of  the 
quartz  is  secondary;   magnetite  is  also 
present.    Porphyritio  feldspars  are  very 
large  and  abundant,  and  include  both 
oligoolase  and  orthoclase,  the  latter  often 
finely  twinned.    The  rock  stands  between 
the  more  silicious  of  the  orthoclase-gab- 
bros  and  the  quartzless  porphyries. 


Felsite  and  quartziferous  porphyry. — Rocks  belonging  under  this  head 
play  a  very  important  rdle  in  the  Keweenaw  Series,  not  merely  as  pebbles 
and  bowlders  in  the  conglomerates,  but  also  as  extensive  and  widely  spread 
original  masses,  a  fact  which  is,  as  already  stated,  now  recognized  for  the 

first  time. 

The  non-porphyritic  felsites  and  true  quartz-porphyries  are  here  con- 
sidered together  for  the  reason  that,  in  the  Lake  Superior  region,  as  else- 


96        COPPER-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 

where,  they  often  form  parts  of  the  same  rock,  the  groundmasses  of  the 
two  rocks  being  identical  in  constitution. 

These  rocks  belong  to  a  class  which  is,  as  is  well  known,  one  of  the  most 
difficult  of  all  the  rock  groups  to  study  with  the  microscope  satisfactorily. 
Very  widely  separated  views  have  been  held  as  to  the  nature  of  their 
aphanitic  matrix,  even  since  the  use  of  the  microscope  in  their  study.  The 
differences  of  opinion  have  been  chiefly  as  to  whether  the  matrix  is  crys- 
talline, partly  crystalline,  or  wholly  uncrystalline.  In  his  discussion  of  this 
subject,  Rosenbusch  has  shown  that  both  completely  crystalline  and  partly 
glassy  matrices  occur,  with  many  intermediate  stages,  and  that  uncrystalline 
and  completely  crj^stalline  material  often  occur  intermingled  in  the  same  rock. 
Calling  all  parts  of  the  matrix  which  are  doubly  refracting,  crystalline — so 
long  as  it  cannot  be  shown  that  the  double  refraction  is  a  result  of  mechan- 
ical strain — Rosenbusch  designates  as  microcrystalline  those  porphyritic 
groundmasses,  or  parts  of  groundmasses,  which  are  made  up  of  mineral- 
ogically  determinable  particles;  as  cryptocrystalline  those  which  are  ag- 
gregates of  mineralogically  indeterminable,  but  still  doubly  refracting 
particles,  and  as  glass  or  glass  hasis,  those  which  show  no  polarization  effects 
whatever,  even  when  examined  under  the  highest  powers.  In  place  of  the 
true  glass  basis,  there  is  often  to  be  seen  interwoven  with  the  microcrys- 
talline and  cryptocrystalline  material,  a  completely  isotropic  substance, 
which  is  colorless,  grayish,  yellowish  or  brownish,  and  which  differs  from 
the  true  glass,  in  that  it  is  not  wholly  structureless,  but  is,  on  the  con- 
trary, made  up  of  extremely  minute  scales,  fibers,  granules  or  aggregates 
of  granules,  and  other  grouped  forms.  This  substance,  "which  is  distin- 
guished from  the  microcrystalline  and  cryptocrystalline  aggregates  by  lack 
of  action  upon  the  polarized  light,  and,  on  the  other  hand,  from  the  true 
glass  by  lack  of  structurelessness" — Rosenbusch  calls  microfelsite  or  micro- 
felsitic  base}  In  the  following  descriptions,  the  Rosenbusch  nomenclature  is 
followed. 

The  matrix  of  the  Lake  Superior  quartz-porphyries  and  felsites,  as  seen 
macroscopically,  presents  usually  some  quite  marked  shade  of  red;    the 

'Conf.  H.  Rosenbusch,  Microscopische  Pliysiograpliie  der  Mineralien  und  Gesteine.    Band  II,  pp. 
51-76. 


FELSITE  AND  QUAETZ-POEPHYEY.  97 

various  shades  mentioned  in  my  notes  being  "brick-red,"  "dark  red,"  "dark 
purplish-red,"  "dark  brownish-red,"  "  brick-red,  banded  with  lighter  shades," 
"dark-purple,  banded  with  indefinite  markings  of  light-red,"  "dark  purp- 
lish-gray, blotched  and  banded  with  brick-red,"  "pale-lilac,"  "pinkish-lilac," 
"pink"  and  "light-gray."  Often  there  is  no  appearance  of  banding,  but  as 
often  one  produced  by  waving  or  contorted  lines  or  rows  of  spots  of  lighter 
material.  Occasionally  this  banding  becomes  very  pronounced,  as  in  the 
lower  part  of  the  flow  at  the  Great  Palisades  on  the  Minnesota  coast, 
where  it  is  plainly  a  flowage  result,  even  as  seen  macroscopically.  In  one 
case,  that  of  a  pink  felsite  on  the  Minnesota  coast,  in  the  SW.  ^,  Sec.  28,  T 
56,  R.  7  W.,  the  flowage  structure  is  brought  out  on  a  grand  scale  by 
curving  bands  and  S-like  forms,  sometimes  several  inches  in  width,  of  a 
darker,  more  highly  ferritic  felsite.  This  rock  is  fully  described  and  pictured 
in  a  subsequent  chapter. 

The  matrix  is  always  aphanitic,  and  has  often  a  pronounced  con- 
choidal  fracture,  but  this  is  never  quite  so  prominent  as  in  the  diabase- 
porphyrites  above  described,  and  is  at  times  entirely  wanting,  the  fracture 
surface  presenting  a  rough  and  even  hackly  appearance.  These  felsites  fre- 
quently come  out  of  the  ledge  in  sharp-edged,  angular  and  even  tabular 
fragments,  while  a  very  common  result  of  weathering  is  the  leaving  of  wedge- 
shaped  or  three-cornered  cores,  which  fall  from  a  cliflf-side  in  showers  at 
the  slightest  blow  of  the  hammer.  The  silica  content  of  the  aphanitic  matrix 
appears  to  be  high,  ranging  in  all  cases  where  a  determination  was  made 
between  72  and  77  per  cent.  The  matrix  is  fusible  before  the  blow-pipe, 
but  with  difficulty. 

Of  the  porphyritic  ingredients  the  feldspars  are  the  most  commonly 
present,  appearing  macroscopically  in  regularly  outlined  crystals,  which  are 
usually  of  a  red  color,  though  occasionally  white  and  porcellaneous,  and 
range  in  size  from  particles  so  minute  as  to  be  barely  visible  to  the  naked 
eye,  to  crystals  a  fourth  of  an  inch  and  even  half  an  inch  in  length.  The 
porphyritic  feldspars  often  show  striated  surfaces.  In  the  banded  porphy- 
ries they  sometimes  conform  roughly  to  the  contorted  banding,  forming 
strings  of  crystals  ;  and  again  they  lie  across  the  course  of  the  bands,  which 
will  then  curve  around  them.     The  whole  structure  is  plainly  one  due  to 

7   L   S 


98        COPPBR-BEAEmG  EOCKS  OP  LAKE  SUPEEIOE. 

flowage  in  a  molten  state.  While  often  present  without  the  porphyritic 
quartzes,  when  at  all  abundant  and  large  the  feldspars  are  almost  always 
accompanied  by  the  quartzes.  These  present  an  invariable  appearance,  i.  e., 
a  glassy  rough  surface,  which  has  a  black  color  owing  to  the  dark  back- 
ground in  which  the  crystals  lie.  They  are  rarely  very  minute,  running  from 
one-twentieth  to  one-fifteenth  of  an  inch  in  diameter.  Both  of  the  poi'phy- 
ritic  constituents  vary  greatly  in  amount,  at  times  sinking  out  of  sight,  when 
the  rock  becomes  a  felsite,  and  again  nearly  equaling  the  matrix  in  com- 
bined quantity. 

In  the  thin  section  the  matrix  of  these  rocks  is  only  rarely  colorless, 
as  seen  in  the  ordinary  light,  being  commonly  more  or  less  thoroughly 
stained  red  by  minute  particles  of  iron  oxide.  Some  sections  show  red  and 
white  material  blotched  or  interbanded  in  waving  non-continuous  bands. 
Very  characteristic  are  the  deep-brown,  and  deep-red  to  black,  translucent 
to  opaque,  irregularly  outlined,  or  needle-shaped  ferrites.  These  range  from 
the  most  minute  particles  to  pieces  which,  with  a  low  power,  will  run  a 
quarter  of  the  way  across  the  field.  They  occur  in  both  red  and  white 
portions,  where  the  sections  present  a  blotching  or  banding  of  these  colors  ; 
but  are  more  abundant  in  the  red.  They  are  at  times  without  any  appa- 
rent arrangement,  and  again  may  show  a  crowding  in  the  neighborhood  of 
the  porphyritic  ingredients ;  while  in  many  sections  they  present  a  pro- 
nounced linear  arrangement,  as  is  shown,  for  instance,  in  Fig.  1  of  Plate  XII. 
In  some  of  the  sections  in  which  the  fluidal  structure  is  brought  out  in 
especial  prominence  by  a  very  marked  interbanding  of  colorless  and  red- 
stained  portions — as  for  instance  in  those  of  the  banded  rock  of  the  Great 
Palisades  of  the  Minnesota  coast — the  ferrite-needles  are  abundant  and 
large-sized  in  the  latter  bands,  and,  while  showing  a  general  tendency  to 
follow  the  courses  of  the  bands,  they  yet  lie  across  each  other  at  small 
angles.  The  appearance  produced  is  strongly  suggestive  of  the  common 
brush-fence,  a  simile  used  by  Zirkel  in  describing  a  similar  arrangement  of 
ferrite-needles  in  some  of  the  western  rhyolites.-'  Figs.  3  and  4  of  Plate 
XII  illustrate  this  peculiarity. 

The  red-stained  material  which  makes  up  the  whole  groundmass  of 

'  Geological  Exploration  of  the  Fortieth  Parallel,  Vol.  VI. 


FELSITE  AND  QUAETZ-PORPHYEY.  99 

many  sections,  and  much  of  nearly  all  sections,  appears  to  be  made  up  for 
the  most  part  of  microfelsitic  and  cryptocrystalline  matter.  Only  very 
rarely — as  in  the  light-gray  and  lilac  quartz-porphyries  of  the  islands 
off  the  north  point  of  Beaver  Bay,  Minnesota — is  there  present  a  non-jiolar- 
izing  material  without  the  red  stain.  Distinctly  recognizable  orthoclase 
and  quartz  particles — as,  for  example,  in  a  brick-red  felsite  from  the  Avest- 
ern  part  of  the  Porcupine  Mountains — are  not  often  to  be  found,  though  in 
some  sections  there  may  be  seen  floating  in  the  isotropic  material,  as  in  a 
cloud,  minute  tabular  crystals  of  some  feldspar.  True  glass  is  supposed  to 
be  present  in  many  sections,  interwoven  in  minute  particles  with  the  crypto- 
crj^stalline  and  microfelsitic  matter,  but  the  determinations  made  are  not 
regarded  as  satisfactory  on  account  of  the  imperfections  of  the  instrument 
employed.  The  large  proportion  of  scaly  and  granular  material  which 
produces  no  effect  on  the  polarized  light,  and  the  rarity  of  true  micro- 
crystalline  matter,  show,  however,  that  we  have  here  to  do  with  a  sub- 
stance very  close  to  the  original  glassy  condition,  even  if  it  does  not  con- 
tain true  glass. 

The  white  areas  and  bands,  which  in  some  sections  interrupt  the  prevail- 
ing red  background,  are  always  found  to  be  made  up  of  completely  crys- 
talline matter,  and  to  include  orthoclase  and  quartz  in  aggregated  irregular 
grains,  although  it  is  often  difficult,  in  a  given  section,  to  separate  these 
materials  from  one  another.  The  peculiar  interlocking  of  this  more  crys- 
talline colorless  material  with  the  prevailing  isotropic,  ferrite-bearing,  reddish 
base,  is  plainly  and  most  beautifully  seen  in  many  sections  to  be  the  result 
of  flowage.  All  of  the  peculiar  and  characteristic  irregularities  which  ac- 
company the  flowage  structure  are  present  here,  viz,  sudden  angular  or 
curving  bends  in  the  bands,  damming  against  the  porphyritic  ingredients, 
streams  of  ferritic  particles,  brief  continuity  of  individual  bands,  etc.  I 
have  attempted  to  illustrate  this  fluidal  structure  in  Figs.  3  and  4  of  Plate 
XII,  and  in  Figs.  8,  9,  10,  11,  and  12  of  Plate  XIII. 

Although  wholly  absent  from  some  sections,  a  very  highly  characteristic 
feature  of  the  sections  of  many  of  these  rocks,  and  more  particularly  of  the 
felsites  without  porphyritic  quartz,  is  a  networked  quartz  which  can  only 
be  regarded  as  of  secondary  origin.     I  find  no  mention  of  such  a  feature 


100       COPPER-BEAEmG  EOCKS  OF  LAKE  SUPEEIOE. 

in  any  of  the  descriptions  of  the  felsites  of  other  regions  that  I  has^e  exam- 
ined. Only  occasionally,  as  in  the  pink  felsite  from  the  S.  W.  ^  of  Sec.  28, 
T.  56,  R  7  W.,  on  the  Minnesota  coast  (see  Figs.  15  and  16,  Plate  XIIl),  is 
this  networked  quartz  coarse  enough  to  be  readily  seen  with  a  low  power, 
in  the  ordinary  light.  Usually  both  a  high  power  and  the  use  of  the  polar- 
ized light  are  required  for  its  detection,  when  it  appears,  in  its  most  charac 
teristic  development,  as  a  delicate  arborescent  tracery  or  frost-woi-k  satu- 
rating the  groundmass  in  all  directions.  In  the  polarized  light  all  of  the 
quartz  network  within  each  of  numberless  irregularly  round  areas,  whose 
existence  would  not  be  suspected  in  the  ordinary  light,  is  found  to  be  simi- 
larly oriented. 

From  these  more  pronounced  developments  the  secondary  quartz  is 
found  through  many  degrees  of  lessening  amount,  and  less  plainly  marked 
character,  until  it  disappears  altogether.  It  is  plainly  of  the  same  nature 
as  the  secondary  quartz  of  the  already  described  orthoclase-gabbro,  diabase- 
porphyrite,  and  quartzless  porphyry,  and  of  the  augite-syenite  described 
below.  It  never,  however,  reaches  in  the  rocks  now  under  description  the 
coarseness,  nor  presents  the  graphic  form,  with  which  it  appears  in  the 
augite-syenites,  its  characteristic  development  here  being  the  delicate  arbo- 
rescent clusters  above  mentioned.  Whether  this  secondary  quartz  may 
ever  be  rather  a  result  of  devitrification  than  a  truly  secondary  or  alter- 
ation-product I  have  no  means  of  deciding,  though  it  is  certainly  the  latter 
often,  and  I  should  suppose  always.  It  surely  can  have  had  no  connec- 
tion with  the  original  solidification  of  the  rock. 

The  absence  in  the  Lake  Superior  felsites  and  felsitic  porphyries,  so  far 
as  my  observations  have  extended,  of  anything  like  a  true  spherulitic  struc- 
ture, such  as  is  so  often  met  with  in  rocks  of  this  class  from  other  regions, 
is  worthy  of  note.  The  occasional  radial  arrangement  of  ferrite  needles  and 
lines  of  secondary  quartz  may  indicate  such  a  structure,  but  these  appear- 
ances are  rare  and  feebly  characterized. 

One  or  two  other  unusual  occurrences  in  the  thin  sections  of  the  matrix 
of  these  rocks  need  description.  One  of  these  is  a  faintly  greenish,  wholly 
isotropic  substance,  which  is  present  in  some  of  the  Beaver  Bay  porphyries 
in  elongated  bands  and  irregular  patches.     Whether  it  is  to  be  regarded  as 


^  ^:.'. 


§^]  ^ 1 ^ 


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UN'ITED  STATES   GEOLOGICAL  SURVEY 


COPPER-BEARING   ROCKS  OF  LAKE  SUPERIOR   PL.  Xlll 


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FELSITB  AND  QUAETZ-PORPHYEY.  101 

partially  altered  glass,  I  am  uncertain.  The  other  occurrence  referred  to  is 
that  of  curvilinear  aggregations  of  brown  and  red  ferrite  particles,  large 
enough  to  be  seen  macroscopically,  in  the  hand  specimen,  as  hair-like 
markings.  These  characterize  the  felsite  from  the  Minnesota  coast  above 
mentioned  as  remarkable  for  the  coarseness  of  its  secondary  quartz  network. 
This  rock  is  illustrated  in  Figs.  15  and  16  of  Plate  XIII. 

The  porphyritic  feldspars  in  the  thin  section  are  found  to  be  either  or 
both  of  orthoclase  and  oligoclase.  They  are  always  turbid  from  decompo- 
sition, and  are  more  commonly  red-stained  than  not.  They  have  always 
crystalline  outhnes,  or,  when  they  have  been  eaten  into  by  the  still  fluid 
matrix,  as  is  not  seldom  found  to  have  been  the  case,  at  least  the  remnants 
of  such  outlines.  In  a  number  of  sections  the  feldspars  are  seen  to  have 
been  not  only  eaten,  but  also  much  shattered  before  the  solidification  of  the 
surrounding  magma. 

The  porphyritic  quartzes  present  all  the  usual  characters  of  the  quartzes 
of  similar  rocks  the  world  over.  They  are  random  sections  of  dihexahe- 
dral  crystals  (double  pyramids  due  to  combination  of  the  two  rhombohe- 
drons),  with  now  and  then  some  development  of  the  lateral  (prismatic)  faces. 
The  rhombohedral  angle  being  only  a  few  degrees  over  90°  (94°.15),  the 
sections  of  these  crystals  present  a  nearly  square  shape.^  Usually  they 
are  more  or  less  rounded  and  eaten  into  by  the  matrix,  many  odd  forms  re- 
sulting from  this  corrosion.  In  nearly  all  cases,  however,  some  traces  of 
the  original  outHne  remain,  with  the  aid  of  which,  along  with  the  behavior 
between  the  crossed  nicols,  it  is  always  easy  to  ascertain  the  crystallo- 
graphic  directions  of  these  eaten  crystals.  In  the  series  of  figures  on  Plate 
XIII,  I  have  placed  a  number  of  these  quartzes  with  the  crystallographic  axes 
in  a  vertical  position  so  that  they  may  be  compared  with  one  another.^ 

Included  in  the  porphyritic  quartzes  are  particles  or  patches  of  the  red- 
stained  microfelsitic  or  cryptocrystalline  groundmass.  In  most  cases  these 
have  had   originally   a   connection  with  the  rest  of  the  groundmass  by 

'N.  H.  Winoltell  (Nintli  Annual  Keport  of  the  Geological  and  Natural  History  Survey  of  Minne- 
sota, pp.  21,  33,  &c.),  has  called  the  dihexahedral  quartzes  of  the  quartziferous  porphyry  of  the 
Palisades  and  other  points  on  the  Minnesota  coast,  adularia. 

2Rutley  (Study  of  Rocks,  p.  210),  speaks  of  roundish  blebs  of  quartz  as  characterizing  quartz- 
porphyries  generally,  but  his  "  blebs"  are  only  rounded  crystals. 


102  COPPEE-BEAEESTG  BOOKS  OF  LAKE  SUPEEIOE. 

channels  through  the  quartz,  either  above  or  below  the  plane  of  section. 
In  some  cases,  however,  I  have  noticed  these  groundmass  inclusions  sur- 
rounded by  the  quartz  in  such  a  way  as  to  render  it  probable,  at  least,  that 
they  are  veritable  inclusions  of  the  groundmass  dating  from  the  time  of 
crystallization  of  the  quartz.  In  several  sections  of  North  Shore  porphyries, 
and  especially  in  those  of  the  Grreat  Palisades  and  of  Baptism  River  point  on 
the  Minnesota  coast,  unmistakably  genuine  inclusions  of  a  true  glass  are 
to  be  seen  in  the  porphyritic  quartzes.  These  glass  inclusions  are  in  doubly 
terminated  "negative  crystals,"  conforming  in  position  exactly  with  the 
crystal  in  which  they  are  found,  and  are  of  sufficiently  large  size  and 
thickness  to  test  satisfactorily  with  the  polarized  light.  They  show  com- 
monly more  or  less  of  a  trichitic  devitrification.  Two  of  these  glass 
"negative  crystals"  are  figured  on  Plate  XIII,  at  Figs.  6  and  8. 

PorphjTitic  augites,  while  far  less  frequent  than  the  quartzes,  are  yet 
not  very  unfrequently  to  be  met  with  in  sections  of  these  rocks.  They 
always  have  crystalline  outlines,  or  remnants  of  them,  being  commonly 
more  or  less  deeply  eaten  into  like  the  other  porphyritic  ingredients.  Their 
chief  characteristic  is  the  ferritic  decay  that  they  have  undergone,  the 
whole  mass  of  the  crystal  being  often  represented  by  an  opaque,  brown,  or 
deep-red,  or  black  mass  of  iron  oxide. 

I  have  already  discussed,  briefly,  the  question  of  the  origin  of  the 
Lake  Superior  felsites  and  quartziferous  porphyries,  and  the  same  question 
is  referred  to  hereafter  in  other  connections.  Here  it  is  sufficient  to  say  that 
the  marked  fluidal  structure  so  often  seen,  both  on  the  large  scale  and  mic- 
roscopically; the  corroded  quartzes;  the  glass  inclusions  in  these  quartzes; 
the  near  approach  of  the  groundmass  to  the  glassy  condition;  the  complete 
identity  of  these  felsitic  roc^s  with  others  universally  conceded  to  be  of 
eruptive  origin,  and  their  very  close  similarity  to  the  undoubtedly  eruptive 
rhyolites — all  combine  to  make  up  an  irresistible  argument  in  favor  of  an 
eruptive  origin  for  these  rocks  also. 

As  typical  localities  for  these  rocks — including  only  places  where  they 
occur  as  original  masses — maybe  mentioned  the  following:  (1)  for  the  non- 
porphyritic  felsites — Mount  Houghton,  Keweenaw  Point;  the  central  area 
of  the  Porcupine  Mountains,  and  especially  the  great  ledges  in  Sec.  35,  T. 


FELSITE  AND  QUAETZ-POEPHYET.  103 

61,  R  43  W.,  and  again  in  Sec.  31,  T.  50,  R  44  W.;  the  Minnesota  coast 
in  the  S.  W.  ^,  Sec.  28,  T.  56,  R.  7  W.;  the  same  coast  immediately  below- 
Grand  Marais;  the  bed  of  the  Devil's  Track  River,  Minnesota,  for  several  miles 
from  its  mouth;  and  the  islands  off  the  harbor  on*the  south  shore  of  Michi- 
picoten  Island;  (2)  for  the  kinds  carrying  porphyritic  orthoclase,  but  no 
quartz — the  central  area  of  the  Porcupine  Mountains,  where  much  of  the 
rock  is  of  this  character;  the  N.  W.  4  of  Sec.  12,  T.  37,  R.  16  W.,  in  the 
Clam  Falls  region,  Polk  County,  Wisconsin;  and  the  Minnesota  coast, 
ten  miles  above  the  mouth  of  Split  Rock  River;  (3)  for  the  quartziferous 
kinds — the  line  of  the  Torch  Lake  Railroad,  Keweenaw  Point,  Sec.  36,  T. 
56,  R  33  W.;  the  hill  known  as  the  "North  Brother,"  near  Rockland,  Mich., 
N.  E.  I,  Sec.  9,  T.  50,  R.  39  W.;  the  bold  bluffs  in  the  northern  part  of  T. 
49,  R.  42  W.,  Mich. ;  the  central  area  of  the  Porcupines,  where,  however, 
the  prevailing  rock  is  without  porjohyritic  quartz ;  the  bed  of  Potato  River, 
S.  E.  1,  Sec.  15,  T.  46,  R.  1  W.,  Wisconsin;  the  mouth  of  Tyler's  Fork  of 
Bad  River,  S.  E.  4,  Sec.  17,  T.  45,  R  2  W.,  Wisconsin;  the  islands  off  the 
north  point  of  Beaver  Bay,  on  the  Minnesota  coast ;  the  Great  Palisades, 
Baptism  River  point,  and  Red  Rock  Bay,  all  on  the  same  coast ;  Bead  Island 
at  the  mouth  of  Nipigon  Straits  on  the  Canadian  coast ;  and  the  east 
shore  of  Michipicoten  Island. 

The  detailed  descriptions  of  the  following  tabulation  cover  a  sufficient 
number  of  occurrences  to  substantiate  the  general  descriptions  above  given. 
Other  thin  sections  of  these  rocks  are  briefly  described  in  connection  with 
the  detailed  descriptions  of  Chapters  VI  and  VII.^ 

'Mr.  M.  E.  Wadsworth,  who  has  described  (op.  cit.,  pp.  113-120)  a  number  of  these  sections  of  peb- 
bles of  felsite  and  of  the  graulte-like  rocks  which  I  describe  below  under  the  name  of  augite-syenite, 
was  the  first  to  note  the  occurrence  of  secondary  quartz,  and  of  an  apparently  spherulitic  structure  in 
these  rocks. 


104 


COPPER-BEAEmG  EOCKS  OF  LAKE  SUPERIOR. 


Tabulation  of  the  results  of  a  microscopic  study  oftlie  felsites  and  felsitic  porphyries  of 

the  Keioeenaw  Series, 


Place. 


Macroscopic    charac- 
ters. 


Microscopic  descriptions  of  thin  sec- 

tions. 


Mount  Honghton,  Ke- 
weenaw Point. 


"W.Une. 


29W. 


190S.. 


1846  <{ 


Mount  Houghton,  Ke- 
weenaw Point. 


Pehble  from  Eagle 
Biver  conglomerate; 
mouth  of  Eagle 
Kiver,  Keweenaw 
Point. 


■W.line. 


24 


58 


29  W. 


NW. 


19 


58 


31  "W. 


1838. 


Torch  Lake  Bailroad, 
Keweenaw  Point. 


S.  part. 


36 


56 


33  "W. 


Aphanitic;  pink  to 
hrick-red;  no  visible 
porphyritic  ingredi- 
ents; very  hard. 
Si02,  76.9  per  cent. 
Difficultly  fusible. 


Aphanitic;  hard, 
light-pinkish;  sharp- 
ly angular  fracture ; 
no  porphyritic  ingre- 
dients.  SiOa,  77.2 
per  cent. 

Matrix  aphanitic,  pur- 
plish-red,  hard; 
abundant  large 
black  quartzes  and 
flesh-red  feldspars. 


Aphanitic;  dark-red, 
hard ;  very  abun- 
dant  and  large, 
black,  porphyritic 
quartzes  reaching 
two-tenths  inch  in 
diameter,  and  red 
feldspars  two-tenths 
to  one-quarter  inch 
in  length.  Some  of 
the  feldspars  are 
plainly  striated.  Ee- 
sembles  1970  and 
1846  d. 


Much  noD -polarizing  matter,  through 
which  are  seen  scattered,  when 
viewed  between  the  crossed  nicols, 
minute  bright  points  and  lines ;  also 
a  few  relatively  large,  scattered  nests 
of  aggregated  particles  of  quartz.  The 
whole  section  is  stained  with  red 
ferrite,  which  is  also  aggregated  in 
numerous  irregular,  opaque  particles. 

This  section  differs  from  the  proceeding 
in  containing  much  less  red  ferrite, 
and  less  non-polarizing  matter;  and 
in  containing  many  polarizing  parti- 
cles often  arranged  in  a  felt-like  mass. 
Many  of  the  particles  are  plainly 
tabular  feldspars. 

The  matrix  is  much  stained  with  red 
ferrite,  and  shows  but  feeble  polariza- 
tion in  flocks  of  small  particles ;  some 
small  non-polarizing  areas.  In  addi- 
tion to  the  general  red  stain  are  abund- 
ant brown  and  black,  opaque  ferrite 
particles.  Porphyritic  quartzes  large, 
in  the  usual  doubly  terminated  crys- 
tals, with  embayments  and  inclusions 
of  the  matrix.  The  porphyritic  feld- 
spars are  oligoclase.  Augite  occurs 
also  porphyritically  in  particles  as 
large  as  the  quartz,  and  with  rounded 
contours ;  these  augites  are  filled 
with  a  brown,  ferritlc  alteration-pro- 
duct. 

Matrix  irregularly  mottled  pink  and 
nearly  colorless,  these  mottlings  being 
so  arranged  as  to  suggest  flowage. 
The  darker  portions  of  the  matrix 
are  thickly  studded  with  minute  fer- 
rite needles,  so  arranged  as  to  empha- 
size the  fiuidal  structure  very  strong- 
ly, especially  in  the  neighborhood  of 
the  porphyritic  quartzes.  The  darker 
portions  of  the  matrix  affect  the  polar- 
ized light  only  in  a  few  minute  points. 
The  lighter  portions,  on  the  contrary, 
appear  to  consist  of  wholly  individu- 
alized quartz  and  orthoclase  confused- 
ly intercrystallized.  Calcite  is  also 
occasionally  seen  in  these  lighter  por- 


FELSITE  AND  QUAETZ-PORPHYET. 


105 


Taiulation  of  the  results  of  a  microscopic  study  of  the  felsites  and  felsitic  porphyries 

of  the  Keweenaw  Series — Continued. 


i 

a 
g| 

Place. 

1 

ti 
o 

■■a 

i 

Macroscopic    charac- 
ters. 

Microscopic  descriptions  of  thin  sec- 
tions. 

& 

CO 

H 

M 

tions.    The  porphyritio  quartzes  are 

very  large  and  commonly  much  eaten, 

with  the  usual  embayments  and  in- 

clusions of  the  matrix.    The  feldspars 

are  chiefly  oligoclase,  rarely  ortho- 

clase;    both   always  much   clouded 

firom  alteration.    There  are  also  pres- 

ent rare  porphyritio  augites,  largely 

replaced  by  a  black  opaque  substance. 

See  Fig.  I,  Plate  XIL. 

1970.. 

Pebble  from  tbe  Calu- 
met   conglomerate, 
Keweenaw  Point. 

23 

56 

83  W. 

Matrix  darli  reddish- 
brown,     aphanitic ; 
sharply  angular  con- 
choidal  fracture; 
very  abundant  pink 
feldspars  up  to  one- 
half  inch  in  length, 
also  black  quartzes 
one-tenth    to    two- 
tenths    inch  in   di- 
ameter ;    resembles 
1846  <2. 

Differs  from  the  preceedlng  only  in  hav- 
ing much  less  of  the  whitish  individu- 
alized areas  in  the  matrix,  nearly  the 
whole  of  which  presents  a  brownish 
staining  and  produces  no  defiuite  effect 
onpolarizedlight.  The ferrite  needles 
are  also  somewhat  more  minute  than 
in  the  proceeding  section.  One  of  the 
quartzes  carries  a  sharply  outlined, 
fresh,  brilliantly  polarizing  augite 
crystal,  the  augite  crystals  of  the  ma- 
trix being  wholly  replaced  by  a  black- 
ish substance.    See  Fig.  2,  Plate  XII, 

2514.. 

Porcupine  Monntains, 
MioMgan,    2000    IT. 
700  W. 

IT.  line. 

5 

50 

43  W. 

Aphanitic ;  pale-lilac ; 
no   porphyritio   in- 
gredients. 

Nearly  colorless,  faintly  tinted  with 
pink;  minute  tabular  feldspars;  some 
networked  secondary  quartz;  some 
non.polarizing  material;  opaque  fer- 
rite particles  not  abundant. 

25S1.. 

Bed  of  Carp   Eiver, 
Porcupine  Monn- 
tains,  Michigan,  1420 
IT.    1400  W. 

XW. 

35 

43 

43-W. 

Aphanitic;  dark  pur- 
plish-red;  no    por- 
phyritio ingredi- 
ents. 

Blotched  red  and  colorless ;  the  reddish 
tinted  portions  chiefly  nou-polarizlng, 
the  white  portions  composed  entirely 
of  individualized,  often  relatively 
coarse  quartz  and  orthoolase;  net- 
worked secondary  quartz  rare;  fer- 
rite particles  not  abundant. 

2574.. 

Porcupine  Mountains, 
Michigan,  500   N. 
1450  W. 

sw. 

20 

51 

42W 

Aphanitic;  dark  pur- 
plish.red;  a  few  mi- 
nute dark  quartzes. 

Thickly  studded  with  minute  brown 
ferrite  particles,  which  are  also  aggre- 
gated into  large  patches;  appear- 
ance in  ordinary  light  pretty  homoge- 
neous. In  the  polarized  light  the  ma- 
trix is  seen  to  be  completely  saturated 
with  secondary  quartz,  which  pre- 
sents itself  in  irregularly  rounded 
areas  each  of  which  is  a  clo-sely  in- 
volved network  of  non-polarizing 
base  and  quartz.  Allof  the  quartz  in 
one  of  these  areas  polarizing  together, 
it  foUows  that  it  all  belongs  to  one 
Individual.  A  few  minute,  altered 
porphyritic  feldspars. 

lOG 


GOPPBE-BBAEIlSrG  BOOKS  OF  LAKE  SUPBRIOE. 


Tabulation  of  the  results  of  a  microscopic  study  of  the  felsites  and  felsitic  por2)liyries 

of  the  Keweenaw  Series — Contiuued. 


IS  o 


1247 


1259.. 


Place. 


Bed  of  Little  Carp 
Kiver,  Porcupine 
Mountains,  M  iclii- 
gan,  1S50N.  eOOW. 


<y 


XE. 


Porcupine  Mountains, 
Michigan,  270  "W. 


1263.. 


1621, 


Porcupine  Mountains,     SE. 
Michigan,  200  W. 


S.  line 


Bed  of  Potato  River, 
Ashland  County, 
"Wiaconsin. 


371. 


SE. 


32 


50 


WW 


WW 


Macroscopic  charac- 
ters. 


Aphanitic;  dark  pur- 
plish-red ;  some  mi- 
nute porphyritic 
quartzes  and  feld- 
spars. 


51     43"W 


15     46 


Mouth  of  Tyler's 
Fork,  Ashland 
County,  Wisconsin. 


SE. 


17 


IW 


45 


Microscopic  descriptions  of  thin  sec- 
tions. 


Aphanitic ;  bright-red 
very  i^lainly  "banded 
with  lighter  shades; 
porphyritic  white 
orthoclase  rather 
abundant,  often  ly- 
ing across  two  or 
three  bands. 


Aphanitic;  dark-red 
closely  banded  with 
lighter  red ;  no  por- 
phyritic ingredi- 
ents. 

Aphanitic;  pale  lilac- 
tinted  base,  thickly 
studded  with  white 
porcellaneous  crys- 
tals of  feldspar 
reaching  oue-eighth 
inch  in  length,  and 
smaller  black  glassy 
quartzes. 


2W 


Much  altered  and  soft- 
ened, aphanitic, 
brick-red  matrix, 
scattered  through 
which  are  minute 
brighter  red  ortho- 
clases  and  very 
abuuda,nt  larger 
quartzes. 


Matrix  stained  with  ferrite,  and  satura- 
ted with  secondary  quartz  as  in  the 
last  described.    There  are  also  some- 
what abundant  larger  quartz  areas, 
apparently    also   secondary,   besides 
which  there   are  the  usual  sharply 
marked  quartzes  and  feldspars,  the 
latter  much  reddened  and  altered. 
Here  and  there  a  quite  perfectly  de- 
veloped augite  crystal  is  seen.    The 
ferrite  particles  are  arranged  so  as  to 
indicate  flowage. 
In  the  thin  section  the  banding  is  seen 
to  be  produced  by  the  piesence  of 
much  oxide  of  iron  in  some  bands  and 
absence  of  it  in  others:    the  latter 
bands  are  also  more  highly  crystalline, 
but  all  of  the  section  presents  an  un- 
usual quantity  of  individualized  mat- 
ter, apparently  both  quartz  and  ortho- 
clase.     The  bands  are  non-continuous 
even  in  the  breadth  of  a  thin  section, 
lu  the  thin  section  the  lighter  bands  are 
seen  to  contain  much  more  and  rela- 
tively coarser  secondary  quartz  than 
the  other  bands,  which  are  in  turn 
relatively  rich  in  ferrite  particles. 
In  ordinary  light  the  matrix  appears  of 
a  general  gray  color,  with  thickly  scat- 
tered ferrite  particles,  which,  for  the 
most  part  transmit  a  reddish  light, 
even  when  very  thick.    In  polarized 
light  this  matrix  appears  to  be  satura- 
ted  with  networked   quartz.     The 
very  abimdant  quartzes  present  all 
the  usual  characters.    The  feldspars 
are  all  turbid  and  appear  to  be  wholly 
orthoclase.    Some  sections  have  the 
ferrite  particles  and    the  secondary 
quartz  arranged  in  indefinite  lines  so 
as  to  suggest  tlowage. 
The  base  is  like  that  of  the  rock  last 
described,  but  is  penetrated  through 
and  through  by  veinlets  of  quartz  and 
caloite.     The   ferritic    particles    are 
more  thickly  crowded  in  the  vicinity 
of  the  porphyritic  ingredients,  and 
now  and  then  show  a  tendency  to  a 
linear  arrangement.    The  quartzes  are 
much  eaten,  and  are  penetrated  to  an 


FELSITE  AND  QUARTZ-POEPHTEY. 


107 


Tabulation  of  the  results  of  a  microscopic  study  of  the  felsites  and  felsitie  porphyries 

of  the  Keweenaw  Series — Continued. 


n 

Place. 

1 

H 

j 

Macroscopic    charac- 
ters. 

Microscopic  descriptions  of  thin  sec- 
tions. 

extraordinary  extent  by  club-shaped 
masses  of  the  base.    The  orthoclases 

are  very  much  reddened  and  kaolinized. 
Several  inclusions  of  partially  devitri- 
fied  glass  were  found  in  the  quartzes. 
A  few  porphyritic  augites,  largely  al- 
tered to  red  iron  oxide  are  contained. 

800  S. 

Clam  Falls   District, 
Polk       County, 

NW. 

12 

37 

16  W 

Aphanitic;  pinkish- 
red  matrix  with  mi- 

The thin  section  shows  a  matrix  com- 
pletely saturated  with  a  network  of 

708... 

■Wisconsin. 

North  shore  Lake  Su- 
perior,   two    miles 
above  mouth  of  Split 
Eock  Elver,  Minne- 
sota. 

irw. 

13 

54 

9W. 

nute  feldspar  faces. 

Aphanitic ;  brick-red, 
much  decomposed 
and  softened.    Car- 
ries minute  pink  or- 
thoclases. 

secondary  quartz ;  the  quartz  network 
is  coarser  than  usual.  Theporphyritic 
feldspars  are  wholly  triclinic. 
Matrix  largely  of  non-polarizing  ma- 
terial staiued  red,  and  containing  min- 
ute tabular  crystals,  also  areas  of  sec- 
ondary  quartz.    Confusedly   inter- 
mingled with   these   deeper  colored 
areas  are  lighter   ones,   occasionally 
colorless,  in  which  there  is  a  larger 
proportion  of  individualized  material, 
apparently    quartz.     The  section  is 
dotted  throughout  with  minute  points 
of  ferrite.    The  porphyritic  ingredi- 
ents are  orthoclase  and  oligoclase  in 
not  very  abundant,  small,  and  mucli 
altered  crystals,  some  of  which  pre- 
sent the  appearance  of  having  been 
much  fractured  before  the  solidifica- 

711... 

North  shore  Lake  Su- 
perior,   one-quarter 
mile  east  of  708. 

irw. 

13 

54 

9W. 

Light-red ;   obscurely 
banded  ■with  dark- 
red  ;  aphanitic;  sub- 
conchoidal  fracture; 
carries      abundant 

tion  of  the  matrix. 
Similar  to  70S,    but   with   very  much 
more  secondary  quartz. 

730... 

Bed   of   SpUt   Eock 
Elver,  Minnesota. 

NW. 

1 

64 

9W. 

irregular,    black, 
hair-like  markings, 
Aphanitic ;  dark  red ; 
conchoidal  frac- 
ture ;    carries   min- 
ute red  feldspars. 

Near  to  708  and  711 ,  with  the  red  staining 
more  general ;  in  other  worda,  there  ia 
but  little  of  the  lighter  tinted,  more 
individualized  material;  secondary 
quartz  throughout  in  minute  arbores- 

790... 

South  shore  of  Beaver 
Bay,  Minnesota. 

SE. 

12 

55 

8W. 

Aphanitic;      light- 
gray,  banded   "with 
non-continuous 
pinkish  bands,  the 
middle    portion    of 
each  baud  being  oc- 
cupied by  a  quartz 
seam. 

cent  clusters. 
Colorless,  merely  varying  in  transpar- 
ency.   The  more  transparentportions, 
which  are  distinctly  arranged  in  bands, 
are  composed  of  quartz  in  relatively 
large  areas.    The  clouded  bands  pre- 
sent, in  the  polarized  light,  a  dark 
background,    thickly   crowded  with 
minute  polarizing  points  which  are  in 

108 


COPPEE-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 


Tabulation  of  the  results  of  a  microscopic  studg  of  the  felsites  and  felsitic  porphyries 

of  the  Keweenaw  Series — Continued. 


a  c 


818. 


820. 


Place. 


Island  on  north  side 
ot  Beaver  Bay,  Min- 
nesota. 


Cedar  Island,  north 
shore  Lake  Super- 
ior, Minnesota. 


852... 


O* 


NE. 


NE. 


North  shore  Lake  Sa- 
perior,  Minnesota. 


55 


55 


SW. 


28 


8  TV. 


SW. 


56 


Macroscopic    charac- 
ters. 


TW. 


Aphanitlc;  conch oi- 
dal ;  dark  pur- 
plish-red, banded  by 
indefinite  waving 
bands  of  light-red. 
Porphyritic  ingredi- 
ents: quartz,  very 
abundant  in  crystals 
one-tenth  to  one- 
twentieth  inch  in 
diameter,  and  pink 
feldspars  one-twelfth 
inch  In  length.  The 
feldspars  tend  to 
have  their  longer 
axes  in  the  direction 
of  the  banding. 
Si02, 76.83  per  cent. 

Matrix  aphanitic, 
dark  purplish-gray, 
blotched  andbauded 
with  red;  very 
abundant  porphy- 
ritic pink  feldspars 
one-tenth  inch  in 
length,  and  more 
minute  quartzes. 
The  arrangement  of 
feldspars,  and  the 
fine  red  banding,  in- 
dicate flowage. 


Aphanitic,  pinkish- 
violet;  highly  con- 
choidal  fracture ;  no 
porphyritic  ingredi- 


Microscopic  descriptions  of  thin  flec- 
tions. 


part  secondary  quartz,  but  are  not  all 
evidently  so ;  these  bands  also  hold 
abundant  minute  opaque  ferrites.  In 
the  transparent  bands  are  quite  large 
irregular  patches  of  a  yellowish-green 
material  which  might  be  altered  au- 
gite  or  epidote,  but  which  all  remain 
dark  between  the  crossed  nicols 
throughout  an  entire  revolution. 
Matrix  nearly  colorless,  faintly  tinted 
pinkish-gray,  cloudy.  In  the  polar- 
ized light  this  matrix  is  seen  to  be 
made  up  of  individualized  material, 
in  large  proportion,  saturated  with 
secondary  quartz  in  an  arborescent 
tracery  suggestive  of  the  most  deli- 
cate frosting;  the  filaments  of  this 
quartz  network  polarize  together  in 
relatively  large  areas.  Excessively 
minute,  opaque  ferrite  particles  abun- 
dant. Bare  non-continuous  bands 
composed  of  quartz  particles,  as  in 
the  last-described  section,  occur,  and 
here  again  occurs  the  greenish-gray 
non -polarizing  substance  above  de- 
scribed. Large-sized,  doubly  termin- 
ated, rounded  quartzes  are  the  chief 
porphyritic  ingredients. 

Matrix  very  close  to  that  of  818.  The 
patches  of  the  peculiar  greenish-gray 
substance  there  described  are  here 
very  plenty ;  they  are  often  drawn  out 
into  long  strings,  and  also  occur  in 
small  particles  dotted  over  the  section 
so  exactly  in  the  manner  and  with 
the  shapes  of  the  usual  ferrite  par- 
ticles, as  to  suggest  the  formation  of 
the  ferrites  frOm  them  by  alteration. 
Porphyritic  ingredients :  quartzes, 
with  the  usual  characters  and  large 
and  size;  oligoclase.  This  rock  and  the 
two  preceding  are  plainly  nearer  to 
the  glassy  condition  than  any  others 
described  in  this  list. 

Colorless,  cloudy  matrix,  completely 
saturated  with  secondary  quartz  in  a 
network  coarser  than  usual ;  thickly 
scattered  through  this  matrix  are  un- 


FELSITE  AND  QUAETZ-POEPHTEY. 


109 


Tabulation  of  the  results  of  a  microscopic  study  of  the  felsites  and  felsitic  porphyries 

of  the  Keweenaw  Series — Continued. 


a^ 


Place. 


I 

o 


Macroscopic    charac- 
ters. 


Microscopic   descriptions  of  thin  sec- 
tions. 


853 . . .   Same  place  aa  852. 


876... 


SW. 


28 


56 


7"W. 


Foot  of  north  clife  of 
the  Grreat  FaUsades ; 
north  shore  Lake 
Superior,  Minne- 
sota. 


NE. 


22 


56 


7W. 


ents.  This  rock  is 
thickly  studded 
with  brown  and 
black,  curving,  hair- 
like markings.  SiOa 
77.12  per  cent. 
Aphanitic ;  dark  pur- 
plish-red and  light 
pinkish-red,  in  inter- 
twisted curving 
bands. 


Matrix  aphanitic ; 
dark  pnrplish-red 
closely  banded  with 
lighter  tinted,  non- 
continuous  bands, 
and  rows  of  lighter 
colored  spots. 
White,  kaolinized 
orthoclases  one- 
tenth  inchinlength, 
are  the  moat  impor- 
tant porphyritic  in- 
gredients.  More 
minute  quartzes  are 
abundant. 


usually  large  particles,  and  strings  of 
particles,  of  red  and  brown  ferrite. 
As  already  indicated,  these  strings  of 
particles  are  sufficiently  large  to  at- 
tract attention  in  the  hand  specimen. 
See  Figs.  15, 16,  Plate  XIII. 
Colorless  matrix,  saturated  with  net- 
worked secondary  quartz,  and  thickly 
studded  with  particles  of  red  and 
black  ferrite ;  the  great  abundance  of 
these  particles  in  portions  of  the  sec- 
tion, and  their  nearly  complete  ab- 
sence in  others,  produces  a  strong 
banding. 
Matrix  very  strongly  banded.  The 
most  abundant  bands  present  a  cloudy, 
gray  appearance,  and  are  seen  in  po- 
larized light  to  be  largely  composed 
of  non-polarizing  matter,  with  which 
are  abundant  polarizing  particles,  some 
of  which,  at  least,  belong  to  secondary 
quartz.  There  are  also  present  in 
these  bands  exceedingly  minute  par- 
ticles of  brown  ferrite.  Other  bands 
are  nearly  colorless  and  transparent, 
and  these  are  made  up  chiefly  of  indi- 
vidualized quartz.  Still  other  bands 
present  much  ofa  brown,  blotchy  stain, 
and  are  thickly  studded  with  long, 
black,  femte  needles.  The  needles 
are  at  times  straight,  but  more  often 
have  a  marked  curvature ;  and,  while 
they  show  a  marked  tendency  to  fol- 
low the  general  directions  of  the 
bands,  they  yet  lie  across  one  another 
In  such  a  way  as  to  suggest  the  ap- 
pearance of  a  brush-fence  (compare 
Zirkel,  in  Fortieth  Parallel  Report, 
ToL  VI).  The  narrow  ones  of  these 
bands  arenotcontinuous  even  through 
thewidthofathinsection.  Allthicken 
and  thin  suddenly,  and  all  are  inter- 
twisted in  various  curving  forms,  mak- 
ing abrupt  turns  when  coming  into  con- 
tact with  the  abundant  porphyritic 
quartzes  and  orthoclases.  The  bands 
containing  ferrite  needles  are  least 
continuous  and  are  sometimes  found 
making  forms  like  the  letter  S  within 


110 


COPPER-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 


Tabulation  of  the  results  of  a  microscopic  study  of  the  felsites  and  felsitic  porphyries 

of  the  Keweenaw  Series — Contiuued. 


1539. 


Place. 


North,  shore  Lake  Su- 
perior, one  mile  he- 
low  mouth  of  Bap- 
tism River,  Minne- 
sota. 


North  shore  Lake  Su- 
perior ;  bay  below 
Grand  Marais,  Min- 
nesota. 


EedBock  Bay,  Indian 
Reservation,  north 
shore  Lake  Super- 
ior, Minnesota  (not 
surveyed). 


s 


SE. 


sw. 


about 


21 


35 


IE. 


5E. 


Macroscopic    charac- 
ters. 


Close  to  876.  The 
porphyritic  white 
kaolinized  feldspars 
are  very  abundant. 
The  arrangement  of 
these  feldspars  and 
of  the  lighter  ma- 
terial in  the  matrix 
tends  to  produce 
carving  lines. 


Aphanitic;  red, 
blotched  with  yel- 
lowish-white; no  por- 
phyritic ingredients, 
hut  minute  flashing 
points,  due  to  the 
secondary  quartz, 
may  be  seen  in  a 
bright  light.  The 
rock  is  much  altered 
and  softened  and 
comes  out  in  sharp- 
edged  tabular  frag- 
ments. 
Aphanitic ;  fleah-red ; 
porphyritic  redfeld- 
spars  and  black  quar- 
tzes. 


Microscopic  descriptions  of  thin  sec- 
tions. 


other  bands.  The  whole  appearance 
of  this  banded  matrix  plainly  indi- 
cates movement  while  in  a  fluid  condi- 
tion, and  is  much  the  same  as  com- 
monly observed  in  the  modern  rhyo- 
lites.  The  porphyritic  quartzes  and 
feldsparspresent  the  usual  characters, 
the  quartzes  carrying  often  the  usual 
embayments  of  the  matrix,  as  also 
well-marked,  partially  devitrifled, 
doubly  terminated  glass  inclusions. 
See  Figs.  3,  4,  Plate  SII ;  also  Figs. 
9, 10, 11, 12,  Plate  XIH. 

In  the  thin  section  the  faint  white 
banding  noted  macroscopically  is  seen 
to  be  produced,  as  usual,  by  the  pres- 
ence in  these  bands  of  relatively 
coarse  quartz  and  their  comparative 
freedom  from  the  red  stain  which  af- 
fects the  rest  of  the  rock.  The  usual 
excessively  fine  quartz  network  af- 
fects the  whole  rock,  and  in  all  por- 
tions, except  the  lightest-colored 
bands,  the  usual  brown,  opaque  fer- 
rites  are  abundant.  The  porphyritic 
quartzes  and  orthoclaaes  present  no 
unusual  characters ;  the  former  show 
very  large,  doubly  terminated  glass  in- 
clusions {negative  crystals) .  See  Figs. 
6  and  8,  Plate  XIII. 

In  the  thin  section  this  rock  is  seen  to  be 
completely  saturated  with  the  usual 
quartz  network,  but  shows  also  num- 
erous polarizing  particles,  apparently 
independent  of  the  secondary  quartz. 
Red  stain  and  ferrite  particles  rather 
less  abundant  than  usual. 


In  the  ordinary  light  the  matrix  of  this 
rock  is  only  faintly  stained,  and  is  pe- 
culiar from  being  strewn  with  irregu- 
lar greenish  blotches.  In  the  polar- 
ized  light  the  nearly  colorless  back- 


FELSITE  AND  QUAETZ-POEPHYEY. 


Ill 


Tabulation  of  the  results  of  a  microscopic  study  of  the  felsites  and  felsitie  porphyries 

of  the  Keweenaw  /Series— Continued. 


a 
g 


1540. 


1728. 


Place. 


Nortli  shore  Lake  Su- 
perior, Minnesota, 
200  paces  east  1539. 


North  shore  Lake  Su- 
perior; north  side 
Bead  Island,  nioath 
of  Nipigon  Straits, 
Ontario,  Canada. 


©> 


About. 


35 


1728  6 


63 


A  few  hundred  yards 
east  of  1828,  the 
same  rock -mass. 


5E. 


Macroscopic  charac- 
ters. 


Microscopic  desciiptions  of  thin  sec- 
tions. 


Aphanitic;  dark  pur- 
plish-red. Very 
abundant,  much  al- 
tered red  feldspars, 
up  to  one-half  inch 
in  length;  also 
quartzes  one-twen- 
tieth inch  in  di- 
ameter. Comes  out 
in  thin  tabular  frag- 
ments. 
Matrix  dark  pur- 
plish-red, aphanitic; 
porphyritic,  flesh- 
colored  feldspars, 
one-quarter  to  one- 
half  inch  long,  ex- 
traordinarily abun- 
dant; quartz  also 
very  abundant,  rare- 
ly exceeding  one- 
tenth  inch  in  diam- 
eter. Resembles 
1838,  1846d,  1970, 
and  Michipicotenll. 


Matrix  brick-red, 
banded  with  vaguely 
defined  bands  of 
lighter  and  darker 
red.  The  feldspars 
are  less  abundant 
than  in  1728,  and 
are  whitish  and 
porcellaneous  from 
alteration. 


ground  shows  only  the  usual  excess- 
ively fine  quartz  network,  while  the 
green  blotches  remain  in  large  meas- 
ure dark  throughout  an  entire  revolu- 
tion. Porphyi'itic  orthoclases  are  of 
very  large  size,  red-stained  and  deeply 
eaten  into  by  the  matrix.  The  por- 
phyritic quari^zes  present  no  unusual 
characters. 
Tn  the  ordinary  light  a  matrix  much 
like  the  lighter  portions  of  1539,  ex- 
cept that  red  and  brown  ferrites  are 
thickly  clustered  in  some  portions. 
In  the  polarized  light  the  distinction 
between  the  quartz  network  and  other 
independently  polarizing  particles  is 
plainly  seen. 


In  the  ordinary  light  the  matrix  pre- 
sents throughout  a  deep  reddish- 
brown  stain,  produced  by  thickly- 
crowded  ferrite  particles,  which,  in 
the  immediate  vicinity  of  the  porphy- 
ritic ingredients,  present  some  indi- 
cations of  a  fluidal  texture ;  but,  for 
the  most  part,  the  matrix  is  without 
such  an  appearance.  In  the  polar- 
ized light  the  matrix  is  for  the  most 
part  dark,  presenting  only  very  mi- 
nute, feebly-polarizing  particles.  The 
usual  quartz  network  appears  to  be 
entirely  wanting.  The  quartzes  are 
much  eaten,  which  is  also  the  case 
with  the  porphyritic  feldspars.  See 
Figs.  4,  7,  Plate  XIII. 
Close  to  1728,  but  in  the  polarized  light 
the  background  presents  more  indi- 
cations of  individualization  and  is  pe- 
culiar for  its  curvilinear  clusters  of 
ferrite  particles.  The  quartzes  are 
extraordinarily  large  and  abundant, 
and  are  eaten  by  the  matrix  into  many 
peculiar  forms.  See  Figs.  1  2,  3, 
Plate  Xin. 


112 


COPPEK-BEAEING  BOOKS  OP  LAKE  SUPEEIOE. 


Tabulation  of  the  results  of  a  microscopic  study  of  the  felsites  and  felsitic  porphyries 

of  the  Keweenaw  Series — Continued. 


Place. 


Macroscopio  charao* 
ters. 


Microscopic  descriptiona  of  thin  sec- 
tions. 


(') 


East  side  Michipico- 
ten  Island. 


m 


Islands  off  harbor, 
soutli  side  of  Michi- 
picoten  Island. 


Matrix  dark  pnr- 
plish-red,  aphani- 
tic;  porphyritic 
quartzes  extraordi- 
narily abundant,  of- 
ten reaching  two- 
tenths  inch  in  diam- 
eter. Ked  porphy- 
ritic  feldspars  also 
very  abundant,  two- 
tenths  to  three- 
tenths  inch  in 
length.  Close  to 
1838,  ISiSd,  1970, 
and  1728. 


Aphanltioi  light 
flesh-red;  very 
rough  fracture;  re- 
sembles the  rock 
from  Mount  Hough- 
ton, Keweenaw 
Point. 


The  groundmass  of  this  rock  is  faintly 
pinkish-tiuted  and  cloudy ;  it  con- 
tains numerous  very  minute  ferrite 
particles,  which,  in  the  vicinity  of 
the  porphyrifcic  ingredients,  show 
crowding  and  a  tendency  to  linear 
directions.  In  the  polarized  light  the 
matrix  shows  a  dark  background 
strewn  with  particles  and  flocks  of 
particles  of  feebly  doubly  refracting 
substances,  but  only  rarely  a  dis- 
tinctly recognizable  quartz  network. 
The  porphyritic  quartzes  are  very 
large  and  abundant,  and  much  eaten. 
The  feldspars  are  also  unusually  large, 
are  both  orthoclase  and  oligoclase, 
and  are  also  much  altered.  See  li'ig. 
5,  Plate  XTrr. 

The  groundmass  is  nearly  colorless, 
cloudy,  and  thickly  dotted  with  very 
minute  ferrite  particles,  which  are  at 
times  aggregated  into  waving  lines. 
In  the  polarized  light  feebly  polarizing 
flecks  dot  a  dark  background,  some  of 
which  are  recognizable  as  quartz  net- 
work clusters.  No  porphyritic  ingre- 
dients in  the  section. 


*Macfarlane's  Michipicoten  Col.  Ifo.  11. 
t  Maofarlane's  Michipicoten  Col.  No.  13. 


'Felsite  porphyry."  Eep.  of  Progress,  Geol.  Snr.  Canada,  1863-1866,  p.  142. 
"  Trachytio  phonolite."    Ibid.,  p.  142. 


Augite-syenite  and  Granitell. — Occurring  abundantly  as  pebbles  and 
boulders  in  the  Keweenawan  conglomerates ;  in  great  irregular  mountain 
masses  in  the  lower  part  of  the  series;  and  again,  in  plainly  intersecting 
masses,  and  even  in  thin  seams  in  the  coarse  gabbros  lying  near  the  base 
of  the  series,  on  both  north  and  south  sides  of  Lake  Superior — are  found 
flesh-red  to  brick-red  rocks,  which  present  a  plainly,  and  often  quite  coarsely 
crystalline  structure  and  general  granitic  appearance.  Red  feldspars,  un- 
striated  alone,  or  both  unstriated  and  striated  together,  appear  always  to 
make  up  the  bulk  of  these  rocks,  and  quite  commonly  are  the  only  macro- 
scopically  recognizable  ingredients.  Quartz,  however,  is  often  visible,  and 
especially  in  the  more  coarsely  grained  and  more  strongly  granite-like 


JNITED  STATES   GF' 


j:-'LKIL'h    ML 


Ori7toc29.feA/J  cyyeiali.  sa/ureu-^e^  -rri'iTL,  eo-ryojton.  £u.a.Tit(sj  ■       7^if.9i  f  clesi^rzeel  io  skoi*  ni»r  ?iumee.r,f 


^'wrfs^T^'wsfm 


fty.S.  J'rom  rein,  irt  yahiro ,  7fice.T'otn&^  J)u2v.in-,  •Ai.inn^, 
OrSina.Ty  tZjrni         Sca2e.  ir dia.me-6e-Tt. 

UrxAoccilSe^   clecoynporc^^   re^c/eneci  oty  iVo-n^  oxz^e ^aitd  stltu.ra^ 
"itel  By   ■fecgnciaT'y  ouArts. 


7zo.'4.  -frQ-yn  Lar*oe.  Q.7*e.CL  oft^ec^  roc^  mike'  ^aoATo    of  Die  Cut  n,  ' 
Jffi'nn.  Polarx'secl  liyTit .  Sca2e^  j.r  AiA?ne-itTs. 

^ua-piz ,    'iTie-n.u.fne-rou.t  area*    of -nlrjcfi  eeconqr'to  only  s-viC 
tn^ivi£ua.7s 


8  OP  LAKE  8UPBEI0E. 


ogcopic  study  of  '^• 
eenaw  Series — < 


A'Juiipsi   mi,]    t, 


Uacroaoopla  <ihai«.< 
ten. 


atriz  dark  pnr- 
plisb-red,  apliaui- 
tioj  porpliyritio 
qoartzes  extraordi- 
narily abundant,  of- 
ten' reaching  twi> 
tenths  inch  if  ' 
eter.  Ked  p- 
ritio  feldspaii^  :»»«*.< 
very  abundant,  two- 
tenths  to  three- 
tenths  inch  in 
lengtb-  Close  to 
1838,  1846(2,  1970, 
and  1728. 


WaiiV.    ,\5^« 


i.si.iQiis  otf  harbor, 
picotcu  island. 


Ijio    pori'.iyr'ic  >iUB  'Ziu 
largo  and  abundant,  and  mauh  e.^i«n. 
The  feldspars  are  also  uuasually  large, 
aro    both   ortboclase  aud  oligoclase, 


*>« 


&^  v«a"^|>  ^.WsAJV   ■OKsi;oS^os'4««'^o 


Occuning  abundai 
merates ;  in  gre 
Jes;  and  again, 
(0  coarse  gabbj 
tth  sides  of  Lai 
isent  a  plainly, 
;initic  ap-^-^  '"• 
aud  stria 
and  quite 
ients.     Quartz 

grained  and  more  stro: 


UNITED  STATES   GEOLOGICAL  SURVEY 


COPPER-BEARING   ROCKS  OF  LAKE  SUPERIOR    PL.XfV 


GRANITIC   POTRPHYRIES  (  HIGHLY  ALTERED  ) 


AUGITE-SYENITE  AND  GEANITELL.  113 

kinds.  In  some  kinds  blackish  and  greenish-black  points  dot  the  rock 
rather  sparsely,  while  occasionally  a  greenish  substance  is  coarse  enough  to 
be  recognizable  as  softened  hornblende  or  augite.  With  diminishing  coarse- 
ness of  grain,  there  comes  to  be  present  in  these  rocks  more  or  less  of  a 
matrix  whose  crystalline  structure  is  not  macroscopically  recognizable,  when 
there  appears  to  be  a  passage  towards  the  uncrystalline  felsites  above  de- 
scribed. An  increasing  amount  of  the  softened  greenish  mineral,  along  with 
an  increasing  amount  of  striated  feldspar,  accompanies  in  other  varieties 
what  appears  macroscopically  to  be  a  passage  toward  the  orthoclase-gabbros 
of  the  preceding  part  of  this  chapter. 

Under  the  microscope,  as  to  the  naked  eye,  these  rocks  are  always 
found  to  be  chiefly  made  up  of  the  feldspars.  These  feldspars  include,  in 
most  cases,  a  triclinic  kind  as  well  as  the  predominating  orthoclase.  In  a 
few  sections  no  plagioclases  were  recognized,  but  this  may  have  been  simply 
on  account  of  their  great  alteration.  The  polarization  angles  obtained,  and 
the  general  appearance  of  these  plagioclases  prove  them  to  be  oligoclase, 
or  low  down  in  the  labradorite  range.  The  feldspars  are  always  turbid, 
and  commonly  also  highly  charged  with  red  iron  oxide.  In  the  larger 
number  of  sections  the  feldspar  crystals  are  charged  also  with  secondary 
quartz,  which  occurs  either  in  rows  of  club-shaped  or  "graphic"  particles^ 
which  often  follow  the  cleavage  directions  of  the  crystals,  or  in  very  fine 
lines  radiating  in  fan-shape  from  a  central  line.  As  in  the  secondary  quartz 
of  the  above-described  felsitic  porphyries,  so  also  here,  large  clusters  of 
adjacent  and  apparently  separate  particles  are  found  to  polarize  together. 
In  this  case,  however,  it  is  the  particles  belonging  to  one  feldspar  crystal 
which  are  thus  similarly  oriented.  Particles  and  needles  of  brown  and 
black  ferrite  are  also  often  present  in  these  altered  feldspars,  and  often  ar- 
ranged in  the  radial  fashion  just  mentioned  as  showing  in  the  quartz. 

In  the  case  of  the  radiating  quartz  and  ferrite  clusters,  it  is  often  diffi- 
cult to  tell  if  we  are  dealing  with  an  alteration  of  a  feldspar  crystal  or  of 
unindividualized  matrix,  but  in  so  many  cases  is  it  evident  from  the  polar- 
ization efi'ects  that  this  alteration  has  progressed  in  orthoclase  crystals,  that 
it  is  reasonable  to  assume  that  the  same  is  true  for  other  sections  present- 
ing a  similar  appearance.  This  quartz  saturation  varies  in  extent,  but  is 
8  L  s 


114       COPPER-BEARING  ROCKS  OP  LAKE  SUPERIOR. 

present  in  all  sections  examined.  It  is  a  curious  fact  that,  although  it  has 
been  found  affecting  the  outer  borders  of  the  plagioclases,  these  are  for  the 
most  part  free  from  the  quartz  saturation.  In  many  cases  it  is  evident  from 
the  existence  of  a  regularly  outlined  core  without  quartz  within  the  feldspar 
crystal,  and  of  an  outer  border,  of  greater  or  less  width,  saturated  with 
quartz,  that  the  replacing  process  has  gone  on  from  without,  inwards.  In 
other  cases,  again,  the  feldspar  crystals  have  plainly  been  more  or  less 
shattered  into  fragments,  and  cracked  across,  before  the  deposition  of  the 
quartz. 

In  those  kinds  which  macroscopically  are  especially  granite-like,  show- 
ing large  feldspar  and  quartz  areas  orie-twentieth  to  one-fifth  inch  across, 
the  larger  quartz  areas  appear  much  like  the  quartz  of  true  granite, 
filling,  as  in  granite,  the  spaces  between  the  feldspars.  But  the  same 
sections  sometimes  show  the  saturating,  club-shaped,  secondary  quartz  along 
with  these  larger  interstitial  quartz  areas,  and  then  the  larger  areas  often 
polarize  together  with  a  number  of  the  smaller,  undoubtedly  secondary 
ones  in  the  immediate  vicinity,  in  which  case  all  must  be  taken  as  secondary. 
In  some  sections  of  these  coarser  rocks,  however,  none  of  the  plainly 
secondary  saturating  quartz  is  found  in  the  feldspars,  and  then  it  cannot  be 
shown  that  the  coarse  quartz  is  not  an  original  ingredient.  In  the  figures 
of  Plates  XIV  and  XV,  I  have  attempted  to  illustrate  the  occurrence  of  the 
secondary  quartz  of  these  rocks.  Figs.  1  and  4  of  Plate  V,  and  Figs.  13, 
14,  15,  16  of  Plate  XIII,  showing  secondary  quartz  as  characterizing  the 
already  described  orthoclase-gabbros  and  felsitic  porphyries,  should  be 
referred  to  for  comparison. 

About  two-thirds  of  the  sections  show  sparsely  scattered  augite  crys- 
tals, which  present  the  same  peculiar  red  and  brown  to  black  ferritic  altera- 
tion that  characterizes  the  augites  of  the  felsitic  porphyries,  as  above  de- 
scribed. The  unaltered  augite  is  commonly  present  only  in  cores.  A  very 
few  sections  show,  instead  of  the  augite,  a  fibrous  green  hornblende,  whose 
uralitic  nature  is  extremely  probable.  Titanic  iron  or  magnetite  occurs  in 
two  or  three  sections. 

A  perusal  of  the  following  tabulation  of  observations  on  these  rocks 
will  serve  to  make  it  plain  that  they  present  a  tendency  to  graduate  in  four 


UNITED  STATES   GEOLOGiCAL  SURVEY 


COPPER-BEARING   ROCKS   OF  LAKE   SUPERIOR    PL.  XV 


A-Hoen-ft  Co.  Lltli . 'Biimiiio] 


AUGITE-S^'ENITE  S   (  TJRALITIC 


•^Xj  i i ^V 3r5^  ji«; ^  3 «V S ^V-    i  ■  V '^^ 


,%TJ»0 


mass,  uiesti  rociiii 


-^  -syenite"  of  Rosenbusch  than  to  any  other  pr. 


•flic      MI'P-rHi'f'l:!!' 


u-1<^      T.^o 


descriijo'i 
fiuartz  is 


^S■^si>'^»M5    Mie'iv'roD^i.    ^Vj'tT  S>^V«>-'>5^s»i  ^^■^o^s^^.'^S^\^  So^ivoVjSaeSv 
W"*  i-0M5l»''«^  «0»^«»t*^^I>    Kx's:^93'V>^  ^VV.\3ii  sisi^'^^t*^ 


Pigi    ZlyalH-vc-  auntie.  J yem6e. .  Diil-uin.,.Min-n.. 
Or^iKayy   lifini.  Scale,  zo  diarne^erf. 

a7«y  o-r  coTfott'ctv  quarit(3J ,  nTii'eli  tome'itTnei  faTma 
2a.-rfe.   a-reaf;    wyali^ie'jiteuSamorpht f'fJ  afie.r'  a-u.^i.ie-  ; 

cAl<i-rzie4S'/  ,   -mag-naiiie-H/. 


J^T.g.  i  ^tioz^t'c  ora.n.%ie.ZZ  ^/totiv  'Che.  ■nort/c .  sliore.  of  lake. 

Superior.     Sec..-3Z^  T.si,'fi.7ff.  -^i^'n..         O r dinar tf  l-ty^t . 

Sca.7^ '9  ixa.-me.icTS 

Or67teciaje.^a-nel  o2iffoc2a.je.^-n-iKc7A-  a.2.ie.redj  g^*ar6a    pofjet- 
i2y  itvpaT&pTzmary  -^  ^ree-n  aXteTo.it/on,  pyociice^i  of 
av.yiit'i'tl  fn  c-Juiiers  it.€rrten  iH&  ZoTyer  J^e.2tLrpa.Tj, 


IF'iQ.3.   Granitic  parp'kyTyjTOTn  TT^onie-n.  Trail ,  ^ iTila-ncl 
CouTttc/.  ?yit.        Orcti-ncirylxyni      Sca.2e  a/  ^ia-me.ier<r 
st^oiois-yte^  jelclspcLrs  saitiTa'te<i.Tv'ti\   j'eco'^cdary   ^u-ar^st-Vj 
~ynei-c/-n.eizte,ij/ ,  ^Teenzsli  aliefoJ-zon.  proSua-i  (m 


T^lff.u   -^itvtic-lifemte.  fro-m. ptJfhle-x-n.  cotty7o7neTa2e^j 
JHi.Soherm'a,  7^e?re.e.-na.-rr Poitii,  ^ick.  Oiritinetrtf  Z-iyAt, 

Sea. Ze.  •  3.r  cZ ia.m. e  te rf. 

Ori7iocZa.se.f7/fi7rzn-yieaU  a-ncZ  Zai/ra^ori-icl-i-'iotJL  satzcT-nie^ 
mill  ■seco-nda.Ti/  gzcarii.  arrtTtye£  tfi  f.ftslinet  (-itj  a.tui  zrt' 
rt9vc7a.r?u  auiZiHeci  artaJ  It^j ;  ferriiic  ahcre.<ionfv/  of  au^ite. 


AUGITE-SYENITE  AND  GRANITELL.  115 

different  directions,  viz:  toward  orthoclase-gabbro  and  the  non-quartziferous 
porphyries  on  the  one  hand,  and  toward  granite  and  the  quartziferous 
porpliyries  on  the  other. 

I  have  been  somewhat  at  loss  for  a  name  for  these  rocks.  In  1878 
Pumpelly  designated  as  "granitic  porphyry"  one  or  two  specimens  of  a 
red  rock,  sent  him  by  the  Wisconsin  Survey,  but  according  to  Rosen- 
busch  and  the  accepted  nomenclature  generally,  a  granitic  porphyry  is  a 
rockwitha  finely  crystalline  admixture  of  the  usual  granitic  minerals— quartz, 
orthoclase,  plagioclase  and  mica  or  hornblende — with  porphyritically  devel- 
oped feldspar  and  quartz,  and  thus  stands  between  the  true  granites  and 
the  felsitic  porphyries.  But  all  of  the  rocks  under  consideration  in  which  the 
quartz  is  wholly  secondary  are  essentially,  or  were,  before  the  deposition  of 
the  secondary  quartz,  aggregates  of  feldspar  crystals,  with  the  orthoclase  pre- 
dominating. The  altered  augite  is  the  only  other  ingredient  worthy  of  any 
consideration  in  determining  on  a  name  for  these  rocks,  while  it  is  always 
so  subordinate  as  to  be  hardly  more  than  an  accessory,  and  from  about  a 
third  of  the  sections  is  entirely  absent.  Regarding  it  as  an  essential  ingre- 
dient, and  the  quartz  in  most  cases  as  a  secondary  product,  resulting  from 
the  leaching  of  the  original  feldspar  mass,  these  rocks  are  nearer  to  the 
"augite-syenite"  of  Rosenbusch  than  to  any  other  previously  described 
species  among  the  pre-Tertiary  rocks.  The  kinds  in  which  the  quartz  is 
possibly  primary  would  be  then  "  augite-granite"  or  "granitell,"  but  no  ex- 
act provision  is  made  for  these  rocks  in  any  system  of  nomenclature.^ 

As  typical  instances  of  the  occurrence  of  these  rocks  may  be  men- 
tioned the  pebbles  of  a  conglomerate  at  the  south  foot  of  Mount  Bohemia, 
Keweenaw  Point,  apparently  included  in  the  Eastern  Sandstone;  the  irreg- 
ular masses  apparently  intersecting  the  orthoclase-gabbro  of  Mount  Bohe- 
mia itself;  many  pebbles  of  the  conglomerate  at  Eagle  River,  Keweenaw 
Point,  where  are  found  both  fine-grained  and  coarse  granite-like  kinds ;  the 
prevailing  pebbles  of  the  Albany  and  Boston  conglomerate  near  Portage 
Lake ;  the  irregular  masses  intersecting  coarse  olivine-gabbro  at  a  number 
of  points  in  the  gabbro  belt  of  the  Bad  River  region  of  "Wisconsin  f  the 

1 "  Granulite"  of  Eosenbnsch  is  without  plagioclase,  or  the  term  might  be  used  for  the  kinds  almost 
ftee  ftom  augitic  ingredient,  and  rich  in  large  quartzes. 

'See  Plate  XXII  of  this  volume,  and  also  Geology  of  Wisconsin,  Vol.  Ill,  pp.  45,195. 


116 


COPPER-BEARING  ROCKS  OF  LAKE  SUPERIOR. 


irregular  masses  and  thin  veins  intersecting  the  coarse  orthoclase-gabbro 
of  Duluth,  Minnesota ;  the  great  granite-like  mass  constituting  the  bold  red 
point  three  miles  above  the  mouth  of  Split  Rock  Kiver,  on  the  Minnesota 
coast;  the  similar  mass  on  the  same  coast  forming  the  south  point  of  Beaver 
Bay;  the  bold  point  on  the  shore  of  section  32,  township  56,  range  7  west; 
the  intersecting  masses  on  the  same  coast  two  miles  below  the  mouth  of 
Baptism  River;  and  the  great  mountain  mass  forming  Eagle  Mountain, 
Minnesota,  thirty  miles  back  from  the  lake  coast. 

Tabulation  of  the  results  of  a  microscopic  examination  of  augite-syenites  and  granitells 

of  the  Keweenaw  Series. 


1 

a 

i 
1 

flaoe. 

1 
1 

-j 

t 

1 

Macroscopic  char- 
acters. 

Microscopic  descriptions. 

Angle    between 
maximum    ex- 
tinctions of 
adjacent  hemi- 
tropic  bands  of 
the  plagioclase 
in  sections  cut 
at   random   in 
the  zone  0 :  ii. 

Angle     on 
opposite 
sides  of 
cross-hair. 

1903  A 

Pebble  from  con- 
glomerate at 
Bonth    foot    of 
Monnt  Bohemia, 
north   shore  of 
Lac   La    Belle, 
KeweenawPolnt, 
Michigan. 

NE. 

32 

58 

29  ■W. 

Minutely  crystal- 
line,- pinkish-reel, 
mottled     with 
minute    green 
spots;  epidote 
in  seams ;  pink- 
ish feldspars  are 
plainly  recogniz- 
able as  the  prin- 
cipal ingredient. 

Appears  to  be  chiefly  made  up 
of  orthoclase  and  plagioclase 
crystals  saturated  with  sec- 
ondary quartz,   which  is  ar- 
ranged in  two  ways:  1st.  In 
relatively     coarse     particles 
shaped  like  the  quartzes  of 
graphic  granite.   These  par- 
ticles commonly   polarize  in 
clusters ;  they  .ire  plainly  sep- 
arate from  one  .another  in  the 
thin   section,    and    therefore 
present  quite  a  contrast  with 
the  networked  quartz  so  com- 
monly found  in  the  matrices  of 
the  felsitio  porphyries.    2d. 
In  excessively  fine  radiating 
lines,  diverging  usually  not 
from  a  point,  but  from  a  Une, 
producing  thus  a  fan-like  ar- 
rangement.   This  fan-lilce  ap- 
pearance is  also  brought  out 
by   the   arrangement  of  the 
minute   ferrite  particles,  by 
which  all   of  the   rock,  ex- 
cept the   large   quartzes,  is 
stained.     That   the   original 
rock  was  entirely  made  up  of 
relatively  coarse  crystals  is  an 

0 

25 
21 
19 
18 
19 
17 
20 

o 
25 
21 
21 
14 
24 
IC 
21 

O 

50 
42 
40 
32 
43 
33 
41 

AUGITE-STENITB  AND  GEANITELL. 


117 


Tabulation  of  the  results  of  a  microseopic  examination  of  augite-syenites,  c£-c.-Continued. 


■b 

g 

g 
a 

1 


Place. 


Macroscopic  char- 
acters. 


1864  C 


North  shore  Bftte 
Grise  Bay,  Ke- 
weenaw Point, 
Michigan.  Con- 
glomerate peb- 
hle  from  Eastern 
Sandstone. 


Near 
north 
line 


35 


58 


29  W. 


Microscopic  descriptions. 


Angle  between 
maximum  ex- 
tinctions of 
adjacent  hemi- 
tropic  bands  of 
tlioplasioclaso 
in  sections  cut 
at  random  in 
the  zone  O :  ii. 


Angle  on 
opposite 
sides  of 
croas-hair, 


inference  from  the  fact  that 
there  is  in  the  most  deeply 
decomposed  places  a  tendency 
to  polarize  in  large  areas ;  it 
is  not   impossible,  however, 
that  this  appearance  is  due  to 
the  polarization  of  the  finer 
secondary    quartz,    and  that 
these  areas  aremcrely  quartz- 
saturated      matrix,      whose 
original     spherulitic     stmc- 
ture  has  produced  the  radial 
arrangement  of  much  of  the 
secondary  quartz.    The  pla- 
gioclases  give  angles  rather 
low  in  the  labradorito  range. 
Where  still  recognizable  as 
plagioclase   they  never  con- 
tain any    secondary   quartz. 
Besides  the  finer  ferrite  par- 
ticles,   there  are  clnsters  of 
coarse  opaque  black  particles 
—some  of  which  may  possibly 
be   magnetite— which,    from 
their  having  associated  with 
them  brightly  polarizing  au- 
gite  particles,  may  be  regarded 
as   alteration-products    from 
augite.     Little     seams     and 
nests  of  secondary  calcite  are 
hei  e  and  there  seen,  and,  with 
the  calcite,  occasionally  par- 
ticles of  cpidote.    Brilliantly 
polarizing   epidote-like    par- 
ticles are   also  seen  dotting 
the  plagioclases,   as  if  from 
their  alteration. 
Closely  similar  to  the  section 
last  described  except  in  con- 
taining less  red  staining  mat- 
ter. 


5t 


9 

8 
20 


118 


COPPER-BEAEmG  EOOKS  OF  LAKE  SUPEEIOE. 


Tabulation  of  the  results  of  it  microscopie  examination  of  augite-syenites,  &c, — Continued. 


a 


1846  & 


1846  A 


Place. 


Pebble  from  Eagle 
Eiver  conglom- 
erate, Keweenaw 
Point,  Michigan. 


17816 


Pebble  from  Eagle 
River  conglom- 
erate, mouth,  of 
Eagle  Elver, 
Keweenaw  Point, 
Michigan. 


N-W. 


NW. 


19 


19 


Pebble  from  the 
Albany  and  Bos- 
ton conglomer- 
ate, Keweenaw 
Point,  Michigan- 


58 


31  "W. 


58 


31  "W. 


N"W, 


55 


Macrosoopio  char- 
acters. 


33  "W. 


Finely  crystalline; 
pinbish-gray; 
shows  abondant 
feldspar  facets 
and  also  dark 
greenish  parti- 
cles. 


Medinm  to  fine- 
grained ;  pink- 
ish;  granite- 
Uke. 


Mioroscopio  descriptions. 


Minutely  crystal- 
line; dark-brown, 
mottled  with 
red ;  abundant 
evident  feldspar 
facets. 


Closely  similar  to  the  two  pre- 
ceding. The  still  recognizable 
plagioclases  are  large  and 
abundant,  aud  some  of  them 
contain  unmistakably  the 
graphically  arranged  second- 
ary quartz.  Irregularly  out- 
lined augites,  largely  altered 
to  ferrite,  arenotunfrequent. 
There  is  but  little  red  stain, 
but  ferrite  needles  of  some 
size  are  rather  abundant  and 
always  arranged  in  a  sort  of 
rude  parallelism  to  the  lines 
of  the  finer  secondary  quartz. 

Made  up  chiefly  of  orthoclaae 
and  quartz,  with  rarer  oligo- 
claaes.  There  is  very  little 
quartz  which  from  its  ar- 
rangement can  be  regarded  as 
secondary,  and  none  what- 
ever of  the  fine  radiating 
quartz  characteristic  of  the 
pEeceding  rocks.  A  few 
large-sized  clusters  of  red  and 
black  ferrite,  associated  with 
brightly  polarizing  particles, 
are  seen;  they  are  supposed 
to  represent  altered  augites. 
Here  and  there  calcite  fills 
spaces  between  the  other  in- 
gredients. Though  present- 
ing some  important  differ- 
ences from  the  previously  de- 
scribed sections,  its  mineral- 
ogical  composition  and  gen- 
eral appearance  are  strongly 
suggestive  of  its  close  affinity 
to  them.  It  is  a  rather  fine- 
grained granite  or  granitell. 
Close  to  1903  A,  1861  C,  and 
1846  6.  In  this  section  there 
is  an  evident  gradation  from 
the  coarser  graphically  ar- 
ranged secondary  quartz  to 
the  finer  kinds ;  while  coarse 


Angle  between 
maximum  ex- 
tinctions of 
a^acent  hemi- 
tropic  bands  of 
the  pla^ioclase 
in  sections  cut 
at  random,  in 
the  zone  0:  il. 


Angles  on 
opposite 
sides  of 
cross-hair. 


15 
14 
12 

20 
24 


n 


29 
22 


AUGITE-SYEOTTE  AND  GEANITELL. 


119 


Tabulation  of  the  results  of  a  microscopic  examination  of  augite-syenites,  &c» — Continued. 


Place. 


cc    H 


Macroscopic  char- 
acters. 


Microscopic  descriptions. 


Angle  between 
maximum  ex- 
tinctions of 
adjacent  hemi- 
tropic  b.inds  of 
the  plagioclase 
in  sections  cut 
at  random  in 
the  zone  O:  il. 


Angles  on 
opposite 
siaes  of 
cross-hair. 


1781  d 


602.. 


617.. 


Pebble  from  Al- 
bany and  Bos' 
ton  conglomer- 
ate, Keweenaw 
Point,  Michigan. 


T^rw. 


8    55 


33  "W. 


Dnlnthf  Minn. 
From  red  vein  12 
inches  wide  cut- 
ting coarse  gab- 
bro,  Eice  Point 
quarry. 


Area  of  red  rock 
in  coarse  gabbro, 
Dnluth,  Minn. 


W^. 


34    50 


14  "W. 


mv. 


27    50 


14  "W. 


Fine-grained; 

highly  ciystal- 
line ;  dark  brick- 
red;  appears  to 
be  entirelymade 
up  of  minute  led 
feldspars. 


Flesh-red;  highly 
crystalline;  me- 
dium -  grained ; 
pinkish  feldspar 
facets  predomi- 
nate. 


Highly  crystal- 
line; medium- 
grained;  red, 
mottled  with 
green. 


and  fine  frequently  polarize 
together  in  large  clusters.  It 
also  appears  evident  in  this 
section  that  the  fine  radially 
arranged  quartz  affects,  in 
part  at  least,  original  crystals. 

Appears  to  have  been  originally 
chiefly  made  of  feldspar  crys- 
tals, in  part  triclinic,  with 
possibly  also  some  finer  ma- 
trix material.  The  crystals 
are  now,  however,  all  much 
dulled  and  altered,  and  more 
or  leas  blotched  with  iron 
oxide.  In  many  crystals  long, 
brownish  ferrite  lines  follow 
the  cleavage  directions.  An 
excessively  fine  secondary 
quartz  fills  some  of  the  feld- 
spars, comers  between  which 
are  sometimes  occupied  by 
coarser  particles  of  quartz; 
but  large  parts  of  the  section 
are  singularly  free  from  the 
usual  quartz  saturation. 
Clusters  of  opaque  black  and 
brownish  particles,  with  now 
and  then  a  biightly  polarizing 
core,  represent  augites. 

Appears  to  have  been  originally 
made  up  entirely  of  feldspar 
crystals;  but  these  are  now 
completely  saturated  with 
secondary  quartz  in  unusually 
coarse  particles,  arranged  in 
a  graphic  form.  See  Figs.  3 
and  4,  Plate  XIV. 

A  granite-like  rock,  in  which 
the  chief  ingredients  are  red- 
dened feldspars  and  quartz. 
The  quartz  is  in  relatively 
large  areas,  filling  spaces  be- 
tween the  feldspars;  hut  in 
the  neighborhood  of  the  larger 
areas,  saturating  the  feldspars. 


120 


COPPEE-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 


Tdbtilation  of  the  results  of  a  microscopic  examination  of  augite-syenites^  (fie— Contiimed, 


Place. 


C3? 


Macroscopic  char- 
acters. 


Microscopic  descriptions. 


Angle  between 
maximum  ex- 
tinctions of 
adjacent  hemi- 
tropic  bands  of 
the  plagioolase 
in  sections  cut 
at  random  in 
the  zone  0 :  ii. 


Angles  on 
opposite 
sictes   of 

croas-hair. 


51 


Mouth  of  Tischer's 

Creek,  north 
shore  of  Lake 
Superior,  Min- 
nesota. 


SE. 


50 


14  W 


Eed;  earthy; 
shows  occasional 
feldspar  facets, 
but  is  for  the 
most  part  with- 
out crystalline 
structure ;  m  i  - 
nute  nests  of 
quartz  abund- 
ant; very  rongli 
fracture. 


ia  a  quartz  in  smaller  areas, 
which  ia  plainly  secondary. 
Since  large  and  small  quartzes 
frequently  polarize  together, 
it  ia  plain  that  all  maat  be 
taken  aa  aecondary.  A  very 
few  of  the  feldspars  show 
banding  in  the  polarized  light. 
There  are  also  present  good 
sized  pale-greonish,  strongly 
dichroic,  much  shattered 
hornblendes,  which  often  pre- 
sent much  the  appearence  of 
nralite.  Titaniferona  iron 
and  apatite  are  not  uncommon 
ingredienta,  and  the  whole 
aapect  of  the  rock  suggests 
the  possibility  of  its  being  an 
orthoclase  -  gabbro,  in  which 
the  uralitic  alteration  of  the 
hornblende  and  the  deposition 
of  secondary  quartz  have  been 
pushed  to  an  extreme.  (Com- 
pare the  description  of  pp. 
50-56.  See  also  Fig.  1,  Plate 
XV.) 
In  the  ordinary  light  the  thin 
section  presents  a  red-atained 
background,  which  ia  com- 
pletely riddled  by  very  coarse 
secondary  quartz  in  such  a 
manner  as  to  suggest  for  parts 
of  the  section  a  complete  shat- 
tering before  the  deposition  of 
the  quartz.  The  uncrystal- 
line  appearance  of  the  red 
matrix  in  the  ordinary  light, 
and  the  networked  arrange- 
ment of  much  of  the  quartz, 
suggest  that  this  rock  might 
more  properly  be  included 
among  the  felsitic  porphyries ; 
but  in  the  polarized  light  a 
considerable  proportion  of  the 
background    resolves    itself 


AUGITE-SYENITE  AND  GRANITBLL. 


121 


Tabulation  of  the  results  of  a  microscopic  examination  of  augite-syenites,  (&c. — Continued, 


Place. 


Macroscopic  char- 
aoten. 


Microscopic  d^oriptions. 


Angle  between 
maximum  ex- 
tinctions of 
adjacent  hemi- 
tropic  bands  of 
the  plagioclaae 
in  sections  cut 
at  random  in 
the  zone  0 .-  ii. 


Angles  on 
opposite 
sides  of 
cross-hair. 


•^  a 


North  shore  of 
Lake  Superior, 
three  miles  be- 
low the  mouth 
of  SpUt  Kock 
River,  Minne- 
sota. 


NE. 


65 


8"W. 


Highly  crystal- 
line;  rather 
coarse-grained; 
flesh-red;  gran- 
ite-like.  feld- 
spar and  quartz 
both  easily  re- 
cognizable to 
the  naked  eye. 


into  fragments  of  orthoclase 
crystals.  Another  portion  of 
the  background,  however, 
presents  no  appearance  of 
crystalline  structure ;  but  has, 
throughout,  a  real  or  apparent 
Bpherulitic  structure, brought 
out  by  the  radiation  of  minute 
quartz  veinlets  and  by  lines  of 
minute  ferritic  particles. 
Although  much  of  the  quartz 
is  coarse  enough  to  be  seen 
with  the  naked  eye,  none  of  it 
is  porphyritic,  but  all  second- 
ary. In  this  rock  we  have 
evidently  a  transition  phase 
between  the  felsitic  porphy- 
ries and  the  more  completely 
crystalline  granitic  porphy- 
ries, or  granitells. 
With  the  exception  of  some  rare 
magnetite  areas,  this  rock  is 
completely  made  up  of  pink- 
ish, turbid,  rather  large-sized 
orthoclase  and  oligoclase  and 
quartz.  The  quartz  makes 
up  fully  half  of  the  section, 
and  is  often  in  areas  one- 
twentieth  to  one-tenth  inch  in 
width  ;  it  is,  however,  wholly 
secondary,  the  coarsest  por- 
tions often  penetrating  deeply 
into  the  mass  of  an  orthoclase 
crystal.  Several  of  the  large 
feldspar  crystals  were  ob- 
served to  have  cores  whose 
outlines  form  abrupt  limits, 
beyond  which  the  secondary 
quartz  does  not  pass,  a  fact 
suggesting  the  decomposition 
and  corrosion  of  the  feldspar 
before  the  advent  of  the 
secondary  quartz. 


122 


COPPEE-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 


Tabulation  of  the  results  of  a  mieroscopie  examination  of  augite-syenites^  &c, — Continued. 


791  and 

787. 


Place. 


Nortli  Bbore  Lake 
Superior,  Min- 
nesota, south 
point  of  Beaver 
Bay. 


North  shore  of 
Lake  Superior, 
Minnesota. 


SE. 


NE. 


32 


SW. 


Microscopic  char* 

acters. 


Pale  flesh -red; 
medium  to  fine- 
grained; a  large 
proportion  of 
the  rock  appears 
to  he  made  up  of 
pinkish  feld- 
spars. 


Microscopic  descriptions. 


56 


7"W. 


Medium  -grained ; 
flesh-red ;  gran- 
ite-like ;  quartz 
and  feldspars 
both  plainly  re- 
cognizable to  the 
naked  eye. 


The  thin  section  is  made  up 
chiefly  of  much  dulled  and 
clouded  feldspars,  a  few  of 
■which  are  obscurely  banded. 
Quartz  occurs  in  little  par- 
ticles, filling  corners,  and  in 
Bome  few  places  is  plainly 
secondary,  but  on  the  whole 
it  is  much  less  abundant  in 
this  section  than  in  most  of 
those  previously  described. 
There  are  areas  in  the  section 
which  present  a  cloudy,  red- 
stained  appearance,  which 
may  be  either  much  altered 
feldspars  or  felsitic  matrix. 
Magnetite  is  rather  coarse 
and  abundant  for  this  class  of 
rocks. 

The  thin  section  is  close  to  that 
of  7G2,  consisting  for  the  most 
part  of  coarse,  turbid,  pink- 
ish-stained feldspars  and  clear 
quartz.  The  feldspars  show 
in  large  part  a  fine  lineatiou 
in  the  polarized  light,  and 
give  the  low  polarization 
angles  of  oligoclase.  Proba- 
bly half  of  the  feldspars  are, 
however,  orthoclase.  The 
feldspar  crystals  are  always 
of  quite  large  size,  measuring 
often  one  to  two  inches  in 
width.  The  quartz  is  not  so 
coarse,  but  stUl  is  in  very 
large  particles  for  this  class 
of  rocks,  and  enters  the  feld- 
spar in  such  a  way  as  to  ren- 
der certain  its  secondary  ori- 
gin, or  at  least  its  deposition 
after  the  feldspars  had  be- 
come thoroughly  eaten  and 
honeycombed.  There  are  one 
or  two  places  in  the  section 


Angle  between 
maximum  ex- 
tinctions  of 
ac^jacent  hemi- 
tropic  bands  of 
the  plagioclase 
in  sections  cut 
at  random  in 
the  zone  O :  U. 


Angles  on 
0  p  p  osite 
sides  of 
cross-hair. 


^3 


7 
24 
27 


AUGITE-SYEOTTE  AND  GEAI^ITELL. 


123 


Tabulation  of  the  results  of  a  microscopic  examination  of  augite-sijenites,  cfcc— Continued. 


Place. 


1035.. 


Kear  Baptism 
Elver,  Mian©. 
eota. 


S.llne 


67 


Microscopic  char* 

actera. 


Mioroscoplo  deBCriptions, 


TW,  Fine-grained; 
highly  crystal- 
line ;  flesh-red ; 
appears  to  be 
mostly  made  up 
of  red  feldspars. 


Angle  between 
maximum  ex- 
tinctions  of 
adjacent  hemi- 
tf  opic  bands  of 
the  plagioclase 
in  sestions  cut 
at  random  in 
the  zone  O :  ii. 


Angles  on 
opposite 
sides     of 

cross-hair. 


where  a  finer  material,  ap- 
parently  feldspathic   in    its 
nature,  has  mingled  with  it 
numerous  particles  of  brightly 
polarizing   augite.       Several 
other  similar  places  present, 
in  place  of  the  augite,  green- 
ish particles,  which  revolve 
dark   between    the    crossed 
nicols,  and  which  are  proba- 
bly an  alteration-product  of 
angite.    There  are  also  small 
angites    filling    comers    be- 
tween   coarser    ingredients, 
and  even  in  places  lying  di- 
rectly within  the  feldspars. 
The  quartz  is  tbickly  crowded 
with  cavities,  many  of  which 
hold  a  bubble-bearing  fluid. 
Appears  to  consist  entirely  of 
feldspar   crystals,    stained  a 
deep-red  with  oxide  of  iron 
and  completely  saturated  with 
"graphic"  secondary  quartz. 
The    quartz    particles    fre- 
quently  follow   the    cleav- 
age  lines  of  the  feldspars ; 
and  even  when  not  doing  so, 
always    show   a   tendency 
to  a  common  direction  within 
one    feldspar  crystal,   which 
usually    differs    from    the 
directions    in    the    adjacent 
crystals.    Some  black,  opaque 
clusters,    including   brightly 
polarizing  particles,  probably 
represent  altered  augite. 
2iTone  of  the  feldspars  are  re- 
cognizable  as   triclinio,    bnt 
several  show  the  character- 
iatio  twinning  of  orthoclase. 


124       COPPEE-BEARING  EOCKS  OF  LAKE  SUPERIOR. 

Tabulation  of  the  results  of  a  microscopic  examination  of  angite-syenites^  &c, — Continued. 


1067. 


Place. 


Eajsrle  Mountain, 
Minnesota 
(township  not 
subdivided). 


about 


North  shore  o  f 
Lake  Superior, 
one  mile  above 
the  mouth  of 
Maw-ske-quaw- 
caw-maw  River. 


G? 


SW. 


23 


63 


12     62 


2W. 


Macroscopic  char- 
acters. 


4E. 


Fine-grained ; 

highly  crystal- 
line; bricb-red; 
blotched  with 
gray  and  black; 
appears  to  b  e 
chiefly  made  np 
of  minute  red 
feldspars. 


Microscopic  descriptions. 


Brick-red;  mi- 
nutely crystal- 
line; appears  to 
be  made  np  of 
minute  red  feld- 
spars ;  crossed 
by  parallel  black 
stripes  of  rela- 
tively large  size. 


Resembles  closely  the  last  sec- 
tion described,  except  that 
there  are  some  coarser  clus- 
ters of  quartz  which  are 
plainly  connected  with  that 
which  saturates  the  feldspars. 
The  latter  attains,  in  this  sec- 
tion, an  extreme  and  most 
beautiful  development.  It 
often  follows  the  cleavages  in 
a  most  regular  manner,  and 
again  forms  groups  of  diverg- 
ing lines.  The  tendency  seems 
to  be  for  each  quartz  particle 
to  have  one  narrow  or  even- 
pointed  end,  while  the  other 
is  broad,  club-shaped,  or  even 
hooked.  The  deposition  o  f 
this  secondary  quartz  has 
plainly  been  from  without  in- 
ward in  each  feldspar  crys- 
tal. This  is  evidenced  by  its 
common  greater  abundance 
and  coarseness  in  the  outer 
portions  of  the  crystals,  some 
of  the  feldspar  crystals  being 
provided  with  a  core,  in  which 
there  is  ni  quartz.  Thi^ 
core,  when  present,  has  linear 
outlines  parallel  to  the  sides 
of  the  original  crystal. 

Allied  to  the  last  two  described. 
It  contains  also  long  needle- 
shaped  crystals  of  augite, 
largely  altered  to  greenish  and 
opaque  ferritic  substances ; 
numbers  of  these  needles  are 
parallel  and  appear  to  be  parts 
of  one  original  crystal. 


Angle  between 
maximum  ex- 
tinctions of 
adjacent  hemi- 
tropic  bands  of 
the  plagioclase 
in  sections  cut 
at  random  in 
the  zone  O:  U. 


Angles  on 
opposite 
sides  of 

cioBB-hair. 


GEANITE.  125 

Granite.— In  the  third  volume  of  the  Geology  of  Wisconsin,  I  have 
spoken  of  granite  as  occurring  in  intersecting  masses  in  the  coarse  gabbros 
which  form  the  base  of  the  Keweenaw  Series  in  that  region,  as  also  in  the 
immediately  underlying  mica- schists  of  the  Huronian;    and  a  number  of 
these  granitic  areas  were  mapped  on  the  accompanying  atlas  plates.     This 
granite^  is  there  described  as  a  red   biotite-granite,  with   orthoclase  and 
liquid-bearing  quartz  as  the  chief  constituents,  and  with  mica,  rarer  white 
plagioclase  and  still  rarer  apatite  and  magnetite,  as  subordinate  constitu- 
ents.    For  the  sections  examined  this  description  is  entirely  correct;  but 
most  of  these  few  sections  were  from  the  granites  of  the  underlying  Huron- 
ian.    For  the  present  memoir  I  have  had  a  number  of  new  sections  cut, 
and  find  that  the  granites  cutting  the  Keweenawan  gabbro  are  more  often 
hornblendic  than  micaceous,  and  that  all  of  these  granites,  including  those 
of  the  underlying  Huronian,  are  very  closely  allied  to  the  coarse-grained 
granitoid  rocks  of  the  immediately  preceding  tables,  from  which  they  merit 
separation  only  from  their  relatively  large  content  of  the  hornblendic  or 
micaceous  ingredient.      The  hornblende  of  these  rocks  is  of  the  basaltic 
variety,  with  an  intensely  deep-brown  color  and  very  strong  dichroism. 
.Here  and  there  an  augite  core  is  to  be  noticed  in  the  hornblende,  which  is 
thus  plainly  a  secondary  product.     It  is  always  in  regularly  outlined  crys- 
tals.    The  feldspars  are  always  much  reddened  and  altered,  and  were  evi- 
dently much  shattered  and  corroded  before  or  during  the  deposition  of  the 
quartz.     The  latter  ingredient  is  very  abundant,  crowded  with  liquid-filled 
cavities,  many  of  which  carry  a  minute  salt  cube,  and  while  not  certainly 
a  secondary  product,  was  subsequent,  as  just  stated,  to  much  breaking  and 
honeycombing  of  the  feldspars.     Besides  the  very  coarse  and  more  abund- 
ant quartz  areas,  there  are  in  some  sections  aggregates  of  numerous  little 
particles  between  the  coarser  constituents,  and  it  is  mingled  with  these 
finer  quartz  particles  that  the  minute  biotite  flakes  are  found  in  those  sec- 
tions carrying  this  mineral. 

Although,  as  already  said,  so  closely  allied  to  the  augite-syenites  and 
augite-granites,  or  granitells,  of  the  preceding  tables,  rocks  so  completely 
granitic  in  character  as  these  have  not  been  observed  elsewhere  in  the 
Keweenaw  Series. 

1  Op.  cit.,  p.  193. 


126  COPPEE-BEAEmG  EOCKS  OF  LAKE  SUPBEIOE. 


Section  in.— SUMMAEY  VIEW  OF  THE  OEIGIifAL  EOCKS  OF  THE 

KEWEENAW  SEEIES. 

To  any  one  who  will  read  through  the  foregoing  part  of  this  chapter 
it  will  become  evident  that  between  the  several  kinds  of  original  rocks 
described  there  are  no  sharp  lines,  and  that  there  is,  in  fact,  a  continuous 
series  of  kinds,  from  the  most  basic  to  the  most  acid.  It  will  also  become 
evident  that,  from  the  necessity  of  following  some  accepted  nomenclature, 
it  has  been  unavoidable  to  use  names  which  cover  very  different  ranges  of 
acidity.  In  the  following  table  an  attempt  is  made  to  classify  the  various 
rocks  with  reference  to  silica  content,  as  well  as  to  texture  and  mineralogical 
composition,  and  thus  to  bring  out  more  plainly  than  has  been  done  in  the 
preceding  pages  the  relations  of  the  different  kinds  to  one  another.  The 
references  B  1,  A  1,  &c.,  are  to  the  several  classes  of  basic  and  acid  rocks, 
respectively,  as  given  on  pages  37  and  91. 


Summary  tabulation  of  tlie  eruptive  roeks  of  the  Keweenaw  Series,  showing  tlieir  mutual  transitions  and  relations. 


9s 

oS 


('oarse'graniilar  kitids. 


■sS 


a 

H 

< 

WH 

35 

o 

" 

3 

OUvinc-sabbro  (li  1). 

Oliviuo. 

Aniirtliiic. 

Diallaj:.-. 

TitaiiifiTOiis  jnaffnMilo, 
Nun  orUwdastic,  olivirie/ree  (jahhro  nnd  duiii" 
(ia  part  only)  (B  1). 

Anortliite  or  labradorite. 

Diallapeor  aiiKite. 

Titaiiifi-niue  magnetite. 
Anorthiteruck;  a  apecial  phase  (U  4). 


lirading  by  iiicreasiug  fiDoiiesa  of 
grain  and'  losa  of  diallagic  tileavage 
in  the  augite,  wliich  remains  coarser 
Mian  the  other  couatitueutA,  into 


Grading  by  loss  of  olivine,  decrpase  in  basicity 
of  the  plagioilasfs,  and,  in  some  kinds,  by 
addition  of  orthoilose.  into 


Nonorthoclaitic  olivine-free  sjabbro  and  diabase 
(in  part  only)  (B  1). 

Labrador]  te. 

Dialla^e  or  aiigito, 

Titaniferoas  magnetite. 
Orthocla-gegabbro  (in  small  part  only)  (B  2). 

Labradorite  or  oligoclasc. 

Orthoclase. 

Diallage  and  augite. 

TitanifiTous  magnutite. 

Apalit«.  uralite.  common  accessories. 

Homblende-gabbro  lB3).  i    '^  *^ 


Grading  by  decreasing  basicity  of  plagioclnse. 
increase  of  orthoclaae.  and  introdaction  of 
secondary  quartz,  into 


Orthodase-gabbro  (the  larger  part)  IB  2). 

Oltgocluee. 

Oiirhoclase. 

Diallage. 

Augite. 

Titanifi'ious  magnetite. 

Apatite,  uralite  and  secondary  quartz,  i 
common  accessories. 
Uralitic  orthoclase'gahbro  (B  2). 
Uornblende'gabbro  (B  3). 


>  Special  phases. 


Grading  by  decrease  of  aiii;itic  constituent,  and 
increase  of  orthoclaae  and  of  quarts,  into 


Augite-syenxte  (in  part)  (A  3). 
Oligoclase. 
Uttboclase. 

Augite  (very  subordinate). 
Fcrriteand  abundaut  quartz  characteristic 


aeceasuries. 


Giading  by  det'rease  of  oligoutaae  and  great  in- 
crease of  quartz,  into 


Augite  granite   (A  4)  nnd   ijranilell  or  granitic 
porphyry  (A  3)  (in  part). 
1  )rtboc]a8B. 

Oligoclaso  (not  always  present). 
Quartz, 

Augite  (always  more  or  less  thoroughly  al- 
tered to  ferrite  or  lioiiiblendo  and  very 
sparse). 


Grading    by    increasing    fineness   of 
giain  iuto  ■ »■ 


Grading  by   increasing   fineness  of 
grain  into  — * 


Gradation  forms  not  known,  int-o  - 


Giading  by  increasing  finem-i 
grain,  into  — » 


iradiiig    by    iucrensing    finmcs 
grain,  iuto *■ 


Fioe-granular  kinds. 


Olirinltic  diabase   and  melnphyr  ("luster-mot- 
tled" rocks),  B  0  (in  large  iiart). 

Anorthite. 

Olivine. 

Augite. 

TitanileniMs  magnetite. 
Pgeudamygdaloids  a  special  phase. 

"Ordinary -type"  diabase  (in  small  part  onlv) 
(B5). 

Lahrndorite. 

Augite. 

Magnetite. 
I'seudamygtialoids  a  special  phase. 


Grading  by  loss  of  olivine  and  decreasing  ba- 
sicity of  plagioclase,  into 


"  Ordinary-type"  diabate  (tho  larger  part)  (B  5). 

Labradorite  or  oligoclase. 

Augite. 

Titauifernus  uiaguetite. 
Pseud  amygdaloids  a  special  phase. 


Grading  by  decreasing  basicity  of  feldspars 
iuto 


"Ordinary  type"  (fi'abaire  (in  small  part  only)  (BSi 
Oligoclase. 

Orthoclase  possibly  in  a  few  kinds (T). 
Augite. 

Titan iferons magnetite.     (These  rocks  have 
never  been  found  to  contain  over  53  or  54 
per  cent,    silica — rarely  so    mnch,   save 
whensilicified  by  quartz  infiltration.) 
Pseudamygdaloids  a  special  phase. 


Gradation  phases  not  known,  into 


Fine-grained  augitesyenite  (A  3). 
Orthoclase. 
Oligoclase. 
Augite. 
Ferrite, 
Secondary  quartz. 


Grading  by  iucreaaing  amount  of  quartz,  into 


i 


Fine-grained  j/ram'/eli  or  granitic  porphyry  (A  3) 
(in  part). 
Orthoclase. 
Olisnclase. 
Quattz. 
Aiigile    (very  sparse,    altered    to    ferrite, 

chlorite,  or  uralite). 
Ferrite. 
Secondary  qaartz. 


Grading,  by  addition  of  residuary  magma,  i 


Poipbyritic  kinds,  i.  e.,  kind^  containing  some 
unindividnalized  matter. 


Grading  bydecrrnne  in  amount  of  augite  and 
change  of  tho  augite  into  aggregaten  of 
rounded  grains,  into 

" A sltbed" -diabase  (B  7)  (in  small  part  only)  and 
this,  by  introduction  of  uniudividualized  ma- 
terial and  increasing  fiuoness  of  grain,  into 


Grading,  as  above,  into 

"A8libed"-diabnae  (in  part)  (B  7),  and  this  as 
above,  into*     > 


Melaphyra  or  "luster  ninltled"  rocks  of  I'um- 

Cefly.    Haveattimi^'snliltle  residuary  magma, 
tit  it  never  amounis  to  much. 
Genuine  porphyritic  kinds  of  high  basicity  are 
unlmown. 


Grading  by  in- 
creaaum  auioaut 
of  uncrystiilliuo 
base,  and  intro- 
duction of  gna 
vesicles, into — ► 


D:abaseporphi/ri'e  (in  small  part  only)  (It  71. 
Taliular  plagiochises. 
Round  augite  particles. 
Magnetite 
Irresolvable  base. 
Porphyritic  plagioclnsea  and  rarer  augites. 


Grading  by  decrease  in  augito  and  general  i 
crease  in  acidity,  into 


Grading  through  increasing  fineness  of  grain, 
h)ss  of  cryHtalline  outlines  to  the  augite.  and 
iutroductiou  of  ii resolvable  base,  into — »■ 


Grading  by  introduction  of  felsitic  matter,  and 
increasing  fineueds  of  grain,  iuto- — *- 


rading  by  increaaing  fineness  of  gniiu,  and 
iutroductiou  uf  felsitic  matter,  into *■ 


Diabase-porphyriU  (tho  larger  part)  {B  7). 
Tabular  oligoclaaes. 
Round  augit«  particles. 
Magnetite. 
Irresolvable  base. 

Large  porphyritic  plagioclasca  and  rarer 
augitea. 


Grading  by  increasing  acidity  of  tho  feldspars, 
decrease  in  the  amount  of  augite,  and  intro- 
duction of  mach  ferritic  matter,  into 


Half  gtasay  vcsicu- 
lar  kinds. 


Grading  by  in- 
creasing amount 
of  uuci^-stalline 
base,  and  intro 
ductiou  of  gas 
vesicles,  into — ► 


Grading  by 
creasing  amount 
of  ancrystalliue 
base,  and  intro- 
duction of  gas 
vesicles,  into- 


ddisb-brown  and  jet-lilaek  kinds  with 
highly  conchoidal  fracture)  (B  7). 

Tabular  plagioclases. 

Round  augite  particles. 

Magnetite. 

Irresolvable  base,  often  in  very  large  pro- 
portion. 

Much  ferritic  material  in  the  base. 

Porphyritic  plagioclases  and  augites. 


(iradiug  by  still  further  increase  in  aci<lity,  in 
troduotiou  uf  ortlioclase  among  the  feldspars 
and  loss  of  augit'j  in  tho  ba^e,  into 


tfnartiUss porphyries  (A  I). 
Groundmiiss: 

Mioro-felsitic  matter. 

Cryptfl-crystalline  matter. 

Ferrite. 

Tabular  fpldspars. 

Secondary  quartz. 
Porphyritic  ingredients: 

Oligoclaso  and  orthoclase. 

Augite  with  ferritic  decay. 


Grading,  by  increasing  acidity,  loss  of  tabular 
feldspars  in  the  matrix,  anil  introduction  of 
porpnyritio  quartz,  iuto 


(^  •nrfzifcrotis  pitrphyry  and  /dsite. 
Groundmass: 

Glass  (very  littlo). 

Crypto-eryatalline  matter. 

Micro- felsitic  matter. 

Micro-crystal! iue  matter  (subordinate). 

Ferrite.  ' 

Secondary  iin.iit)!, 
Porpbvritii  ingn-ili-iits: 

Quarts  {ioimdii!  .liliexabedral  pyramids). 

Orthoi-lase  and  uligcclase. 
Augite  (rare). 


?  v-||  fag. 


(§  b  p.a*  at  ^ 

■gS'S'|Ss| 


MS  2 

«,  g   * 


In  1 11 


Grading  by  in- 
creasing amount 
of  uncrystalline 
base,  and  iolro- 
duction  of  gas 
vesicles,  into — » 


I'ace  puge  126,  Vol.  V,  Irving. 


UNITED  STATES   GEOLOGICAL  SURVEY 


COPPER -BEARING   ROCKS  OF  LAKE  SUPERIOR   PL.  XVI 


SANDSTONES 


i'^sCT-aS.^  -sT^w^  *  V^ 


J^DS^    >0.«i^  ^»^l!,«SV.     ^3«^?.%VOj  ,Tf^^>?^''^     -.^^'^ 


o3-^>snr«-  »^^\«  *^s*»i«\«-v\  ■Av^^a^i-s-^^''^  is^oSs.s^ 

•A. 


:£,c^.,-::£^r^i^^£3S^«^ 


Fi^.J..  CtxpriffTTrus  somcliioyie,,    ./yonesu<;.n^  ~^ne-,  yfCK^ru 
Ordinccrif  ZzpTii .  Scale  l£  citciTnecert. 


Ordvpzayy  2t.y?ii .  Scale.  JE/'  dicnne-iert . 


-^^zcTi.  0-r^^nxtr*y  ?igno .         Scale  J^B    i£z'ayTie€ers 


^y^^9 


Jjiiihaste.  d^irt ius (i:) i  fraymen6t  of  iTuR  -matnctt (ai ,  gua-rites (3/,  an3  fcTcl.rpciT'S  fuj  of  fe2sHic  jioypTtyries ;  fraf^tnii 
from  a  gr-ra-n-iixc  ji  o-i-pTtyr'y  Iji) ;  ytia^Tze&iie.  frayme-nh  UJ;  paTiicles  of  an  amyfcidloi^  ■ma.^ri'xlrj  ;  o-ri[yi-:ial2y  c2e- 
j>ottieS  riaizve.  co/tptr-La)  a^cZ  ca^czie/g/y    c7i2oTiit.(io/   as  an  alifra^xovi  pro3icc6. 


DETKITAL  BOOKS.  127 


Section  IV.— DETEITAL  BOOKS. 

The  fragmental  rocks  of  the  Keweenaw  Series  include,  as  ah-eady 
shown,  both  conglomerates  and  sandstones.  The  former  have  already  been 
sufficiently  described  in  a  general  way,  the  several  kinds  of  bowlders  and 
pebbles  of  which  they  are  made  up  having  been  also  described  in  detail  in 
the  foregoing  pages  of  this  chapter  in  connection  with  the  descriptions  of 
the  original  massive  rocks  from  which  they  have  been  derived.  Descrip- 
tions of  the  more  important  conglomerate  beds  are  also  given  in  connection 
with  the  local  descriptions  of  Chapters  VI  and  VII.  It  is  therefore  unnec- 
essary to  give  any  further  account  of  them  here. 

It  may  merely  be  said,  in  review,  that  the  ordinary  conglomerates  are 
•  for  the  most  part  made  up  of  pebbles  and  bowlders  worn  from  the  several 
kinds  of  acid  rocks,  viz :  quartzless  porphyry,  quartziferous  porphyry  and 
felsite,  augite-syenite,  granitell  and  granite;  that  the  same  band  of  con- 
glomerate will  vary  when  followed  along  its  course  as  to  which  of  these  kinds 
is  the  predominant  one;  that  there  are  often  pebbles  of  the  basic  rocks, 
but  in  greatly  subordinated  quantity;  that  the  finer  material  between  the 
pebbles  is  composed  of  a  detritus  of  the  same  rocks  as  the  pebbles  them- 
selves, the  only  difference  being  that  the  greater  fineness  of  these  particles 
has  often  resulted  in  separating  crystals  of  feldspar  and  quartz  completely 
from  the  matrix;  that  one,  or  more,  of  calcite,  chlorite,  epidote  and  cop- 
per— brought  in  by  infiltrating  waters — ^has  often  been  deposited  in  the 
interstices  of  these  conglomerates,  or  rather  has  replaced  their  constituent 
particles;  and  that  these  conglomerates  tend,  even  when  quite  thin,  to  run 
laterally  into  sandstone  or  shale,  the  coarser  fragments  faihng  altogether. 

The  sandstones  seem  in  large  measure  to  be  made  up  of  particles  of 
the  same  acid  rocks  whose  fragments  form  the  conglomerates.  When  they 
closely  overlie  beds  of  basic  rocks,  these  sandstones  often  contain  a  greater 
or  less  proportion  of  basic  detritus.  This  basic  material  presents  itself  in 
the  shape  of  pieces  of  more  or  less  altered  amygdaloid  matrix,  pai-ticles  of 
the  more  completely  crystalline  rocks  with  the  constituent  minerals  still 
adhering  to  one  another,  and  pieces  of  these  minerals  broken  away  from 


128 


COPPEE-BEAEING  BOOKS  OF  LAKE  SUPEEIOR. 


the  mass.  Magnetite,  being  the  least  destructible  constituent  of  the  basic 
rocks,  is  theii-  most  common  representative.  In  the  case  of  one  belt  of  sand- 
stone— the  Nonesuch  belt — which  lies  far  above  any  basic  flow,  the  basic 
detritus  becomes  unusually  abundant,  at  times  almost  wholly  excluding  the 
usual  acidic  detritus. 

On  the  other  hand,  there  are  sandstone  beds,  as  those  of  Black  and 
Nipigon  bays,  on  the  north  side  of  Lake  Superior,  in  which  a  large  pro- 
portion of  the  constituent  particles  have  been  derived  from  gneiss  and 
granite,  and  in  these  cases  the  quartz  particles  are  unusually  abundant.  In 
the  same  region  some  of  the  sandstone  beds  are  largely  charged  with  mag- 
nesian  and  calcareous  carbonates,  which  are  even  concentrated  at  times 
in  thin  seams  of  limestone,  a  thing  unknown  elsewhere  in  the  Keweenaw 
Series. 

As  in  the  conglomerates,  so  also  in  the  sandstone  beds,  secondary 
calcite,  chlorite  and  epidote,  and  even  copper,  are  occasionally  to  be  met 
with. 

The  following  tabulation  includes  enough  examples  to  illustrate  the 
different  varieties  of  sandstone  which  occur  in  the  Keweenaw  Series.  A 
number  of  other  thin  sections  will  be  found  described  in  connection  with 
the  local  details  of  Chapters  VI  and  VII. 


Tabulation  of  microscopic 

observations  upon  sandstones  of  the  Keweenaw  Series. 

a 

Si 

r 

Place. 

s 

S 

P. 
1 

Maorosoopio  descrip- 
tions. 

Microscopic  descriptions. 

1 

3 

to 

^ 

1 

1943.. 

Eagle  Harbor,  Ke- 
weenaw Point, 
Michigan. 

sw. 

6 

58 

SOW. 

Dark  purplish-red ; 
m  e  d  i  nm -grained ; 
firm;   gives    slight 
effervescence   with 
hydrooUoric  acid. 

Subangnlar,  or  only  very  slightly 
rounded  fragments,  worn  from  the 
matrix  and  porphyritic  ingredients 
of  a  quaxtziferous  porphyry,  along 
with  fragments  plainly  derived  from 
an  augite-syenite,  make  up  most  of 
the  rock.    The  fragments  referred  to 
the    porphyritic    ingredients    of   a 
quartziferous  porphyry    are    almost 
whoUy  feldspars,  both  orthoclase  and 
oligoclase,  quartz  particles  being  al- 
most wholly  absent.    The  particles  re- 
ferred to  angite-syenile  are  pieces  of 
feldspar   affected    by  the    peculiar 

DETEITAL  EOCKS. 


129 


Tabulation  of  microscopic  observations  upon  sandstones,  &c. — Continued. 


Place. 


O" 


Macroscopic  descrip- 
tions. 


Microscopic  descriptions. 


1852.. 


Near  Copper  Falls 
mine,  Keweenaw 
Point,  Micliigan. 


NH. 


12 


31 W. 


Medium-grained; 
dark  purplish-red ; 
of  open  texture ; 
effervesces  briskly 
with  hydrochloric 
acid;  rather  more 
coarsely  grained 
than  1943;  contains 
an  occasional  peb- 
ble of  felsite  or  por- 
phyry. 


graphic  secondary  quartz,  which  is 
characteristic  of  that  rock  throughout 
the  Lake  Superior  region.  There  are 
also  present  rarer,  but  not  uncommon, 
particles  worn  from  basic  rocks. 
These  are  recognizable  as  for  the 
most  part  derived  from  the  matrices 
of  amygdaloids,  showing  the  peculiar 
feldspar  microliths  and  deep-brown 
ferritic  alteration  characteristic  of 
these  matrices.  There  are  also 
present,  however,  particles  plainly 
derived  from  the  more  completely 
crystalline  portions  of  the  basic  rocks, 
but  always  in  a  much  altered  condi- 
tion. There  are  here  and  there 
magnetite  particles.  The  constituent 
particles  raoge,  for  the  most  part, 
from  0.  55  •°'"  to  0. 70  ■»"  in  length. 
There  is  little  or  no  original  intersti- 
tial matter ;  but  the  spaces  between 
the  particles  are  often  occupied  by 
infiltrated  calcite,  with  a  little  quartz, 
both  presenting  themselves  in  very 
fine  particles. 
The  particles  of  which  this  rock  is  com- 
posed are  from  angular  to  sub-angu- 
lar. Although  generally  showing 
some  signs  of  attritioD,  they  are 
never  much  rounded.  They  often 
reach  1.00  "n™  or  more  in  greatest 
length.  They  have  been  derived,  for 
the  most  part,  from  the  matrix  and 
porphyritic  feldspars  of  a  quartzifer- 
0U3  porphyry ;  but  particles  referable 
to  the  augite-syenites  are  common, 
showing  every  phase  of  the  character- 
istic secondary  quartz  of  these  rooks ; 
whilst  smaller  particles  from  the 
basic  rocks,  as  in  1943,  are  not  wholly 
wanting.  Quartz  particles,  plainly 
derived  from  the  quartzes  of  a  quartz- 
iferous  porphyry,  occur,  but  are  not 
abundant.  There  is  no  original  inter- 
stitial matter;  but  the  often  wide 
spaces  between  the  grains  are  now 
wholly  occupied  by  a  coarsely  crys- 
talline calcite.    Fig.  3,  Plate  XVX 


9  L  S 


130 


COPPER-BEAEING  EOCKS  OF  LAKE  SUPERIOE. 


Tabulation  of  microscopic  observations  upon  sandstones,  &c. — Continued. 


I 
a 


Place. 


Macroscopic  descrip- 
tions. 


Microscopic  descriptioiis. 


1857- 


rrom  the  base  of  the 
"Great  Conglom- 
erate" near  Copper 
Falls,  Keweenaw 
Point,  Michigan. 


KB. 


12 


58 


31 W. 


Fine-grained ;     very 
dark-red;  firm. 


1969. 


Sandstone  of  "Calu- 
met conglomerate," 
Calumet  mine, 
Keweenaw  Point, 
Michigan. 


23 


S6 


33  "VP". 


Fine-grained;  dark- 
reddish;  carries  mi- 
nute red  felaite  peb- 
bles; effervesces 
with  hydrochloric 
acid. 


1792. 


Kear  the  Atlantic 
mill,  Portage  Lake, 
Michigan. 


N-W*. 


34 


34-W. 


Medium-  to  coarse- 
grained; dark  pur- 
plish-red, banded 
with  light-red;  some 
portions  effervesce 
briskly,  others  not 
at  all. 


The  larger  particles  of  this  rock  are 
fragments  worn  from  qaartziferoua 
porphyries  and  augite-syenites,  as  in 
the  two  preceding  rocks ;  except  that 
the  particles  are  smaller  and  the 
quartzes  more  abundant  in  this  case. 
Thickly  strewn  among  these  larger 
particles  are  numerous  smaller  ones 
worn  from  basic  rooks.  These  part- 
icles make  up  more  than  half  the  bulk 
of  the  rock,  and  to  their  presence  the 
dark  color  is  due.  Much  the  larger 
proportion  of  these  basic  particles  are 
fragments  of  magnetite,  but  there  are 
also  present  numerous  fragments  of 
matrices  of  amygdaloids  and  of  a 
nearly  opaque  red  oxide  of  iron,  which 
may  be  an  alteration-product  from 
some  of  the  constituents  of  a  basic 
rock.  The  latter  particles  are  so 
opaque  as  to  be  readily  mistaken  for 
magnetite  in  the  transmitted  light. 
TSo  infiltrated  calcite  is  to  be  seen. 

Composed  of  coarser  particles  imbedded 
in  a  matrix  of  fine  particles,  moat  of 
the  former  and  all  of  the  latter  being 
highly  angular.  The  larger  particles 
consist  chiefly  of  fragments  of  the 
matrix  and  porphyritic  feldspars  of  a 
quartziferous  porphyry,  along  with 
which  are  rarer  ones  of  amygdaloid 
matrix.  The  finer  portion  consists  of 
the  same  materials  with  a  larger  pro- 
portion of  basic  detritcs,  abundant 
magnetite  particles,  rather  plentiful 
quartz  particles,  and  some  infiltrated 
calcite  and  epidote.  See  Plate  XTI, 
Fig.  4. 

Larger  particles,  running  from  0.5°""  to 
1.5™"  in  greatest  length,  and  angular  to 
sub-angular — rarely  much  rounded — 
are  imbedded  in  a  matrix  composed  of 
fine  particles.  The  larger  particles, 
which  tend  to  aggregate  along  certain 
lines,  are  mostly  of  bubble-bearing 
quartz.  There  are  also  present  among 
these  larger  particles  pieces  of  red- 
dened and  clouded  feldspars,  for  the 
most  part  orthoclase,  others  of  por- 
phyry matrix  of  the  usual  characters, 


DETEITAL  ROCKS. 


131 


Tabulation  of  microscopic  observations  upon  sandstones^  <&c. — Continued. 


a-^ 


Place. 


C? 


Macroscopic  descrip- 
tions. 


Microscopic  descriptions. 


1976.. 


2504.. 


2506.. 


From  quftrry  on  north 
side  of  Portage 
Lake,  Michigan. 


Foot-wall  sandstone 
at  Nonesuch  mine, 
Porcupine  Mount- 
ains,  Michigan. 


Nonesuch  '*vein 
rock,"  Porcupine 
Hountains,  Michi- 
gan. 


SB. 


SE. 


SB. 


55 


34^". 


43  "W. 


50 


43  "W. 


Excessively  fine- 
grained; dark-gray; 
thin  -laminated; 
hard. 


Light  reddish-brown, 
streaked  with  still 
lighter  shades ;  fine- 
grained. 


Bark  greenish-gray ; 
fine-  to  rather 
coarse- grained; 
oariies  small  peh* 
bles  of  red  felsite. 
Abundant  minute 
flakes  of  copper  are 
Tisihle. 


and  still  other  much  rarer  ones,  de- 
rived from  the  basic  rocks.  The  fin© 
matrix  is  largely  composed  of  angular 
quartz  fragments,  along  with  which 
are  others  again  of  porphyry  matrix, 
and  still  others  stained  of  so  deep  a  red 
that  it  is  difficult  to  tell  their  original 
nature.  Calcite  has  filtered  in  along 
certain  seams,  where  it  is  found 
coarsely  crystalline. 

A  very  fine-grained  rock.  Sabangnlar 
to  rounded  particles  of  quartz  pre- 
dominate. The  other  constituent 
particles  are  porphyry  and  basaltic 
detritus,  minute  fragments  of  mus- 
covite,  infiltrated  epidote  and  quartz, 
and  magnetite. 

Angular  to  rounded  quartz  particles 
are  predominant.  Associated  with 
these  are  numerous  fragments  of 
porphyry  matrix  and  a  still  larger 
proportion  of  basaltic  detritus.  There 
is  also  a  good  deal  of  fine  quartz  that 
appears  to  be  infiltrated. 

This  rock  is  composed  of  mingled  por- 
phyry and  basaltic  detritus.  The 
former  presents  itself  in  the  shape  of 
quartz  grains  (which  are  quite  abun- 
dant), and  fragments  of  the  matrix 
and  of  porphyritic  feldspars.  Jn 
several  instances  fragments  of  a 
quartz-porphyry  stiU  containing  por- 
phyritic quartzes  are  to  be  seen.  The 
basaltic  detritus,  to  whose  presence 
the  dark  color  of  the  rock  is  due,  in- 
cludes pieces  of  amygdaloid  matrix, 
particles  of  the  coarser  grained  dia- 
bases, with  the  several  constituents 
together;  fragments  of  triclinic  "feld- 
spar; magnetite  particles  ;  and  green- 
ish particles,  which  are  at  times  re- 
cognizable as  highly  altered  augite 
and  diallage.  In  the  interstices  be- 
tween the  grains,  which  are  from  an- 
gular to  round  in  shape,  a  fine  detrital 
material  is  sometimes  seen ;  but  for 
the  most  part  these  spaces  are  occu- 
pied by  infiltrated  matter,  in  the 
shape  of  epidote,  a  gi^eenish  chlorite, 


132 


COPPEE  BEAEING  EOGKS  OP  LAKE  SUPEEIOE. 


Tabulation  of  microscopic  observations  ttpon  sandstones,  &c. — Continued. 


a 

1 

g| 

Place. 

S 

f 

o 

•1 

E. 

Macroscopic  descrip- 
tions. 

Microscopic  descriptions. 

1 

CO 

1 

CO 

i 
^ 

o^ 

calcite,  and  native  copper,  all  of  which. 

with  the  exception  of  the  last  named, 

are  very  irregularly  distributed 

through    the   section.      The  copper 

molds  itself  sharply  around  the  con- 

stituent fragments,  having  in  most 

cases  a  core  of  magnetite,  an  occur- 

rence suggesting  its  precipitation  by 

the  ferrous  oxide  of  the  last-named 

mineral.    The  copper  also  is  occasion- 

ally to  be  seen  penetrating  into  the 

interiors  of  the  more  readily  decom- 

posable fragments.  Plate  SVI,  Fig.  1. 

2608.. 

Silver-bearing  rook 
underlying  None- 
Buch    vein,    Porcn- 
pine  Mountains, 
Michigan. 

NW. 

S12 

54 

43  W. 

Very    fine-grained; 
dark-gray;    quartz- 
ite-like ;    firm    and 
hard ;  effervesces  in 
places  with  hydro- 
chloric acid. 

This  rock  is  composed  of  predominant 
quartz  fragments,  with  a  smaller  pro- 
portion of  porphyry  detritus,  and  a 
a  large  one— probably  making  up  half 
the  rock— of  infiltrated  quartz.  There 
is  also  some  infiltrated  calcite,  which 
is  very  irregularly  distributed. 

2524.. 

Union  mine  lode,  Por- 
cupine  Mountains, 
Micliigan ;    top    of 
sandstone  underly- 
ing  "outer  trap:" 
from  bed  of  stream. 

NW. 

27 

51 

42  W. 

Very    fine-grained ; 
dark  reddish-hrown; 
firm;     effervesces 
readily  with  hydro- 
chloric acid. 

A  very  fine-grained  rook;  much  re- 
sembling 1792.  Larger  fragments  of 
feldspar  and  porphyry  matrix  are  im- 
bedded in  a  groundmass,  composed  of 
the  same  materials,  with  a  large  pro- 
portion of  quartz.  There  is  also 
present  a  notable  proportion  of  basalt- 
ic detritus.  The  particles  are  for 
the  most  part  quite  angular.  The 
interstices  are  everywhere  filled  with 
infiltrated  calcite,  along  with  some 
epidote. 

2535.. 

Bed   of  tipper   Carp 
Eiver ;  immediately 
OYerlying    amygda- 
loid; Porcupine 
Mountains,    Michi- 
gan. 

sw. 

19 

51 

42 -W. 

Excessively  fine- 
grained; dark  par- 
plish-red;  firm;  ef- 
fervesces readily 
with    hydro-chloric 
acid;  finely  handed 
with    darker    and 
lighter  shades. 

Very  much  like  the  last  rock  described, 
except  that  it  is  much  finer  in  grain, 
and  contains  a  small  proportion  only 
of  basaltic  detritus. 

44  1. 

Falls  of  Bad  Eiver, 
Ashland  County, 

25 

47 

3'W. 

Medium  to  coarse- 
grained; reddish. 

Larger  angular  grains  of  quartz  and 
feldspars    are    imbedded  in   a  fine 

Wisconsin. 

SiOa  69.78  per  cent. 

matrix  composed  of  the  same  mate- 
rials, along  with  much  porphyry 
detritus,  largely  in  a  decomposed 
condition,  and  an  abundant  brown 
oohei-ous  cement. 

DETEITAL  EOCKS. 


133 


Tabulation  of  microscopic  olservations  upon  sandstones,  &c. — Continued. 


Place. 

1 

s 

A 

Macroscopic  descrip- 
tions. 

Microscopic  descriptions. 

'S 

1 

% 

1 

c. 

P! 

o 

a 

CO 

& 

& 

H 

W 

15161. 

Bed   of  Montreal 
Kiver,     Ashland 
County,  'Wisconsin. 

uw. 

20 

47 

IE. 

Fine-grained;  dart- 
gray  ;  very  hard  and 
firm;  effervesces 
■with  hydrocUorio 
acid.     SiOj,  55.91 
per  cent. 

The  predominating  fragments  are  from 
basic  rocks.  "With  these  are  rarer 
ones  of  a  felsitic  porphyry.  Quartz 
particles  are  very  rare.  A  cement  of 
coarsely  crystalline  calcite  pervades 
the  whole  rock,  and  makes  up  prob- 
ably as  much  as  one-fourth  of  its 
mass.  See  Plate  XVI,  Fig.  2;  also 
Vol.  Ill,  Geology  of  'Wisconsin,  p. 
200,  and  Plate  "JTIX  A. 

1709.. 

Silver  Islet  Landing, 

Very    iine-grained ; 
light  yellowish- 

SmaU  angular  quartz  particles  make  up 
the  larger  part  of  the  rock.     There 

north  shore  of  Lake 

Superior,     Ontario, 

gray;  hard  and  firm; 

are  a  few   fragments   of  porphyi-y 

Canada. 

has  a  semi-crystal- 
line aspect.  A  simi- 
l.ar  sandstone  from 
the  same  bed  yielded 
Macfarlane :    silica, 
72.89 ;    iron    oxide, 
0.91;    carbonate   of 
lime,  13.04;  carbon- 
ate   of     magnesia, 
11.94.   (Canadian 
ITaturalist,  new- 
series,   Vol.  IV,  p. 
39.) 

matrix.  Infiltrated  calcite  and  do- 
lomite, along  with  some  infiltrated 
quartz,  occupy  the  interstices  of  the 
fragments.  The  cleavage  lines  of  the 
two  former  are  only  very  rarely  to  be 
observed. 

1709A 

Same  place  as  1709. 

Dark-red;     fine- 

Predominating   angular    quartz    frag- 

grained;  effervesces 

ments  are  imbedded  in  an  abundant 

with     hydrochloric 

red-stained  earthy-looking  matrix,  in 

acid.    Macfarlane's 

which  must  lie  the  calcium  and  mag- 

analysis of  a  similar 

nesium  carbonates  indicated  by  the 

rock  from  this  vicin- 

analysis. 

ity   (Loc.  cit.)  gives: 

silica,  73.45 ;  ferrous 

oxide,  2.41 ;  carbon- 

ate of  lime,   12.54; 

carbonate  of  mag- 

nesia, 10.94. 

OHAPTEE    IV. 

STRUCTURAL  FEATURES  OF  THE  THREE  CLASSES  OF 
ROCKS   OF  THE   KEWEENAW   SERIES. 

The  basic  rocks;  their  arrangement  in  distinct  beds  or  flows.— Division  of  the  finer-grained  rooks  into 
distinct  portions.— The  amygdaloids.— Stratiform  amygdaloids.— Aahhed  amygdaloids.— Notice- 
able absence  of  beds  of  true  volcanic  ash  in  the  Lake  Superior  region.— The  massive  basic  rocks, 
or  so-called  "traps."— Indications  of  viscous  flow  in  the  basic  rocks.- Proof  that  the  bedded 
basic  rooks  are  "  contemporaneous." — Minor  undulations  in  the  beds  of  basic  rocks. — Lateral  ex- 
tent of  the  beds  of  basic  rooks.— Lateral  extent  of  the  groups  of  beds.— Thickness  of  individual 
Ijeds.— Effect  upon  the  topography  of  the  structure  of  the  basic  rocks  illustrated  by  occurrences 
on  the  North  Shore;  on  the  South  Shore.— Occurrence  of  cutting  masses  or  dikes.— Character  of 
these  dikes  as  seen  on  the  Minnesota  coast ;  these  dikes,  and  those  of  the  underlying  slates,  fill 
the  fissures  through  which  came  the  bedded  basic  rocks.— The  massive  coarse-grained  gabbros  of 
the  Duluth  and  Bnil€  Lake  regions  of  Minnesota,  and  the  Bad  River  region  of  Wisconsin;  are 
they  great  flows,  or  do  they  represent  the  deep-seated  portions  of  masses  from  which  flows  have 
come?— Structural  features  of  the  acid  rocks.— Granite,  augite-syenite  and  granitic  porphyry  in 
the  Bad  River  country  of  Wisconsin  and  in  the  neighborhood  of  Duluth. — The  same  in  north- 
eastern Minnesota,  and  on  the  Minnesota  coast.— Quartz-porphyry  as  seen  on  the  North  Shore;  its 
occurrence  at  the  Great  Palisades.— The  Palisade  porphyry  east  of  Baptism  River.— Important 
theoretical  bearing  of  these  occurrences  of  porphyry.— Other  occurrences  of  quartzose  porphyry 
on  the  North  Shore. — Quartzose  porphyry  on  the  South  Shore;  at  Mount  Houghton;  in  the  Onton- 
agon country;  in  the  Porcupine  Mountains.— General  conclusions  as  to  the  origin  and  relative 
ages  of  the  porphyries  and  basic  rocks. — Structural  features  of  the  detrital  rocks. 

The  basic  crystalline  rocks  make  up  the  greater  part  of  the  thickness 
of  the  series.  They  occur  for  the  most  part  in  distinct  beds,  from  a  few 
feet  to  several  hundred  feet  in  thickness,  which,  while  sharply  defined  from 
each  other,  do  not  commonly  possess  any  subordinate  bedding  structure, 
though  such  a  structure  is  at  times  to  be  observed,  as  noted  below. 

Often  these  beds  present  an  easily  recognized  twofold  division,  into  an 
upper,  narrower,  amygdaloidal  portion,  and  a  lower,  compact,  non-amyg- 
daloidal  portion.  This  subdivision  is  one  characterizing  especially  the  beds 
composed  of  the  finer-grained  diabases,  which  are,  however,  much  the  most 
abundant  of  the  basic  rocks.  The  coarser-grained  diabases  and  gabbros 
are  never,  so  far  as  my  observation  has  extended,  furnished  with  amygda- 
loids. Of  the  finer-grained  kinds,  the  olivine -free  diabases  are  perhaps 
somewhat  more  commonly  supplied  with  amygdaloids  than  those  carrying 

134 


AMYGDALOIDS  AND  PSEUD-AMYGDALOIDS.  135 

olivine,  and  have  the  amygdaloids  more  strongly  developed.  However, 
the  olivine-bearing  kinds  are  often  provided  with  very  highly  vesicular 
amygdaloids,  as  for  instance  most  of  the  succession  of  beds  seen  on  the 
Minnesota  coast  between  Knife  River  and  Split  Rock  River,  described  on 
a  subsequent  page  under  the  name  of  the  Agate  Bay  Group. 

The  amygdaloidal  and  compact  portions  appear  to  grade  into  each 
other  through  an  intermediate  stage,  in  which  the  amygdules  are  less  plen- 
tiful. This  intermediate  stage,  recognized  by  Pumpelly  and  Marvine  in 
their  descriptions  of  Keweenaw  Point  geology  as  "amygdaloidal  mela- 
phyr,"  the  first-named  geologist  ha,s  since  shown  to  be  essentially  different 
from  the  upper  amygdaloid,  in  that  in  the  latter  most  of  the  amygdules  fill 
sharply  defined  pre-existing  cavities,  while  in  the  former  they  occupy  the 
positions  of  primary  rock  constituents.  A  threefold  division  of  these  beds 
into  true  amygdaloid,  pseud-amygdaloid,  and  compact  portion  is  thus  to  be 
recognized.  In  a  number  of  cases  I  have  myself  observed  still  a  fourth 
division,  viz:  a  true  amygdaloid,  occupying  the  base  of  the  bed.  This  has 
little  or  no  gradation-zone  into  the  overlying  compact  portion,  is  thinner 
than  the  top  amygdaloid,  and  shows  sparser  and  larger  amygdules,  which, 
moreover,  occupy  cavities  whose  walls  are  unusually  dense  and  sharply 
defined.  All  of  these  divisions  are  often  present,  but  one,  or  all  save  one, 
may  fail.  When  one  division  only  is  recognizable,  it  is  commonly  the  massive 
portion,  but  in  the  thinner  beds  the  pseud-amygdaloid  or  alteration-zone 
not  unfrequently  extends  all  the  way  to  the  base,  there  being  then  no 
massive  unaltered  portion. 

The  amygdaloidal  portions  of  these  beds  present  many  complicated 
and  much  varied  phases,  the  complexities  arising  chiefly  from  molecular 
alterations  subsequent  to  the  solidification  of  the  rock,  from  admixtures  of 
sediment,  or  from  both  of  these  causes  at  once.  Still,  by  a  study  of  fresher 
conditions,  it  is  easy  to  recognize  certain  constantly  recurring  main  charac- 
teristics, viz:  a  matrix  always  different  from  that  of  the  more  compact  por- 
tion of  the  bed,  in  that  it  is  much  denser  and  often  much  less  perfectly 
crystalline,  and  always  much  more  prone  to  alteration;  and  amygdules  of 
one  or  more  of  calcite,  chlorite,  quartz,  epidote,  prehnite,  laiimontite,  cop- 
per, orthoclase,  or  the  alteration-products  of  these,  filling  sharply  defined 


136       COPPEE-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 

and  distinctly  pre-existing  cavities.  The  amygdules  vary  very  greatly  in 
abundance.  Sometimes,  as  in  the  upper  amygdaloids  of  the  Montreal  River, 
and,  yet  more  strikingly,  in  some  of  the  minutely  vesicular  amygdaloids  ot 
the  Minnesota  shore  in  the  vicinity  of  Agate  Bay,  they  nearly  exclude  the 
matrix,  showing  that  the  original  rock  must  have  been  as  vesicular  as  a 
sponge.  The  amygdules  at  times  take  on  a  cylindrical  form,  with  the  axis 
at  right  angles  to  the  bedding,  sometimes  extending  in  this  way  to  a  length 
of  several  inches.  I  have  noticed  such  "spike  amygdules"  in  a  number  of 
cases  in  the  thin  basal  amygdaloids.  Again,  the  amygdaloidal  cavities  are 
found  elongated  laterally  in  a  common  direction,  this  being  carried  some- 
times to  such  an  extent  that  the  amygdules  are  thinned  to  mere  strings. 
This  is  to  be  finely  observed  in  some  amygdaloids  on  the  lake  shore  at 
Duluth,  as  well  as  at  a  number  of  points  on  the  coast  further  to  the  east- 
ward, and,  in  general,  some  traces  of  this  elongation  are  more  often  to  be 
made  out  than  not.  It  suggests  a  flow  of  the  vesicular  matrix  while  in  a 
viscous  condition. 

The  internal  alterations  that  these  amygdaloids  have  commonly  under- 
gone have  been  described  in  some  detail  in  the  last  chapter.  The  altera- 
tion has  in  some  cases  been  an  extreme  one;  "large  parts  of  the  bed  have 
lost  their  amygdaloidal  character,  and  now  consist  of  quartz,  epidote,  calcite, 
prehnite,  chlorite  and  decomposed  amygdaloid  associated  in  the  most  irreg- 
ular manner.  Such  are  the  beds  worked  for  copper  on  Keweenaw  Point, "^ 
and  similar  beds  occur  throughout  the  entire  extent  of  the  series. 

A  kind  of  amygdaloid  very  interesting  in  its  structural  relations  is  to  be 
seen  largely  exposed  along  the  Minnesota  shore  between  French  and  Split 
Eock  rivers,  in  the  Agate  Bay  Group  of  beds.^  This  is  a  highly  vesicular, 
true  amygdaloid,  occurring  in  two  phases  which  graduate  into  each  other. 
In  the  one  a  crumbling  light-brownish  matrix  holds  large  amygdules  of 
radiating  laumontite,  to  which  zeolite  the  matrix  itself  has  often  partly 
altered.  In  the  other  phase  a  reddish-brown,  iron-stained,  hard  matrix  in- 
cludes thickly  studded  minute  amygdules  of  saponite  and  laumontite.  The 
point  of  especial  interest  with  regard  to  these  amygdaloids  is  their  stratiform 

'E.  Pumpelly,  Geology  of  Wisconsin,  Vol.  Ill,  p.  32. 
s  See  chapter  VII,  pp.       , 


STRATIFORM  AMYGDALOIDS.  137 

condition.     They  lie  in  layers  one  inch  and  upwards  in  thickness.     The 
layers  are  quite  irregular  and  non-continuous,  but  afifect  the  whole  mass- 
This  structure  is  brought  out  with  especial  prominence  on  weathered  cliff 
sides,  the  thinner  layers  falling  away  in  fragments  as  from  a  cliff  of  shaly 
limestone.     Occasionally  seams  of  red  sandstone  are  interleaved  or  overlie 
a  thickness  of  the  stratiform  amygdaloid,  into  which  the  sandstone  pene- 
trates at  times  through  fissures  and  irregular  openings.     This  amygdaloid 
graduates  laterally  into  kinds  without  stratification.     It  occurs  in  thick- 
nesses running  from  less  than  a  foot  up  to  as  much  as  15  or  20  feet,  but 
the  usual  thickness  hes  between  5  and  8  feet.     Interbedded  with  and  grad- 
ing into  these  amygdaloids  are  massive  layers  of  fine-grained  olivine-dia- 
base  or  melaphyr  with  a  quite  pronounced  vertically  columnar  structure. 
A  fine  show  of  both  is  to  be  seen  on  the  west  side  of  Agate  Bay,  on  the 
Minnesota  shore,  where  a  total  thickness,  measured,  of  101.5  feet  included 
seven  massive  layers  running  from  2  to  15  feet  in  thickness,  and  seven 
layers  of  stratiform  amygdaloid  running  from  3  to  20  feet  in  thickness.     A 
single  seam  of  red  shaly  sandstone  2  to  3  feet  in  thickness  is  included. 

The  resemblance  of  these  amygdaloids  to  beds  of  sedimentary  origin, 
however  striking  at  the  first  glance,  is  nevertheless  lost  on  close  inspection, 
for  the  vesicular  structure  is  seen  to  be  identical  with  that  of  the  ordinary 
amygdaloids,  while  no  trace  of  a  fragmental  nature  can  be  detected  either 
with  the  naked  eye  or  in  the  thin  section  with  the  microscope.     The  thin 
section  shows  a  completely  interlocked  crystalline  texture,  and  a  composi- 
tion precisely  the  same  as  the  underlying  massive  rock,  except  as  to  the  al- 
terations and  amygdules,  and  the  presence  of  a  considerable  amount  of 
unindividualized  altered  magma  substance.     The  stratiform  condition  may 
be  in  part  due  to  a  succession  of  thin  scoriaceous  flows,  and  in  part  to  a 
true  "fluidal"  structure.     It  appears  often  to  result  from  an  arrangement 
of  amygdules  more  plentifully  on  certain  planes,  subsequent  molecular 
changes  bringing  out  an  apparent  stratification  by  following  the  planes  on 
which  the  amygdules  were  thickest.     But,  however  this  may  be,  there  can 
be  no  question  as  to  the  identity  of  origin  of  these  amygdaloids  with  all 
others  of  the  series,  although  in  Owen's  Geological  Survey  of  Wisconsin, 


138       COPPBE-BEAEING  BOOKS  OF  T.AKF.  SUPEEIOE. 

Iowa,  and  Minnesota/  Norwood  has  called  them  "  metamorphic  shales,"  in 
which  he  has  since  been  followed  by  N.  H.  Winchell.^  The  possibility  of 
a  sedimentary  origin  for  them  is  absolutely  excluded  by  the  completely 
crystalline  interior  texture,  the  highly  vesicular  character,  the  presence  of 
unindividualized  magma,  the  microscopic  flowage  structure,  and  the  gradua- 
tion of  each  bed  downward  into  vertically  columnar,  non-vesicular  olivine- 
diabase.^ 

The  peculiar  type  of  amygdaloid  characterizing  the  so-called  Ash- 
bed  of  Keweenaw  Point  is  somewhat  related  to  the  last-described.  It 
appears  as  a  peculiar  and  irregular  mixture  of  red  sand  and  amygdaloidal 
material,  and  bears  at  first  sight  some  resemblance  to  those  conglomerates, 
already  described,*  in  which  the  pebbles  are  amygdaloidal.  But  in  the 
Ashbed  the  apparent  pebbles  appear  on  close  observation  to  be  mostly 
connected,  and  I  am  disposed  to  follow  Wadsworth^  in  considering  that  it 
represents  a  very  scoriaceous  and  open  layer,  upon  and  within  which  more 
or  less  sand  was  subsequently  deposited. 

In  this  connection  it  may  be  said  that  materials  to  which  the  term 
"ash"  could  be  applied,  in  the  sense  of  volcanic  detrital  material,  are  almost 
wholly  wanting  in  the  Lake  Superior  country.  It  is  barely  possible  that 
the  basic  ingredient  of  the  dark-gray  sandstone  and  black  shale  of  the  belt 
above  mentioned  as  extending  from  the  Gratiot  River,  on  Keweenaw  Point, 
westward  to  Bad  River,  in  Wisconsin,  may  be  in  part  of  this  nature,  as  also 
the  material  of  which  the  somewhat  allied  gray  sandstone  at  Duluth  is  com- 
posed; but,  with  these  very  doubtful  exceptions,  I  have  met  with  nothing 
in  the  entire  Keweenaw  Series  which  could  have  originated  as  volcanic 
ash.  I  had  supposed  in  the  field  that  some  of  the  crumbly,  more  or  less 
obscurely  stratiform  beds  of  the  North  Shore  might  be  of  this  nature,  their 
weathered  surfaces  presenting  at  first  sight  the  appearance  of  aggregated 

'Pages  345  to  361.  In  Norwood's  descriptions  of  the  Minnesota  coast  this  term  constantly  recurs, 
and  the  coast  line  is  represented  as  chieiiy  formed  by  "metamorphic  shales,"  which  term,  having  ap- 
plied it  first  to  the  bedded  amygdaloids,  he  spread  to  nearly  all  other  amygdaloids,  as  well  as  to  some 
of  the  more  compact  diabases.  I  could  find  no  rock  between  Duluth  and  Grand  Portage  Bay  to  which 
the  term  would  in  any  sense  be  applicable. 

^Reports  of  the  Geological  Survey  of  Minnesota  for  1878  and  1879. 

=  See  further  as  to  these  amygdaloids,  Chapter  VII. 

<  Chapter  II,  p.  29. 

•Notes  on  the  Iron  and  Copper  Districts  of  Lake  Superior,  p.  112. 


INDICATIONS  OP  FLOW  AGE  IN  THE  BASIC  EOCKS.  139 

angular  particles,  but  all  turned  out  to  be  completely  crystalline  rocks, 
owing  their  appearance  of  a  fragmental  nature  to  decomposition. 

The  more  massive  beds  of  basic  rocks,  or  the  lower  massive  portions 
of  such  beds  as  possess  the  crowning  amygdaloids,  are  often  without  any 
definite  structure.  This  is  the  common  case  in  the  typical  region  of 
Keweenaw  Point,  where  a  few  beds  only  show  a  tendency  to  a  transverse 
columnar  structure.  On  the  Montreal  River,  however,  this  structure  is 
well  developed  in  a  series  of  quite  thin  beds,  which  are  furnished  with  very 
perfect  amygdaloids,  and  stand  about  vertically.  On  the  Minnesota  shore, 
a  columnar  structure  is  the  rule,  and  since  the  beds  lie  at  a  very  flat  angle, 
cliffs  of  vertically  columnar  rocks  are  a  prominent  feature.  Still,  the  struc- 
ture never  reaches  a  great  perfection. 

Allusion  has  been  made  above  to  the  elongated  vesicles  of  some  of  the 
amygdaloids.  Other  indications  of  flowage  sometimes  to  be  seen  in  con- 
nection with  the  more  massive  rocks  are  a  slaggy  or  ropy  texture,  and  an 
appearance  as  if  a  once  solidified  crust  had  broken,  and  the  several  frag- 
ments reunited  by  further  solidification,  suggesting  the  "clinkers"  of  mod- 
em volcanic  regions.  These  appearances,  while  noted  at  a  number  of 
points,  are  not  common.  They  are  merely  additional  indications  of  the 
once  fluid  condition  in  which  all  of  these  basic  rocks  are  sufficiently  proved 
to  have  been  by  the  common  division  into  amygdaloidal  (vesicular)  and 
compact  portions;  by  the  mineralogical  composition — augite,  plagioclase, 
olivine,  magnetite;  by  the  completely  crystalline  condition;  by  the  flowage 
lines  formed  by  the  tabular  plagioclases  in  some  of  the  finer-grained  kinds, 
and  by  the  occurrence  in  some  of  the  kinds  of  remnants  of  the  original 
glass  magma.  The  interstratification  of  these  rocks  with  wholly  unaltered 
sandstones  and  shales  must  preclude  at  once  any  thought  of  a  metamorphic 
origin.  As  already  indicated,  I  can  only  regard  these  beds  as  true  lava 
flows,  owing  their  interstratification  with  the  accompanying  sediments  to 
their  having  been  poured  out  at  surface  while  the  sediments  were  forming. 

A  very  beautiful  additional  proof  of  contemporaneousness  is  furnished 
by  the  peculiar  relation  of  a  sandstone  to  its  immediately  underlying  crys- 
talline rock,  the  sand  penetrating  the  openings  and  cracks  in  the  latter.  A 
number  of  places  were  noted  on  the  North  Shore  where  the  overlying  .sand- 


140       COPPEE-BEAEING  E0CK8  OP  LAKE  SUPEEIOE. 

stone  bed  has  been  removed  and  large  surfaces  of  the  underlying  diabase, 
sometimes  many  hundreds  of  feet  in  length,  present  the  singular  appearance 
of  being  intersected  by  veins  of  sandstone,  the  seeming  veins  crossing  each 
other,  zigzagging  and  branching,  like  true  vein-formed  material. 

On  the  Minnesota  shore,  where  single  beds  can  be  followed  on  ex- 
posure for  long  distances,  numerous  minor  bowings  and  corrugations  are 
seen  to  affect  the  layers,  which,  nevertheless,  preserve  a  constant  lakeward 
slant  at  a  low  angle.  Individual  layers  and  sets  of  layers  can  be  followed 
for  miles,  rising  into  arches,  sometimes  of  short  span,  and  again  sinking 
out  of  sight  to  reappear  in  a  short  distance.  These  minor  undulations  are 
the  only  possible  foundation  Norwood  could  have  had  for  his  profile  of 
the  North  Shore,^  on  which  he  represents  a  series  of  much-folded  rocks. 
This  peculiar  warped  structure  I  conceive  to  be  in  a  measure  due  to  the 
original  irregularities  of  the  beds.^ 

Laterally,  the  beds  are,  of  course,  not  of  indefinite  extent.  They 
must  of  necessity  be  far  less  extensive  than  sedimentary  beds  of  the  same 
thickness,  and  these,  too,  wedge  out  laterally.  With  a  succession  of  beds 
much  like  each  other,  it  is  commonly  difficult  to  prove  the  continuity  or 
non-continuity  of  single  flows  over  any  great  distance.  On  the  Minnesota 
shore,  however,  I  was  able  to  trace  individual  layers  for  ten  to  fifteen  miles 
with  certainty,  and  with  great  probability  much  further  than  this.  The 
Greenstone  of  Keweenaw  Point,  a  melaphyr  or  olivine-diabase  flow  of 
considerable  thickness,  which  can  be  readily  traced  by  reason  of  its  pecu- 
liar character  and  marked  effect  on  the  topography,  runs  beyond  question 
from  near  the  end  of  Keweenaw  Point  westward  to  the  Allouez  mine,  a 
distance  of  nearly  30  miles,  and  in  all  probability  is  I'epresented  at  Portage 
Lake,  30  miles  further  southwest,  by  a  coarse-grained  bed  seen  near  the 
Atlantic  mine.  The  so-called  Ashbed  of  Keweenaw  Point  appears  to 
have  been  recognized  at  points  80  miles  from  each  other,  but  the  reference 
of  the  rocks  of  the  two  places  to  the  same  bed  is  made  on  stratigraphical 
and  lithological  evidence  only,  no  actual  continuity  having  been  proved. 

Groups  of  layers  of  allied  lithological  characters  are  recognizable  over 

'Atlas  to  Owen's  Geological  Survey  of  Wisconsin,  Iowa,  and  Minnesota. 

'See  Chapter  VII  for  more  specific  descriptions  of  these  irregularities,  with  illustrations. 


THICKNESS  OF  BEDS.  141 

mucli  longer  stretches.  The  belt  of  thin  amj-gdaloids  and  diabases  lying 
above  the  great  conglomerate  of  Eagle  River,  Keweenaw  Point,  with  a 
thickness  of  some  1,500  feet,  must  run  uninterruptedly  from  the  eastern 
extremity  of  Keweenaw  Point  to  the  Wisconsin  boundary,  a  distance  of 
150  miles.  Several  of  the  groups  into  which  I  have  divided  the  rocks  of 
the  Minnesota  coast  can  also  be  recognized  for  many  miles,  and  there  is  a 
strong  probability  that  one  or  two  of  them  exist  on  Isle  Royale. 

The  thickness  of  the  individual  beds  of  these  basic  rocks  has  been 
already  given  as  ranging  from  ten  or  under  to  several  hundred  feet,  but 
for  the  greater  portion  of  the  series  the  thickness  is  less  than  100  feet. 
Towards  the  base  of  the  series,  among  the  older  flows,  however,  beds  occur 
of  great  thickness,  and  present  massive  exposures  in  which  it  is  very  hard 
to  see  any  sign  of  structure.  At  this  horizon  in  the  Bad  River  region  of 
"Wisconsin  are  immense  structureless  masses  of  a  very  coarse-grained  gray 
gabbro,  and  similar  rocks  have  a  great  development  at  a  similar  hori- 
zon in  Minnesota.  Fine-grained  diabases  also  occur  largely  at  these  low 
horizons  without  any  such  distinct  bedding  as  is  characteristic  of  most  of 
the  series;  for  instance,  on  the  so-called  "South  Copper  Range"  of  Mich- 
igan, between  the  Montreal  and  Ontonagon  rivers,  and  in  the  Duluth 
Group  of  the  Minnesota  coast.  The  apparent  lack  of  bedding  in  the 
latter  region  is  doubtless  due  to  the  much  greater  thicknesses  of  the  in- 
dividual beds,  and  to  their  nearly  vertical  position,  but  it  is  possible 
that  the  coarse-grained  rocks  of  the  Bad  River  and  Duluth  regions  may 
owe  their  want  of  structure  to  a  different  cause,  as  is  subsequently  indicated. 

The  effect  of  the  prevalent  bedded  basic  rocks  upon  the  topography  is 
everywhere  very  marked,  having  a  common  character,  varied  only  by  the 
varying  dips  and  varying  thicknesses  of  the  individual  layers.  Since  the 
dip  is  almost  always  lakeward,  the  common  effect  is  a  longer  lakeward  or 
front  slope,  and  a  steep  or  precipitous  back  slope.  Where  the  dip  is  flat,  as 
all  along  the  Minnesota  shore,  the  front  slope  coincides  with  the  dip  slope, 
and  the  shore  line  of  hills  ascends  at  an  angle  of  from  5°  to  10°,  to  drop  off 
suddenly  in  the  rear.  The  valleys  of  the  streams  entering  the  lake  nearly 
at  right  angles  divide  these  hills  into  detached  blocks.  Such  a  series  of 
blocks  is  well  seen  as  one  looks  towards  the  west  from  Grand  Marais. 


142 


COPPEE-BBAEING  ROCKS  OF  LAKE  SUPEEIOE. 


From  Grand  Marais  westward  to  Poplar  Eiver  the  trend  of  the  strata  is 
more  easterly  than  that  of  the  coast,  so  that  in  looking  in  that  direction 
these  hill  blocks  are  seen  succeeding  each  other  in  such  a  way  as  to  have 
suggested  the  very  apposite  epithet  of  "Saw  Teeth  Mountains."  An 
attempt  to  represent  this  appearance  is  made  in  the  accompanying  outline 
sketch. 


Fig.  1. — Outlines  of  coast  hills  for  20  miles  above  Grand  Marais,  Minn. 

In  the  woods  of  the  North  Shore  away  from  the  lake,  the  same  feature 
is  constantly  repeated;  ridges  are  everywhere  met  with  trending  with  the 
strike  of  the  formation,  sloping  gradually  to  the  southeast,  and  dropping 
off  abruptly  to  the  northwest.  This  structure  has  been  attributed  by  N. 
H.  WinchelP  to  faulting,  each  drop  being  regarded  as  the  result  of  a  fault. 
There  may  be  a  few  such  faults,  but  it  is  evident  enough  that  the  case  is 
just  such  as  is  found  in  every  region  of  flat-dipping  hard  rocks,  and  espe- 
cially where  softer  layers  are  interleaved,  as  in  this  case. 

On  the  south  shore  of  the  lake  the  dip  is  commonly  higher,  and 
although  the  same  structure  occurs  the  front  slope  is  often  flatter  than  the 
dip  slope.  In  the  eastern  part  of  Keweenaw  Point,  where  the  dip  flattens, 
the  structure  comes  out  finely  in  a  series  of  bold  ridges.  Towards  Portage 
Lake,  however,  the  dip  becomes  as  high  as  50°  or  more,  and  the  several 
ridges  merge  into  one  broad  swell.  This  holds  until  the  Porcupine  Mount- 
ains are  reached,  where,  though  the  dip-angle  is  as  high  as  30°,  the  struct- 
ure is  most  beautifully  illustrated  in  the  outer  ridge.      This  ridge  rises 

■i-Eeport  of  the  Geological  Survey  of  Minnesota  for  1878,  p.  12. 


DIKES.  143 

from  the  lake  shore  somewhat  more  gradually  than  the  dip,  to  a  height  of 
over  1,000  feet,  and  then  drops  off  in  a  bold  escarpment  of  400  feet  into 
the  valley  of  Carp  Lake.  Further  west  again,  as  far  as  Bad  River,  the 
dips  are  high,  often  reaching  90°,  and  the  harder  rocks  constitute  merely 
rounded  ridges  and  knobs  with  the  cliffs  facing  indifferently  in  all  direc- 
tions. Beyond  Bad  River,  and  all  across  Wisconsin  to  the  Saint  Croix,  the 
dips  flatten  once  more,  and  the  "saw-tooth"  shape  in  the  ridges  is  every- 
where well  marked. 

True  cutting  masses,  or  dikes,  of  the  basic  rocks  occur,  though  never 
a  prominent  feature  of  the  series.  They  appear  to  characterize  only  its 
lower  portions,  and  have  been  seen  chiefly  along  the  Minnesota  shore,  and 
on  the  east  coast  in  the  region  of  Mamainse.  On  the  South  Shore  they  are 
nearly  unknown,  though  in  all  probability  occurring  largely  in  the  lower 
portions,  where  the  exposures  are  commonly  not  good.  Throughout  the 
entire  basin  they  have  never  been  observed  more  than  one-third  the  way 
above  the  base  of  the  series. 

As  seen  on  the  Minnesota  coast  ^  these  dikes  are  always  small,  com- 
monly under  ten  feet  in  width,  though  seen  occasionally  wider  than  this, 
while  a  number  were  noticed  under  two  feet  in  width.  They  are  usually 
provided  with  a  well-marked  cross- columnar  structure  and  coarser-grained 
middle  portion  as  compared  with  the  sides.  They  consist,  so  far  as  yet  ex- 
amined microscopically,  wholly  of  augite-plagioclase  rocks,  and  are  not  in 
any  essential  point  different  from  the  bedded  diabases  and  melaphyrs  that 
constitute  the  bulk  of  the  series,  the  differences  being  only  those  that  would 
arise  from  the  different  situations  in  which  the  rocks  have  cooled,  and  the 
different  decomposition  effects.  Indeed,  the  thin  sections  of  the  dike  rocks 
are  often  indistinguishable  from  those  of  the  bedded  rocks.  The  dikes  of 
the  Minnesota  coast  trend  usually  with  the  strike  of  the  formation,  less 
commonly  directly  across  it.  I  cannot  doubt  that  these  dikes  and  others 
like  them  all  around  the  rim  of  the  Lake  Superior  basin,  now  unseen  only 
because  of  unfavorable  conditions  of  exposure,  have  been  the  source  of  the 

'  It  should  be  said  here  that  Norwood's  descriptions  of  the  Minnesota  coast  are  entirely  mislead- 
ing as  to  the  number  and  size  of  the  dikes.  As  already  said,  small  dikes  occur  somewhat  abundantly 
at  low  horizons,  but  the  greater  number  of  Norwood's  dikes  are  merely  erosion  points  of  the  harder  and 
more  compact  portions  of  the  diabase  flows. 


144       COPPEE-BEARING  EOCKS  OP  LAKE  SUPERIOR. 

upper  flows.  The  immediately  underlying  series  of  slates  known  to  the 
Canadian  geologists  as  the  Lower  Copper-bearing  Group,  called  by  Hunt 
the  Animikie  Group,  and  regarded  in  this  memoir  as  unquestionably  the 
equivalent  of  the  iron-bearing  Huronian  rocks  of  the  South  Shore,  is 
intersected  everywhere  by  a  very  much  more  powerful  system  of  dikes. 
These  are  spoken  of  more  especially  on  a  subsequent  page,  and  are  cited 
here  merely  that  I  may  express  my  belief  that  in  them,  and  in  the  smaller 
dikes  of  the  Keweenaw  Series  itself,  .we  see  the  source  of  the  volcanic 
strata  of  the  lake  basin,  all  around  whose  rim  I  conceive  the  eruptions 
to  have  taken  place,  rather  than  from  any  one  vent,  as  some  have  supposed,^ 
or  from  within  any  restricted  portion  of  the  basin. 

The  great  structureless  masses  of  coarse  gabbro,  which  in  the  Bad 
River  region  of  Wisconsin,  and  again  in  the  Duluth  region  of  Minnesota, 
and  thence  northeastward  to  the  Bruld  Lake  country,  constitute  so  marked 
a  feature  in  the  geology  of  those  regions,  have  been  above  alluded  to  as 
possibly  owing  their  lack  of  structure  to  the  enormous  thickness  of  the  out- 
flow. There  are  some  things  about  them,  however,  that  suggest  another 
origin.  The  great  coarseness  of  grain,  the  perfection  of  the  crystalliza- 
tion, the  abrupt  terminations  of  the  belts,  the  complete  want  of  structure, 
and  the  presence  of  intersecting  areas  of  crystalline  granitoid  rocks — all 
suggest  the  possibility  that  we  have  here  to  do  with  masses  which  have 
solidified  at  great  depths.  They  certainly  can  not,  however,  be  regarded 
as  intrusive  in  the  ordinary  sense  of  the  word ;  so  that,  unless  we  regard 
them  as  great  outflows,  we  should  be  forced  to  look  upon  them  as  the  now 
solidified  reservoirs  from  which  the  ordinary  Keweenawan  flows  have  come. 
The  acid  rocks  cutting  these  coarse  gabbros  are  clearly  intrusive. 

Of  the  original  acid  rocks  of  the  Keweenaw  Series,  true  granite  has 
been  observed  only  in  the  Bad  River  region  of  Wisconsin,  where  it  is  seen 
intersecting  the  coarse  gabbro  of  the  base  of  the  series,  and  also  the  under- 
lying slates.  It  is  there  a  coarse,  flesh-colored,  completely  developed 
granite,  cutting  the  gabbro  in  irregular  masses  and  in  broad  bands.  In  the 
same  region  a  brick-red  granitic  porphyry  or  granitell  occurs  in  the  same 

'  See  A.  R.  C.  Selwyo,  in  Report  of  the  Geological  Survey  of  Canada  for  1877-'78,  p.  15  A. 


ACID  EOCKS  m  THE  BEULE  LAKE  COUNTEY.       145 

position,  as  do  also  thin  seams  of  a  material  midway  between  true  granite 
and  granitic  porphyry. 

Closely  similar  occurrences,  both  as  to  the  containing  and  cutting  rocks, 
obtain  at  Duluth,  save  that  here  we  have  none  of  the  true  granite,  a  brick- 
red  augite-syenite  being  the  principal  intrusive  rock.  As  seen  at  Duluth, 
this  rock  often  verges  closely  on  granite  on  the  one  hand,  and  on  the  other 
can  be  traced  through  finer  kinds  into  felsite  and  true  quartziferous  por- 
phyry. It  intersects  the  gabbro  in  the  most  irregular  manner,  forming 
great  irregularly  outlined  patches  in  it,  often  many  hundred  square  feet  in 
area,  and  again  occurring  in  sharply  defined  veins,  a  few  feet,  or  even 
inches,  in  thickness.  Some  three  or  four  miles  north  and  west  from  the 
lake  shore  at  Duluth,  N.  H.  Winchell  reports  that  most  of  the  hills  are  com- 
posed of  a  crystalline  red  rock,  similar  to  that  of  Duluth.^ 

Mr.  W.  M.  Chauvenet  has  carried  the  Duluth  gabbro,  with  its  accom- 
panying red  rocks,  westward  for  some  ten  miles  and  northward  for  thirty- 
five  miles,  to  the  Cloquet  River,  where  the  gabbro  appears  in  large  expo- 
sures. To  the  northeast  N.  H.  Winchell  reports  a  similar  rock  with  a  similar 
accompaniment  at  the  headwaters  of  Poplar  River,  some  20  miles  back  from 
the  lake,  while  Mr.  Chauvenet  has  examined  a  large  area  of  like  character 
at  the  headwaters  of  the  Brul^  and  Cascade  rivers,  and  about  Bruld  Lake, 
in  townships  62  and  63,  ranges  1,  2,  and  3  W.,  where  the  red  rock  rises  into 
mountain  masses,  a  prominent  instance  of  which  is  Eagle  Mountain.  This  lies 
near  the  middle  of  T.  63,  R.  1  W.,  and  rises,  a  bald  mass  of  red  rock,  to  a 
height  of  450  feet  above  the  small  lake  at  its  western  foot  and  1,500  or 
1,600  feet  above  Lake  Superior.  Similar  granitoid  rocks  occur  at  higher 
horizons  along  the  Minnesota  coast,  and  are  always  in  intersecting  masses 
except  when  tending  towards  the  quartziferous  porphyries  in  character,  when 
they  appear  to  form  flows,  of  a  similar  nature  to  those  of  the  latter  rock. 
Intrusive  augite-syenites  are  also  met  with  in  the  Bohemian  Range  of  Ke- 
weenaw Point. 

Quartziferous  porphyries,  in  association  with  bedded  diabases  and  mela- 

'The  Building  Stones  of  Minnesota,  1880,  p.  7;  also  Annual  Report  of  the  Geological  Survey  of 
Minnesota  for  1879,  p.  24. 
10  L  S 


146       COPPEE-BBAEING  EOCKS  OP  LAKE  SUPEEIOE. 

phyrs,  are  met  with  at  a  number  of  points  on  the  North  Shore  between  Du- 
luth  and  Saint  Ignace  Island,  south  of  Nipigon  Bay. 

One  of  the  most  instructive  occurrences  of  true  quartz-porphyry  that 
I  have  yet  met  with  throughout  the  entire  lake  basin  is  at  the  Great 
Palisades,  on  the  Minnesota  shore,  six  miles  below  the  little  hamlet  of 
Beaver  Bay,  Sec.  22,  T.  56,  R.  7  W.  The  Palisades,  which  constitute  the 
most  striking  feature  of  North  Shore  scenery  between  Duluth  and  Pigeon 
River,  rise  in  a  sheer  precipice  from  the  water's  edge  to  a  height  varying  from 
150  to  300  feet.  The  length  of  the  precipice,  whose  main  front  looks 
southeast  by  east,  and  runs  almost  exactly  with  the  general  trend  of  the 
strata  in  the  vicinity,  is  about  three-fourths  of  a  mile.  At  the  north  end, 
at  the  mouth  of  Palisade  Creek,  the  cliff  turns  to  the  westward,  and  facing 
first  north,  and  then  west,  is  nearly  as  difficult  in  ascent  in  the  woods  as  on 
the  lake  front.  At  the  south  end  the  shore  of  the  lake  runs  back  with  an 
east  and  west  course,  in  such  a  manner  as  to  expose  the  mass,  with  the 
underlying  rocks,  in  cross-section.  The  same  underlying  rocks  are  cut 
down  into  by  Palisade  Creek  below  the  west  cliff,  so  that  the  Palisade  rock 
is  a  detached  mass  of  porphyry  three-fourths  of  a  mile  long,  one-fourth 
wide,  and  from  300  to  400  feet  thick. 

The  Palisade  porphyry  is  a  hard  and  dense  rock,  purplish-red  on  a 
fresh  fracture,  weathering  to  a  light-red  and  even  pink  on  the  face  of  the 
cliff.  The  matrix  is  thickly  studded  with  sharply  outlined,  white  and  par- 
tially kaolinized  orthoclase  crystals,  running  up  to  0.15  inch  in  length,  and 
somewhat  rarer  and  smaller  black  quartzes.  In  many  cases  the  matrix 
shows  a  fine  banding,  due  to  faint  differences  in  shades  of  color,  lighter 
colored  laminae  appearing  to  alternate  with  darker.  Sometimes  these  appar- 
ent laminae  are  more  definite,  and  from  one  to  two-tenths  of  an  inch  in 
width.  More  commonly  they  are  exceedingly  indefinite,  and  due  to  rows 
of  lighter-colored  indefinite  spots  following  one  direction.  Only  rarely  does 
the  hand  specimen  show  this  lining  plainly,  but  on  the  exposed  and  weath- 
ered surfaces  it  stands  out  prominently.  In  some  places  a  decomposition 
akin  to  kaolinization  has  followed  these  lines  on  weathered  surfaces,  and 
then  there  is  a  cleavage  parallel  to  them,  and  the  resemblance  to  a  material 
of  sedimentary  origin  is  still  more  pronounced.     The  lines  preserve  only  a 


THE  PALISADES  POEPHYEY.  147 

general  parallelism  to  the  bedding  of  the  formation,  being  subordinately 
twisted  and  contorted  into  all  sorts  of  shapes,  so  that  on  a  clifP-side  their 
inclination  often  changes  from  a  nearly  horizontal  one  to  one  of  20°  or 
even  40°  within  a  few  feet.  Where  the  lines  are  developed  the  crystals  of 
quartz  and  orthoclase  are  noticed  to  bear  no  relation  to  them  at  all,  com- 
monly cutting  across  them,  and  at  all  angles,  the  larger  orthoclases  even 
extending  across  two  or  three  of  the  lines.  Very  much  of  the  rock  is 
observed  to  be  without  any  sign  of  this  lining. 

Under  the  microscope,  as  shown  in  the  previous  chapter,  the  Palisade 
rock  presents  the  common  characters  of  a  quartz-porphyry,  and  there  is  no 
trace  of  a  clastic  nature.  The  quartzes  are  all  in  doubly  terminated  crys- 
tals, and  streams  of  black  particles  run  around  them  as  if  pushed  aside 
when  the  rock  was  in  a  viscous  condition.^  The  whole  rock  presents  a  ver- 
tically columnar  structure,  not  developed  so  completely  as  in  some  of.  the 
diabases,  but  still  very  noticeable.  The  striicture  lies  at  right  angles  to  the 
dip,  which  is  some  10°  to  20°,  east-south-east,  varying  between  those  figures. 
In  the  main  clifi",  since  it  trends  with  the  strike,  the  columns  appear  at  first 
sight  much  more  nearly  vertical. 

As  already  said,  both  in  the  Palisade  Creek  and  in  the  bay  at  the 
south  end  of  the  Palisades,  the  underlying  rocks  are  in  sight.  At  the  lat- 
ter place  a  single  cliff-side  shows  the  great  porphyry  overlying  a  thickness 
of  100  feet  of  diabase  and  diabase-amygdaloid.  These  are  disposed  in  two 
great  flows  44  and  56  feet  thick,  respectively,  while  the  top  of  a  third  is  seen 
rising  just  above  the  sand.  Each  of  these  diabases  has  most  perfectly  de- 
veloped crowning  and  basal  amygdaloids.  We  have  thus  a  most  unequivo- 
cal case  of  the  superposition  of  true  quartziferous  porphyry  upon  the  typical 
diabases  and  amygdaloids  of  the  series. 

The  same  layer  that  forms  the  Palisades — or  another  one  closely  like 
it — appears  again  in  a  bold  point  of  bare  rock  150  feet  high,  on  the  north- 
east side  of  Baptism  River  Bay,  a  mile  and  a  half  below  the  Palisades. 
Here  the  underlying  diabases  and  amygdaloids  are  again  seen  in  position. 
The  top  of  the  point  slopes  off  lakeward  with  the  dip  (about  8°).  The 
more  minute  peculiarities  of  the  rock,  and  its  rude  columnar  sti'ucture  at 

'  See  Figs.  1  and  2  of  Plate  XII;   also  pp.  99  and  109  of  this  Memoir. 


148       OOPPEE-BBAEING  BOOKS  OF  LAKE  SUPEEIOE. 

right  angles  to  the  dip  of  the  mass,  are  here  just  as  in  the  Palisades.  The 
same  fine  banding  also  appears  in  places,  but  is  warped  up  and  down,  so 
as  to  change  from  horizontal  to  vertical  within  short  distances.  This  rock 
is  traced  along  the  coast  for  about  three-fourths  of  a  mile,  when  it  again 
disappears  underneath  another  series  of  diabases  and  amygdaloids,  inter- 
leaved with  which  are  thin  seams  of  red  shale,  and  one  bed  of  curiously 
intermingled  shale  and  amygdaloid,  precisely  resembling  the  Ashbed  of 
Keweenaw  Point.  Still  three-fourths  of  a  mile  beyond,  to  the  northeast, 
these  diabases  are  overlaid  by  a  pebble  conglomerate,  in  which  the  matrix 
is  red  shale  and  the  pebbles  rolled  fragments  of  diabase  and  amygdaloid. 

The  occurrence  of  the  porphyry  of  the  Palisades  and  of  Baptism 
River  has  thus  been  given  at  some  length  in  this  connection  because  the 
relations  of  the  basic  and  acid  rocks  are  here  so  unmistakably  plain,  and 
because  upon  these  relations  rest  important  conclusions.  There  can  be  no 
doubt  whatever  that  we  have  here,  between  two  sets  of  perfectly  and 
typically  developed  diabases  and  amygdaloids,  those  on  the  upper  side  car- 
rying the  customary  seams  of  red  shale,  a  true  layer  of  quartziferous  por- 
phyry several  hundred  feet  in  thickness,  while  in  the  previous  chapter  it 
has  been  shown  that  this  porphyry  is  as  truly  an  eruptive  rock  as  the  asso- 
ciated basic  kinds,  presenting  as  it  does  all  characters  of  an  eruptive  por- 
phyry, viz:  a  base  made  up  of  imperfectly  individualized  or  incompletely 
devitrified  material,  doubly  terminated  quartzes  with  inclusions  of  the  base 
and  of  unmistakable  glass,  and  flowage  lines. 

Although  these  rocks  show  at  numbers  of  other  points  on  the  North 
Shore,  the  relations  of  the  porphyry  to  the  diabase  flows  are  not  always  to 
be  so  well  made  out  at  first  sight  as  at  the  Palisades.  Often  the  porphyry 
exposures  occur  separated  from  any  exposures  of  other  rocks  by  long  peb- 
ble beaches.  Again  they  form  merely  projecting  points,  or  restricted  areas 
between  dark-colored  rocks,  apparently  owing  their  preservation  to  a 
sudden  bowing  down  of  the  strata,  so  that  they  represent  subordinate 
synclinals  impressed  upon  the  general  lakeward  dip.  In  other  cases  they 
come  into  abrupt  and  vertical  contact  with  dark-colored  diabases,  and  form 
masses  which  have  been  faulted  into  their  present  positions.  "Wherever  these 
less  plain  occurrences  have  been  closely  studied,  however,  it  has  become 


QUAETZ-POEPHYEIES  OF  TEDS  NOETH  SHOEE.  149 

evident  that  in  them,  too,  we  have  to  do  merely  with  toteAedded  porphyry 


masses. 


As  to  the  more  minute  oharaoteirs  ot  these  other  porphyries  of  the  North 
Shore  we  may  say  that  granitic  porphyries  are  not  common;  that  where 
they  occnr,  except  when  .almost  granite  in  te..nre,  they  - /»«-  '^  ^^ 
carried  directly  into  true  ,na*-porphyries,  often  showing  the  A™  «— 
bandino-  of  the  Palisade  rock;  and  that  a  very  common  feature  of  the  qtmt. 
porphyries  and  felsites  is  a  close,  angular  jointing,  which,  "^  -'^'''ered 
surface,  is  often  developed  to  such  a  degree  that  it  is  imposs.b  e  o  oMam 
hand  specimen  ot  any  size,  or  to  get  a  fresh  fracture,  a  shgh   blow  ot  the 
hrmmer  against  the  cliff  bringing  down  showers  of  small  angular  fragment. 
Where  the  toe  banding  mentioned  is  developed  it  is  frequent  y  possible  to 
lacHhe  rock  with  colrted  banding  into  massive  and  wholly  unhanded 
Muds  wHh  very  large  porphyritic  qnart.es  and  orthoclases.     Th.s  change  . 
^  be  seen  beautifully  at  several  points  on  the  Canada  shore  ^e  "-n  BUck 
ly  and  Saint  Ignace,  notably  at  Bead  Maud,  opposite  Lamh  Ts«h  - 
house,  at  the  mouth  of  Nipigon  Straits.     At  one  po.nt  on  he  North  Shore 
between  Beaver  Bay  and  the  Palisades,  was  noticed  a  pmkish  quartzose 
porphyry,  in  which  the  lighter  matrix  is  filled  with  a  darker  porphyry,  ar- 
rrnged  fn  aU  sorts  of  fantastic  and  snake-like  forms,  smgle  figures  ex- 
ending  sometimes  for  several  feet  with  a  width  ot  two  or  three  mches,  and 
unning  from  these  dimensions  down  to  mere  lines,  the  whok  presentmg 
he  appearance  of  a  somewhat  irregularly  figured  carpet.     This  pecnhsr 
tu  ture  I  attribute  to  flowing  in  a  viscous  condition.     At  several  places,  as 
r  instance  on  the  Devil's  Track  Eiver,  near  the  lake  shore,  and  again  on 
the  shore  of  the  Pigeon  Eiver  Indian  reservation,  the  porphyry  wa   no- 
ticed with  a  tendency  to  come  out  in  thin,  flat  pieces,  or  pieces  with  a 
Irrugated  surface,  but  in  this  case,  as  usual,  it  runs  into  the  compact  sfuct- 

""Irous  porphyries  are  again  '-.^ly  devdoped  on  the  ^outh 
Shore  with  the  same  structural  features  as  noted  on  the  North  Shore^  Moun 
Houghton  and  the  Bare  Hills  ot  the  eastern  end  of  Keweenaw  Pomt  a 
p„7ons  of  abeltofbandedquartziterous  poi-phyry.     Thesewere  longsinee 
mentioned  by  Foster   and  Whitney,  who  describe  the  rock  as  hardened 


150       COPPEE-BEAEINa  BOOKS  OF  LAKE  8UPEEIOE. 

sandstone,  but  I  have  been  able  to  detect  no  trace  of  fragmental  origin,  the 
rock  under  the  microscope,  as  well  as  on  the  large  scale,  presenting  all  the 
characters  of  an  original  porphyry.  The  Mount  Houghton  porphyry  seems 
to  constitute  a  belt  interstratified  with  the  prevailing  diabases  and  standing 
at  a  very  high  angle.  It  has  a  considerable  lateral  extent,  and  I  am  dis- 
posed to  place  with  it  the  quartzose  porphyry  of  the  Torch  Lake  Railroad, 
south  of  the  Calumet  mine,  which  is  evidently  the  source  of  the  pebbles  of 
the  Calumet  conglomerate. 

In  the  region  between  the  Ontonagon  River  of  Michigan,  and  the  Bad 
Eiver  of  Wisconsin,  true  massive  quartziferous  porphyries  are  largely  de- 
veloped. The  exposures  appear  to  lie  in  certain  horizons,  at  least  two  of 
which  have  been  recognized,  and  so  to  constitute  layers  in  the  series,  though 
evidently  very  much  less  regular  ones  than  those  of  the  diabase. 

In  the  Porcupine  Mountains  a  quartzose  porphyry  constitutes  the  cen- 
tral mass  of  the  mountains,  the  beds  of  sandstone,  conglomerate  and  slate 
dipping  away  from  it  on  all  sides  save  where  it  connects  on  the  south  with 
the  Main  Trap  Range.  Here  the  subordinate  features  of  the  porphyries  of 
the  North  Shore  are  seen  constantly  repeated,  but  the  mass  as  a  whole  is 
without  any  trace  of  stratification.  The  structure  of  these  mountains  is 
described  and  illustrated  in  some  detail  in  a  subsequent  part  of  this  report. 

As  already  indicated,  I  conclude  with  regard  to  all  of  these  porphyries 
that  they  are  unquestionably  of  eruptive  origin.  That  they  were  formed 
both  after  and  before  the  more  common  basic  eruptive  rocks  is  shown  by 
such  occurrences  as  that  of  the  Grreat  Palisades  of  the  North  Shore.  These 
porphyry  eruptions  were  evidently  more  plenty  in  the  earlier  part  of  the 
time  of  formation  of  the  series.  Still,  in  the  Ontonagon  region  they  occur 
at  quite  a  high  horizon.  The  structure  of  the  Porcupine  Mountains  has  so 
much  in  common  with  that  of  the  laccolitic  mountains  of  Southern  Utah, 
described  by  G.  K.  Gilbert,  that  they  might  be  supposed  to  owe  their  exist- 
ence to  an  eruption  of  the  porphyry  of  their  central  portions,  in  which 
case  we  should  have  to  believe  in  an  eruption  of  acid  rocks  at  a  time  quite 
subsequent  to  that  of  the  formation  of  the  latest  of  the  basic  flows.  But, 
as  shown  subsequently,  these  mountains  owe  their  existence  in  all  prob- 


STETJCTUEAL  FEATURES  OF  THE  DETKITAL  EOCKS.  151 

ability  to  a  fold,  the  porphyry  of  the  central  portions  being  one  of  the 
usual  embedded  masses  laid  bare  by  subsequent  denudation. 

The  detrital  members  of  the  series  do  not  present  any  structural 
features  that  are  peculiar  to  them  as  compared  with  similar  mechanical  sed- 
iments of  other  regions.  The  conglomerate  layers  reach  sometimes  an 
extraordinary  thickness,  and  are  often  made  up  so  completely  of  large-sized 
pebbles  that  it  is  necessar}^  to  believe  in  the  existence  in  such  places  of  very 
powerful  currents.  Traced  laterally,  these  conglomerate  belts  have  often 
a  great  extent,  but  they  never  remain  constant  as  to  coarseness  or  exact 
nature  of  materials.  The  outer  conglomerate  of  Keneenaw  Point  for  in- 
stance— that  seen  from  Copper  Harbor  eastward  to  the  extremity  of  the 
point — is  undoubtedly  the  same  as  the  outer  sandstone  and  conglomerate, 
immediately  beneath  the  black  slate  of  the  Porcupine  Mountains,  and  is  the 
same  as  the  great  conglomerate  of  the  Montreal,  Potato  and  Bad  rivers, 
in  Wisconsin.  It  has  thus  a  continuous  lateral  extent  as  a  single  layer  of 
at  least  170  miles,  and  possibly  a  much  greater  extent;  but  in  this  long 
distance  it  varies  from  less  than  100  to  4,000  feet  in  thickness,  is  now  pure 
sandstone,  now  nearly  all  conglomerate,  now  a  sandstone  with  conglomerate 
bands,  and  again  a  coarse  bowlder-conglomerate.  Moreover,  as  already 
noticed,  the  nature  of  the  pebbles,  while  always  predominatingly  of  some 
one  of  the  acid  rocks  of  the  series,  presents  many  variations  along  the 
course  of  the  belt,  depending  upon  similar  variations  in  their  source  of  sup- 
ply. The  inner  broad  conglomerate  of  Keweenaw  Point,  which  is  devel- 
oped so  largely  at  the  mouth  of  Eagle  River,  is  also  represented  in  the 
Ontonagon  and  Porcupine  Mountain  regions,  but  on  the  Montreal  has  nar- 
rowed down  to  a  mere  sandstone  seam. 

Of  the  thinner  conglomerates  which  are  intercalated  at  all  horizons  in 
the  Keweenaw  Point  region,  one,  the  so-called  Albany  and  Boston,  lying 
immediately  underneath  the  Greenstone,  has  been  traced  for  a  distance 
of  as  much  as  50  miles,  but  varies  along  its  course,  from  a  full  conglom- 
erate to  a  mere  red  shale  seam.  From  what  I  have  seen  I  have  little  doubt 
that  a  number  of  the  thinner  conglomerates  have  even  a  greater  extent 
than  this. 


CHAPTEE  V. 

GENERAL  STRATIGRAPHY  OF  THE   KEWEENAW  SERIES. 

Separation  of  the  series  into  a  Lower  and  an  Upper  Division. — The  Upper  Division ;  its  thickness ;  its 
■wide  spread  underneath  the  waters  of  Lake  Superior;  its  surface  spread  and  general  characters 
between  Keweenaw  Point  and  the  Montreal  Eiver. — In  Northern  Wisconsin. — The  sandstones  of 
the  Apostle  Islands  and  the  Bayfield  coast  do  not  belong  here. — The  Upper  Division  on  Isle  Roy- 
ale  and  Caribou  Island. — The  Lower  Division ;  from  the  nature  of  the  case  no  subordinate  strati- 
graphy can  be  laid  down  for  the  whole  extent. — Characteristics  of  certain  broad  horizons. — The 
very  coarse  gabbros  of  Bad  and  Saint  Louis  rivers. — The  sandstones  of  the  east  side  of  Thunder 
Bay. — Enormous  total  thickness  of  the  Lower  Division. — Variations  in  total  thickness  connected 
with  the  origin  of  the  constituent  beds. — Remarkable  thinning  on  Bad  River. — Thickness  on 
Keweenaw  Poiut. — Between  Keweenaw  Point  and  the  Ontonagon;  west  of  the  Ontonagon. — On 
Bad  Eiver. — The  thickness  between  Bad  River  and  the  Saint  Croix. — In  the  Saint  Croix  Valley. 
— On  the  Minnesota  coast. — On  Isle  Royale. — About  Black  and  Nipigon  bays. — In  the  valleys  of 
Black  Sturgeon,  and  Nipigon  rivers. — On  Miohipicoten. — At  Cape  Gargantua. — At  Mamainse. 

The  most  prominent  fact  in  regard  to  the  stratigraphy  of  the  Kewee- 
naw Series  is  its  separation  into  two  grand  divisions:  an  Upper  member, 
made  up  wholly  of  detrital  material,  for  the  most  part  red  sandstone  and 
shale;  and  a  Lower  member,  made  up  chiefly  of  a  succession  of  flows  of 
basic  rocks,  but  including  layers  of  conglomerate  and  sandstone  nearly  to 
the  base,  and  more  or  less  of  original  acid  rocks. 

The  line  of  separation  between  these  two  divisions  has  to  be  adopted 
somewhat  arbitrarily,  since  the  sandstone  gradually  increases  in  quantity 
upward;  but  placing  it  at  the  base  of  the  outer  conglomerate  of  Keweenaw 
Point — which  corresponds  to  the  top  of  the  upper  amygdaloid  of  the  Por- 
cupines, and  to  the  base  of  the  great  conglomerate  of  the  Montreal,  and 
which  is  above  any  known  occurrence  of  eruptive  matter — I  estimate  the 
Upper  Division  to  attain  a  maximum  thickness  of  about  15,000  feet  in  the 
middle  portion  of  the  Lake  Superior  basin.  Towards  the  eastern  and  west- 
ern ends  of  the  basin  the  thickness  must  be  much  less  than  this. 

The  sandstones  of  the  Upper  Division  are  largely  concealed  beneath  the 

152 


THE  IJPPEE  DIVISIOIfr  OF  THE  KEWEENAW  SERIES.  153 

waters  of  Lake  Superior,  whose  basin  has  been  in  some  measure  carved  in 
them,  but  they  form  the  outer  part  of  Keweenaw  Point,  attaining,  in  the 
neighborhood  of  Portage  Lake,  a  considerable  surface  width.  Westward 
from  Portage  Lake  they  underhe  the  flat  land  north  of  the  Trap  Range, 
sloping  at  a  low  angle  lakeward,  and  are  well  exposed  at  a  number  of  points 
along  the  shore  to  the  Porcupine  Mountains.  The  Porcupine  Mountain 
fold  throws  them  well  out  into  the  lake,  but  farther  west  they  form  a  belt 
along  the  edge  of  the  land,  and,  since  they  are  here  standing  at  high  angles, 
a  larfife  thickness  is  embraced  in  this  narrow  belt.  On  the  Montreal  the 
angle  reaches  ninety  degrees,  and  nearly  the  whole  thickness  of  the  Upper 
Division  is  crossed  by  the  lower  reaches  of  the  river.  Here  the  Upper  Divi- 
sion consists  of  some  12,000  feet  of  red  sandstone  and  shale,  about  600  feet 
of  black  shale  alternating  with  hard,  gray,  nearly  quartzless  sandstone,  both 
shale  and  sandstone  being  composed  largely  of  basic  detrital  material,  and 
about  1,200  feet  of  very  coarse  bowlder-conglomerate.  The  same  succes- 
sion seems  to  hold  all  the  way  to  where,  on  Keweenaw  Point,  the  base  of 
the  Upper  Division  runs  out  into  the  lake — ^the  only  difference  being  that 
the  basal  conglomerate  at  times  passes  into  a  sandstone. 

Westward  from  the  Montreal  River,  also,  the  same  succession  seems  to 
hold,  the  conglomerate  and  shale  at  the  base,  however,  thinning  out  The 
last  seen  of  these  northward-dipping  sandstones  and  shales,  which  have 
thus  been  traced  all  the  way  from  Keweenaw  Point,  is  on  the  Brunschweiler 
River,  in  the  western  part  of  Ashland  County,  Wisconsin.  West  of  this 
they  have  not  been  observed,  the  country  which  would  be  occupied  by 
them  being  everywhere  drift-covered.  The  underlying  diabases,  however, 
appear  all  across  Wisconsin  with  a  flattened  dip,  while  on  the  ixpper  Saint 
Croix  River  there  is  a  large  development  of  sandstones  dipping  southward, 
which  are  evidently  the  same  that  we  have  been  following,  forming  now  the 
other  side  of  the  great  synclinal  which  here  crosses  the  whole  northern  part 
of  Wisconsin.  The  same  southward-dipping  sandstones  are  to  be  seen  in 
the  northern  part  of  Ashland  County. 

The  Apostle  Islands  and  the  adjoining  coast  of  Wisconsin  are  under- 
lain by  horizontal  sandstones.  These  are  to  be  regarded  as  an  overlying, 
unconformable  formation  belonging  with  the  horizontal  sandstones  of  the 


154       COPPEK-BEAEING  EOCKS  OP  LAKE  SUPEEIOE. 

east  side  of  Keweenaw  Point  ;^  but  it  is  to  be  observed  that  it  is  not 
structurally  impossible  that  these  horizontal  sandstones  should  be  the  same 
as  those  I  have  been  considering,  since,  according  to  the  structure  I  have 
worked  out  for  this  part  of  the  basin,  the  upper  Keweenawan  sandstones 
should  themselves  have  here  a  position  not  appreciably  different  from 
horizontality.^ 

The  upper  sandstones  appear  to  view  again  on  the  south  side  of  Isle 
Royale,  with  a  considerable  development.  Caribou  Island,  south  of  Mich- 
ipicoten,  appears  also  to  be  made  of  them,  but  unless  the  sandstones  east 
of  Keweenaw  Point  represent  their  upward  extension,  a  point  which  is 
subsequently  discussed,  and  which  I  cannot  doubt  should  be  decided  in  the 
negative,  they  do  not  appear  to  view  at  any  other  point.  Still  they  must 
have  a  very  extensive  development  underneath  the  waters  of  Lake  Su- 
perior in  its  eastern  as  well  as  in  its  western  portions. 

The  Lower  Division  of  the  series  must,  from  the  nature  of  the  case, 
present  very  considerable  variations  in  subordinate  stratigraphy  and  total 
thickness.  For  portions  of  the  basin,  and  even  over  areas  50  to  200  miles 
in  length,  it  is  possible  to  recognize  a  pretty  constant  subordinate  arrange- 
ment, the  constancy  increasing,  of  course,  in  inverse  ratio  with  the  size  of 
the  district  considered.  It  is  not  possible,  however,  to  lay  down  any 
scheme  of  subordinate  stratigraphy  which  shall  hold  for  the  entire  extent 
of  the  series.  This  results  not  only  from  the  mode  of  formation  of  the 
rocks,  but  from  the  great  similarity  of  the  beds  at  different  horizons,  and 
through  great  thicknesses.  Nevertheless  it  is  possible  to  make  a  number  of 
generalizations  as  to  the  characteristics  of  broad  horizons  which  will  hold 
throughout  most  of  the  geographical  extent  of  the  Lower  Division  of  the 
series.     These  are  enumerated  in  the  next  paragraph. 

(1)  Coarse-grained  rocks,  including  both  orthoclase-free  and  ortho- 
clase-bearing  kinds,  though  occurring  now  and  then  well  up  in  the  Lower 
Division  (e.  g.,  some  of  the  beds  of  the  Greenstone  Group  of  Keweenaw 
Point),  are  very  much  more  common  at  low  horizons,  and  the  very  coars- 

'  Geology  of  Wisconsin,  Vol.  Ill,  Part  I. 

'See  structural  map,  Plate  XVIII,  and  the  explanations  of  Chap.  IX. 


LOWEE  DIVISION  OF  THE  KEWEENAW  SERIES.  155 

est  kinds  have  been  observed  only  at  the  base  of  the  series  (Bad  River 
gabbros ;  Duluth  gabbros).  (2)  Very  heavy  bedding  is  also  much  more 
common  at  low  horizons ;  and  this  statement  affects  both  coarse-grained  and 
fine-grained  kinds  (e.  g.,  the  fine-grained  diabases  of  the  Duluth  Grroup  of 
the  North  Shore).  (3)  The  amygdaloidal  texture  is  more  frequent  and 
more  highly  developed  at  high  horizons  than  at  low;  the  thinner  beds  gen- 
erally having  the  most  strongly  developed  amygdaloidal  or  vesicular  por- 
tions. (4)  As  to  the  distribution  of  the  different  kinds  of  basic  rocks,  the 
fine-grained,  olivine -free,  or  "ordinary"  diabases  affect  very  decidedly  the 
higher  horizons  (e.  g.,  Keweenaw  Point),  though  occurring  throughout ; 
olivine-bearing  kinds,  both  the  coarse-grained  gabbros  and  fine-grained, 
luster-mottled  kinds,  are  as  decidedly  more  common  at  low  horizons,  though 
as  before  not  restricted  to  them  (e.  g.,  the  Greenstone  Group  of  Keweenaw 
Point) ;  the  ashbed-diabases  and  diabase-porphyrites  are  also  very 
much  more  common  at  low  horizons  (e.  g.,  Lester  River  Group;  Duluth 
Group,  &c.);  and  the  same  is  true  of  the  orthoclase-gabbros.  (5)  Of  the  acid 
rocks  all  kinds  affect  especially  low  horizons,  rarely  reaching  above  the 
middle  of  the  Lower  Division.  The  porphyries  of  the  region  between  the 
Ontonagon  and  Bad  rivers  on  the  South  Shore  seem  to  be  an  exception  to 
this  rule ;  but  in  this  case  their  appearance  at  so  high  a  horizon  may  be 
due  in  some  measure  to  the  thinning  out  of  overlying  beds.  A  more  cer- 
tain exception  to  the  rule  is  probably  to  be  found  in  the  case  of  the  red  fel- 
site  of  the  islands  of  the  harbor  on  the  south  side  of  Michipicoten  Island. 
(6)  Detrital  beds,  chiefly  porphyry-conglomerates  and  red  sandstones,  oc- 
cur throughout  the  series,  having  been  seen  all  the  way  from  the  base  to 
the  summit,  but  they  are  rare  in  the  lower  third  of  the  series,  and  as  a  rule 
increase  in  thickness  and  frequency  towards  the  top,  only  one  instance,  to 
be  noted  hereafter,  being  known  of  a  heavy  bed  at  a  low  horizon. 

The  coarse  gray  gabbros  so  largely  developed  in  the  Bad  River  coun- 
try of  Wisconsin,  at  the  base  of  the  series,  present  the  appearance  of  a  cer- 
tain sort  of  unconformity  with  the  overlying  beds.  These  gabbros,  which 
lie  immediately  upon  the  Huronian  slates,  form  a  belt  which  tapers  out  rap- 
idly at  both  ends,  and  seems  to  lie  right  in  the  course  of  the  diabase  belts  to 
the  east  and  west,  since  these   belts,  both   westward  toward  Lake  Nu- 


156       COPPBE-BEARING  EOCKS  OP  LAKE  SUPERIOE. 

makagon,  and  eastward  toward  the  Montreal  River,  lie  directly  against  the 
older  rocks,  without  any  of  the  coarse  gabbros  intervening.  The  great  ex- 
tent of  coarse  gabbros  in  Minnesota  seems  to  sustain  somewhat  the  same 
relations  to  the  more  regularly  bedded  portions  of  the  series.  The  Minne- 
sota gabbro,  with  its  accompanying  syenite  and  granitic  porphyry,  occu- 
pies a  belt  from  5  to  20  or  more  miles  in  width,  extending  from  the  Saint 
Louis  River  at  Duluth  northward,  to  and  beyond  the  Cloquet,  and  thence 
eastward  to  the  region  of  Bruld  Lake,  in  township  63;  but  where  the  base 
of  the  series  comes  out  to  the  lake  at  Grand  Portage  the  gabbro  is  absent, 
the  regularly  bedded  diabases  and  amygdaloids  resting  directly  upon  the 
older  slates. 

For  these  reasons  I  was  at  one  time  somewhat  inclined  to  place  these 
gabbros  with  the  Huronian,  and  to  regard  them  as  possibly  the  equivalents 
of  the  great  flows  that  crown  the  so-called  Animikie  slates  in  the  region 
of  Thunder  Bay.  In  the  Bad  River  region,  however,  where  the  rocks  all 
dip  at  a  high  angle  to  the  northward,  the  coarse  gabbro  appears  to  cut 
across  the  Huronian  at  a  very  small  angle,  and  in  such  a  manner  as  to  come 
into  contact  with  successively  lower  members  of  the  Huronian,^  when  the 
rocks  are  traced  on  the  strike  westward.  This  appears  like  an  unconformity 
with  the  Huronian;  and  on  this  account,  as  also  because  of  the  close  lith- 
ological  relationship  between  the  gabbros  and  the  typical  Keweenawan  dia- 
bases, many  of  which  approach  gabbro  in  character,  and  because  in  a 
series  made  up  chiefly  of  eruptive  flows  dovetailing  into  one  another,  there 
must  often  be  sudden  breaks,  which  in  a  sedimentary  series  would  argue 
unconformity,  I  prefer  to  regard  these  coarse  rocks  as  the  earliest  of  the 
Keweenawan  flows.  I  have  already  referred  in  the  previous  chapter  to  the 
possibility  of  their  representing  the  slowly  solidified  and  subsequently  de- 
nuded reservoirs  from  which  the  later  flows  may  have  been  in  part  derived. 

I  have  alluded  above  to  an  exception  to  the  general  fact  of  the  thin- 
ness of  the  detrital  beds  at  low  horizons.  The  reference  was  to  a  set  of 
dolomitic  sandstones,  running  upward  into  red  marly  clays  and  limestones, 
which  are  largely  developed  in  the  peninsula  between  Thunder  and  Black 

'  See  Map  of  Plate  XXII,  Atlas  of  the  Geology  of  Wisconsin. 


THICKNESS  OF  THE  LOWER  DIVISION.  157 

bays,  and  reach  a  thickness,  according  to  Bell,^  of  about  1,300  feet.  These 
are  the  sandstones  to  which  Hunt  has  proposed  to  restrict  the  term  Nipi- 
gon  Group,*  and  which  he  considers  as  newer  than  the  Keweenawan,  but  I 
'satisfied  myself  on  the  ground  that  Logan  was  correct  in  placing  them 
directly  beneath  the  whole  mass  of  Keweenawan  diabases  and  amygdaloids 
of  the  east  side  of  Black  Bay,  and  that  they  rest  with  slight  discordance 
upon  the  nearly  horizontal  Thunder  Bay  slates.  It  is  these  slates  that  Hunt 
has  called  the  Animikie  Grroup,^  he  regarding  them  also  as  newer  than  the 
Keweenawan  proper.  I  look  upon  them,  however,  as  beyond  question  the 
equivalents  of  the  iron-bearing  Huronian  of  the  South  Shore.  The  rela- 
tions of  these  several  groups  are  considered  more  especially  on  a  subse- 
.  quent  page,  and  are  merely  mentioned  here,  because  I  know  of  no  other 
instance,  in  the  entire  extent  of  the  formation,  of  the  existence  of  such 
a  thickness  of  detrital  rocks  at  so  low  a  horizon.  Only  forty  miles  south- 
westward  from  their  occurrence  on  the  east  side  of  Thunder  Bay,  at  Grand 
Portage  Bay,  the  intervening  space  being  water-covered,  the  Keweenawan 
diabases  rest  directly  upon  the  slates  of  the  Animikie  Group,  without  any 
intervening  sandstone. 

The  thickness  of  the  Lower  Division  is  always  enormous,  and  may  be 
placed,  in  round  numbers,  at  from  25,000  to  30,000  feet.  From  this  figure 
there  are  of  course  some  great  variations,  and  yet,  considering  the  way  in 
which  most  of  the  series  has  been  built  up,  the  variations  must  be  regarded 
as  surprisingly  small.  With  the  exception  of  the  unusually  great  and  sud- 
den thinning  in  the  Bad  River  region  of  Wisconsin,  it  does  not  appear 
probable  that  throughout  all  of  its  geographical  extent  the  thickness  of  the 
Lower  Division  ever  sinks  much  below  25,000  feet. 

In  the  eastern  part  of  Keweenaw  Point  the  maximum  thickness  of  the 
Lower  Division  at  surface  is  some  25,000  feet.  This  measurement,  however, 
does  not  go  to  the  base  of  the  series,  but  only  to  the  junction  with  a  newer 
sandstone,  which  overlies  the  continuation  downwards  of  the  Keweenaw 
Series,  or  rather  both  a  repetition  of  more  or  less  of  the  thickness  included 

'  Report  of  the  Geol.  Survey  of  Canada,  for  1867-'69,  p.  319. 

«  Second  Geological  Survey  of  Pennsylvania  "Azoic  Eocks"  "E,"  pp.  240,  241. 

»Loc.  cit.,  p.  240. 


158       COPPEE-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 

in  the  above  25,000  feet,  and  the  downward  continuation  of  the  series.    The 
amount  to  be  added  for  this  downward  continuation  can  only  be  guessed  at. 

Eastward  from  the  eastern  part  of  Keweenaw  Point  to  the  Ontonagon 
River  the  total  thickness  of  the  Lower  Division  seen  is  from  12,000  to  17,000 
feet,  but  in  all  this  distance  only  the  upper  limit  of  this  is  in  sight,  the 
same  fault  line  and  newer  sandstone  as  met  with  on  Keweenaw  Point 
bounding  the  exposures  on  the  south,  and  rendering  uncertain  the  total 
thickness.  ^ 

West  of  Lake  Agogebic  the  Eastern  Sandstone  does  not  extend  far, 
and  the  two  ranges  of  Keweenawan  rocks,  which  to  the  eastward  bounded 
it  on  the  north  and  south  respectively,  come  together;  so  that  we  have, for  the 
first  time,  the  whole  thickness  of  the  Lower  Division  at  surface,  with  its  lower  . 
limit  well  defined.  On  the  Montreal  River,  taking  the  surface  width  and 
dip  angles  together,  the  apparent  thickness  is  as  much  as  33,000  to  35,000 
feet,  but  how  much  of  this  may  be  due  to  the  continuation  westward  of  the 
Keweenaw  fault,  or  whether  this  fault  extends  so  far  as  this,  it  is  impossible 
to  say.  It  certainly  does  not  extend  much  farther,  and,  from  its  evident 
rapid  decrease  in  throw  from  the  Ontonagon  River  westward,  it  seems  prob- 
able that  its  influence  on  the  Montreal  cannot  be  great. 

On  Bad  River,  1 8  miles  southwest  of  the  Montreal,  the  Lower  Division 
has  a  surface  width,  from  the  Huronian  slates  below  to  the  sandstones  of 
the  Upper  Division,  of  only  17,000  feet.  Since  the  dip  here  is  perpendic- 
ular, or  nearly  so,  the  thickness  is  not  much  less  than  this.  As  shown  in 
another  place,^  this  extraordinary  thinning  is  connected  with  the  presence 
below  of  a  great  belt  of  the  coarse  gabbro  described  in  a  preceding  para- 
graph of  this  chapter.  This  coarse  gabbro — whether  with  or  without 
interbedded  fine-grained  beds  is  not  now  known — usurps  most  of  the  thick- 
ness, leaving  only  some  5,000  feet  for  the  usual  thin-bedded  flows  of  the 
Lower  Division.  The  explanation  perhaps  lies  in  the  view  that,  while  early 
in  the  history  of  the  series  there  was  poured  out  here  an  immense  thickness 
of  a  rock  which  solidified  into  the  coarse  gabbro,  later  in  its  growth  the 
vents  were  removed  from  here  to  either  side.  The  coarse  gabbro  mass 
must  have  stood  up  to  a  great  height,  and  the  later  flows  terminated  against 

'Chapter  VI. 


THICKNESS  OP  THE  LOWEE  DIVISION.  159 

it  on  either  side,  until  they  had  accumulated  sufficiently  to  overflow  its 
upper  surface. 

Twenty  miles  west  of  Bad  River  the  coarse  gabbro  has  thinned  out, 
and  beyond  it  to  the  west  and  southwest,  all  the  way  to  the  Saint  Croix 
Valley,  the  Lower  Division  must  have  its  old  thickness  of  25,000  feet,  to 
judge  from  the  country  covered  and  dip  angles  observed. 

On  the  west  side  of  the  Saint  Croix  Valley  the  thickness  must  be  as 
great,  the  surface  width  from  east  to  west  on  Snake  and  Kettle  rivers  being 
as  much  as  nine  miles,  which,  with  the  high  easterly  dips  in  the  lower  layers 
(45*^  to  70°),  must  mean  a  thickness  of  nearly  25,000  feet,  without  reaching 
either  the  upper  or  lower  limit  of  the  division. 

The  Copper  Range  of  Douglas  County,  Wisconsin,  again,  appears  to 
indicate  by  its  exposures  nearly  as  great  a  thickness,  for,  although  the  belt 
of  exposures  is  relatively  narrow,  its  course  is  oblique  to  the  courses  of  the 
constituent  beds.  The  dip  of  these  beds  is  southeasterly,  and  in  going  along 
the  range  eastward  one  is  steadily  descending  in  horizon.  Here  again  the 
lower  limit  must  be  several  thousand  feet  below  the  lowest  rock  seen. 

On  the  Minnesota  coast  I  estimate  the  total  thickness  above  the  Huro- 
nian  or  Animikie  slates  to  be  some  22,000  to  24,000  feet  on  the  west  end 
of  the  coast,  and  not  more  than  16,000  at  the  east  end.  This  difference  is 
due  in  large  measure  to  the  total  disappearance  of  the  coarse  gabbro  belt 
of  the  Saint  Louis — a  case  analogous  to  that  of  the  coarse  gabbros  of  Bad 
River  in  Wisconsin — but  also  to  the  thinning  eastward  of  higher  layers. 
Several  thousand  feet  must  intervene  between  the  highest  rocks  of  the  Min- 
nesota coast  and  the  summit  of  the  Lower  Division. 

Isle  Royale  is  formed  of  a  succession  of  beds  dipping  southward  at  an 
angle  which  increases  in  amount,  as  the  series  is  crossed  from  south  to 
north,  from  8°  or  10°  to  a  much  higher  but  undetermined  figure.  The 
southernmost  or  highest  layers  belong  to  the  Upper  Division,  while  most  of 
the  island  is  made  up  of  layers  of  the  Lower  Division,  with  a  total  thickness 
which  cannot  be  much  less  than  10, 000  feet,  and  may  be  much  more.  To 
judge  from  the  courses  of  the  rock  belts  on  Isle  Royale  and  the  eastern  end 
of  the  Minnesota  coast,  the  higher  beds  of  the  latter  are  the  same  as  the 


160       COPPEE-BEAEING  BOOKS  OF  LAKE  SUPEEIOE. 

lower  of  tlie  island,  and  the  two  together  seem  to  indicate  a  total  thickness 
of  as  much  as  25,000  feet 

About  Black  and  Nipigon  bays  the  Keweenawan  flows  dip  at  a  low 
angle  to  the  southeast  and  south,  and  reach  a  total  thickness  which  does 
not  exceed  7,500  to  8,000  feet,  measuring  from  the  older  rocks  upward. 
The  greater  part  of  the  Lower  Division  must  here  be  concealed  beneath  the 
lake. 

Michipicoten  Island,  according  to  Macfarlane's  measurements,  displays 
a  total  thickness  of  18,500  feet,  all  of  which  belongs  to  the  Lower  Division, 
neither  limit  of  which  is  in  sight.  From  the  position  of  the  island  it  appears 
that  the  thickness  here  must  be  25,000  feet  or  over.  At  the  promontory  of 
Mamainse,  again,  according  to  the  same  authority,  there  are  16,000  feet  of 
Keweenawan  strata  displayed,  all  belonging  to  the  Lower  Division.  This 
thickness  is  measured  from  the  base  of  the  series,  and  does  not  reach  as 
high  even  as  that  of  Michipicoten.  Judging  from  the  lithological  characters 
of  the  Michipicoten  and  Mamainse  successions  Macfarlane  considers  the 
rocks  of  the  former  to  rise  to  a  higher  horizon  by  4,000  feet,  and  so  esti- 
mates the  thickness  of  the  Mamainse  series  at  20,000  feet,  without  any  in- 
dication that  the  upper  limit  is  reached. 


CHAPTER  VI. 

THE    KEWEENAWAN   ROCKS    OF   THE    SOUTH    SHORE    OF 

LAKE   SUPERIOR. 

Introductory. — Eocks  of  the  South  Shore  east  of  Keweenaw  Bay. — Spread  of  the  Keweenawan  rocks 
on  the  South  Shore. — The  North  Wisconsin  synclinal. — Keweenaw  Point  selected  as  a  typical 
region. 

Section  I.  Keweenaw  Point. — Sources  of  information. — Dimensions  and  position  of  Keweenaw 
Point. — Topography  of  the  Point. — Connection  between  the  topography  and  the  rock  structure. 
— Thickness  of  the  rocks  on  Keweenaw  Point. — Section  of  Keweenaw  Point  from  the  mouth  of 
Eagle  River  to  the  Eastern  Sandstone. — Eocks  of  Keweenaw  Point  east  of  the  Eagle  Eiver 
section. — The  Lake  Shore  Trap. — The  Outer  Conglomerate. — Eocks  of  the  median  valley. — The 
Bohemian  Eange. — Summarized  section  of  the  eastern  part  of  Keweenaw  Point. — Changes  in 
the  geology  of  Keweenaw  Point  southwest  of  Eagle  Eiver. — Section  across  Keweenaw  Point 
in  the  vicinity  of  Portage  Lake  described  in  detail. — Eocks  of  Keweenaw  Point  between  Portage 
Lake  and  Gratiot  Eiver. — Stannard's  Eock. 

Section  II.  The  Region  between  Portage  Lake  and  Ontonagon  Eiver. — Continuance  of  the 
Portage  Lake  conditions  through  this  district. — Differences  between  the  Portage  Lake  and  the 
Ontonagon  sections. — The  Nonesuch  shale  belt  in  the  Ontonagon  region. — The  sandstones  of 
the  Upper  Division  in  this  district. — Thickness  of  Keweenawan  rocks  on  the  Ontonagon. 

Section  III.  The  South  Eange.— Position  and  extent  of  the  South  Eange.— Its  rocks  are  often  over- 
laid by  the  newer  sandstone. — Silver  Mountain. — Exposures  on  the  Ontonagon  Eiver ;  on  the  west 
branch  of  the  Ontonagon. — Eelation  of  the  South  Range  rocks  to  those  of  the  Keweenaw  Point 
Eange. 

Section  IV.  The  Eegion  between  the  Ontonagon  River  and  Numakagon  Lake  of  Wisconsin.— 
Peculiar  interest  of  this  region. — Sources  of  information  with  regard  to  its  geology. — Course  and 
surface  spread  of  the  formation  between  the  Ontonagon  and  the  Montreal. — The  Porcupine 
Mountains. — Porphyry  of  the  Porcupine  Mountains. — Its  continuations  east  and  west, — Other 
belts  of  the  Porcupine  Mountains.— Their  east  and  west  continuations.— The  Montreal  River 
section. — The  Potato  Eiver  section. — The  rocks  west  of  Bad  Eiver. — The  sandstones  of  the  Upper 
Division  in  the  Bad  Eiver  country. 

Section  V.  Northwestern  Wisconsin  and  the  Adjoining  part  of  Minnesota. — Sources  of  infor- 
mation with  regard  to  the  geology  of  this  region. — The  Keweenawan  rocks  of  this  region  lie  in  two 
parallel  ranges,  between  which  is  a  synclinal  trough. — The  southern  belt ;  Numakagon  Lake  to 
the  Saint  Croix. — Important  bearing  of  the  exposures  of  the  Saint  Croix  Valley  on  the  question 
of  the  age  of  the  Keweenaw  Series. — Continuity  between  the  Saint  Croix  rocks  and  those  of 
Keweenaw  Point. — Thickness  of  the  Keweenawan  rocks  in  this  region. — Snake  and  Kettle  River 
districts  of  Minnesota. — District  of  the  Upper  Saint  Croix. — Douglas  County  Copper  Eange. 
— The  peculiar  phenomena  of  the  contact  in  the  Douglas  County  region  between  the  Keweenawan 
rocks  and  the  Western  Sandstone. 

From  the  Sault  westward  to  Marquette  the  formations  of  the  country 
skirting  the  south  shore  of  Lake  Superior  are  flat-lying  hmestones  and  sand- 

11  L  S  161 


162  COPPEE-BEAEIlfG  EOCKS  OP  LAKE  SUPEEIOE. 

stones,  ranging  from  the  Niagara  above  to  a  sandstone  below,  which,  in 
common  with  many  preceding  geologists,  I  regard  as  belonging  with  the  ' 
fossiliferous  Cambrian  sandstone  that  forms  the  base  of  the  Paleozoic 
column  of  the  Mississippi.  At  Marquette  the  older  granites  and  schists 
come  out  to  the  lake,  and  from  here  to  Keweenaw  Bay  form  the  country 
immediately  back  of  the  coast,  leaving  in  front,  however,  a  narrow  band  of 
the  flat-lying  sandstone  just  mentioned. 

The  most  eastern  exposure  of  the  Keweenawan  rocks  of  the  South 
Shore  is  found  in  the  isolated  reef  known  as  Stannard's  Rock,  lying  forty-three 
miles  N.  9J°  E.  from  Marquette  and  twenty-nine  miles  from  the  eastern  end 
of  Keweenaw  Point.  Keweenaw  Point  is  formed  of  the  same  rocks,  which, 
as  already  explained,  stretch  from  here  westward  continuously  into  the  State 
of  Minnesota.  They  form  the  south  shore  of  the  lake  as  far  as  the  Montreal 
River,  beyond  which  to  the  head  of  the  lake  they  are  bordered  along  4he 
immediate  coast  by  a  newer  horizontal  sandstone,  but  spread  over  a  great 
width  in  the  northern  part  of  Wisconsin. 

From  Keweenaw  Point  to  the  Montreal  River  the  Keweenawan  rocks 
dip  northward  toward  the  lake.  West  of  the  Montreal  they  form  two  belts ;  a 
more  southern  one — the  continuation  of  the  Keweenaw  Point  belt — in  which 
the  northern  dip  is  retained,  and  a  northern  one  in  which  the  dip  is  to  the 
south.  These  two  belts  form  the  opposite  sides  of  a  synclinal  trough,  which 
has  its  western  termination  in  the  Snake  and  Kettle  River  district  of  Min- 
nesota, where  the  two  belts  are  found  uniting.  Beyond  the  neighborhood  of 
Snake  River,  the  Keweenawan  rocks  do  not  extend  to  the  westward,  older 
formations  taking  their  place. 

In  the  detailed  descriptions  of  this  chapter  I  find  it  convenient  to  divide 
the  region  traversed  by  the  Keweenawan  rocks  into  several  districts,  which 
are  considered  in  order  from  east  to  west. 

The  mining  region  of  Keweenaw  Point  serves  as  the  best  point  of 
departure  for  these  more  especially  local  descriptions,  since  it  has  been  so 
long  known  in  geological  literature,  and  so  thoroughly  opened  up  by  mining 
enterprise.  The  exposures  here  are  often  large,  and,  for  the  most  part,  the 
structure  is  easily  read.  This  district,  too,  embraces  the  only  portions  of 
the  entire  extent  of  the  Keweenaw  Series  that  have  been  subjected  to 


UNITED  STATES    T-ECLTvi.A:  SiJFVEY 


PPEF    BEARING   .-.C":k_   OF  LAKE   Z-.JPE 


KEWEENAW  POINT.  163 

careful  and  minute  stratigraphical  measurement  and  study.  The  region 
thus  furnishes  a.  type  to  which  the  less  minutely  known  occurrences  in 
other  portions  of  the  Lake  Superior  basin  may  be  compared,  and  its  name 
has  been  appropriately  given  to  the  series  which  forms  the  subject  of  this 
volume. 


Section  I.— KEWEENAW  POINT. 

In  the  following  descriptions  of  the  Keweenaw  Point  district  I  draw 
freely,  of  course,  from  previous  publications,  and  more  especially  from 
the  reports  of  E.  Pumpelly  and  A.  R.  Marvine.  My  own  examinations  of 
this  district  were  devoted  both  to  the  obtaining  a  more  thorough  under- 
standing of  the  published  results  of  others,  and  to  the  study  of  points  left 
obscure  by  former  geologists. 

The  accompanying  maps  and  sections  (Plates  XVII  and  XVIII)  will 
serve  to  illustrate  the  main  points  in  the  topography  and  geology  of  the 
Keweenaw  Point  district.  The  topography  is  from  the  charts  of  the  United 
States  Lake  Survey,  including  a  large  scale  unpublished  map  for  that  part 
east  of  Eagle  River.  The  geology  is  compiled  from  Foster  and  Whitney's 
map  (1850),  from  a  map  by  W.  H.  Stevens,  S.  P.  Hill  and  C.  P.  Williams 
(1863),  from  the  maps  by  R.  Pumpelly,  A.  R.  Marvine  and  L.  G.  Emerson 
in  the  atlas  of  the  Geological  Survey  of  Michigan  (1873),  and  from  my 
own  observations. 

Measured  along  its  middle  line  from  a  base  line  running  from  the  head 
of  the  Keweenaw  Bay  at  L'Anse  N.  60°  W.  to  Fourteen  Mile  Point  on  the 
lake  coast,  Keweenaw  Point  has  a  total  length  of  68|  miles  to  its  eastern 
extremity.  At  its  base,  on  the  line  just  mentioned,  the  point  is  34| 
miles  in  width.  From  the  base  the  middle  line  trends  N.  32°  E.  for  21 J 
miles  to  the  north  side  of  Portage  Lake,  where  the  width  is  19^  miles; 
thence  it  runs  N.  40°  E.  27  miles  to  Gratiot  bluff — width  12^  miles;  thence 
N.  63°  E.  6|  miles  to  the  bluff  south  of  Mosquito  Lake — width  6 J  miles ; 
thence  due  east  10  miles  to  a  point  half  a  mile  west  of  Schlaffer's  Lake — 
width  4J  miles;  thence  S.  66°  E.  3  miles  to  the  eastern  extremity.     This 


164       COPPER-BEARING  ROCKS  OF  LAKE  SUPERIOR. 

extremity  lies  not  far  from  the  middle  of  Lake  Superior,  being  in  air  line 
distances  170.5  miles  from  Sault  Sainte  Marie,  2 1 2  from  Duluth,  60  miles  from 
Isle  Royale,  38  miles  from  the  south  coast  in  a  southerly  direction,  and  68 
miles  from  the  same  coast  in  a  S.  17°  E.  direction. 

From  the  head  of  B^te  Grrise  Bay  eastward  the  entire  width  of  Ke- 
weenaw Point  is  made  up  of  bold  ridges.  Further  west,  however,  the 
ridges  that  form  the  northern  and  northwestern  side  are  bordered  on  the 
south  by  a  belt  of.  flat  country,  which  extends  in  the  neighborhood  of  Port- 
age Lake  to  a  width  of  some  10  miles,  and  at  the  head  of  Keweenaw 
Bay  to  a  much  greater  width.  This  flat  expanse  rises  to  only  inconsiderable 
elevations  above  the  lake  level,  and  ends  abruptly  against  the  high  land  on 
the  north  and  northwest.  Three  lakes  of  some  size.  La  Belle,  Grratiot  and 
Torch,  lie  in  this  low  area  just  below  the  south  face  of  the  high  land,  while 
Portage  Lake  consists  of  a  wide  portion  in  the  low  land  and  a  narrow  exten- 
sion cutting  entirely  through  the  high  land. 

East  of  Gratiot  River,  the  higher  portion  of  Keweenaw  Point  is  made 
up  of  a  series  of  sharp  and  narrow  ridges  lying  parallel  with  the  curving 
trends  of  the  outer  coast  line,  and  coinciding  perfectly  with  the  trends  of 
the  strata.  A  line  running  directly  northward  across  the  point  from  Lac  La 
Belle  will  cross  four  of  these  ridges.  On  the  north  shore  of  this  lake  the 
Bohemian  Range  rises  abruptly  to  a  height  of  over  850  feet.  About  a  mile 
further  north  this  range  sinks  rapidly  for  another  mile  to  the  valley  of  the 
Little  Montreal  River,  which  the  line  crosses  at  an  elevation  of  300  feet 
above  Lake  Superior.  A  little  more  than  a  mile  beyond  the  river  is  met 
the  bold  southern  cliff  of  the  Grreenstone  Range,  along  whose  southern 
foot  are  strung  for  more  than  twenty  miles  the  active  and  abandoned 
vein  mines  that  gave  this  district  its  reputation  in  former  years.  The  Grreen- 
stone Range  on  this  line  rises  to  between  650  and  600  feet  above  lake  level. 
Its  northern  face  is  a  gradual  slope  conforming  with  the  dip  of  the  strata. 
The  next  depression  is  crossed  at  4^  miles,  altitude  44  0  feet ;  at  5 J  miles 
the  "  Grreat  Conglomerate"  ridge — altitude  700  feet — is  crossed;  and  at  5f 
miles  the  next  valley — altitude  90  feet.  The  next  or  "North  Trap"  ridge,  the 
northernmost  of  the  series,  is  broken  down  where  the  line  crosses  it  at  6^ 
miles,  but  2  miles  to  the  east  it  reaches  an  altitude  of  over  700  feet.     The 


TOPOGEAPHY  OF  KEWEENAW  POINT.  165 

north  side  of  this  last  ridge  is  again  a  gradual  slope  to  the  lake,  which  is 
reached  at  seven  miles.  All  of  these  ridges,  save  the  Bohemian,  or  southern- 
most, which  presents  a  rounded  contour,  show  the  same  structure,  viz,  a 
steep,  often  cliff-like,  southern  face,  and  a  gradual  northern  slope,  coin- 
ciding more  or  less  with  the  dip  of  the  strata. 

East  of  the  line  of  profile  just  described  these  ridges  continue  well 
marked,  some  of  them  quite  to  the  end  of  the  point.  Curving  to  the  south- 
ward with  the  strata  the  more  southerly  ranges  reach  the  lake  soonest  on  the 
south  side  of  the  point.  Westward  also  the  same  ridge  structure  continues 
well  marked  to  beyond  Eagle  River,  or  well  around  the  southwestern  turn 
in  the  trend  of  the  point.  The  most  northern  of  the  ridges  named  die  out 
soonest,  to  the  westward,  running  into  the  lake  between  Agate  and  Eagle 
Harbors;  whence,  for  some  miles  westward,  the  trends  of  the  strata  and  of 
the  coast  line  make  small  angles  with  each  other  in  such  a  manner  that  suc- 
cessively lower  layers  appear  on  the  shore  as  it  is  followed  westward.  The 
Greenstone  Range  lasts  the  longest  of  the  four,  running  as  far  west  as 
Gratiot  River. 

Beyond  Gratiot  River  the  several  ridges  all  merge  into  one  broad  swell, 
witha  steep  southeastern  slopefacing  the  eastern  lowland,  and  aflat  northwest- 
ern slope  towards  Lake  Superior.  As  Portage  Lake  is  approached  the  north- 
western slope  grows  broader  and  flatter,  until  in  the  neighborhood  of  that 
lake  there  is  a  western  as  well  as  an  eastern  lowland,  with  this  prominent 
difference,  that  the  eastern  lowland  ends  abruptly  against  the  central  ridge, 
while  that  on  the  west  merges  gradually  into  it.  The  same  conditions  con- 
tinue to  the  westward  as  far  as  the  Porcupine  Mountains,  viz :  a  low  area 
bordering  the  lake,  reaching  ten  or  twelve  miles  in  width,  which  merges 
southward  into  a  high  ridge,  which  again  presents  towards  a  southern  low- 
land a  bold  south-facing  cliff. 

All  of  the  topographical  features  thus  described  result  directly  from  the 
underlying  rock  structure. 

The  abrupt  break  between  the  eastern  or  southern  lowland  and  the 
central  ridge  is  the  junction  line  between  a  flat-lying  sandstone  and  an  older 
series  of  northward-dipping  resistant  crystalline  rocks  of  great  aggregate 
thickness.     This  line  marks  also  the  course  of  a  great  fault.     The  west- 


166       COPPEE-BEAEmG  EOCKS  OF  LAKE  SUPEEIOE. 

ern  or  northern  lowland  is  again  underlain  by  a  sandstone,  which  in  this 
case,  however,  is  the  highest  member  of  the  same  series  that  gives  rise  to 
the  central  ridge,  whence  the  merging  into  one  another  of  this  ridge  and 
the  western  lowland.  In  the  Portage  Lake  region  and  westward  from  that 
vicinity  there  is  one  broad  central  ridge  rather  than  a  series  of  parallel  ridges, 
on  account  of  the  comparatively  high  dip  of  the  strata — 38°  to  65°.  The 
western  lowland  only  begins  in  the  vicinity  of  Gratiot  River,  because 
further  east  the  upper  sandstones  lie  beneath  the  lake.  The  eastern  part 
of  Keweenaw  Point  is  marked  by  a  series  of  parallel  ridges  with  cliffy 
southern  and  flat  northern  faces,  because  here  the  rocks  lie  flatter,  the  softer 
amygdaloids  and  more  easily  decomposable  diabases  wearing  into  valleys, 
while  the  more  resistant  melaphyrs,  coarse  diabases  and  bowlder-conglom- 
erates are  left  in  relief. 

Many  other  connections  between  the  topography  and  geological  struc- 
ture of  this  region  might  be  shown  to  exist.  Not  the  least  interesting  of 
these  is  the  relation  of  the  peculiar  line  of  long  narrow  lakes  and  bays, 
which  flank  the  northern  side  of  Keweenaw  Point  in  its  eastern  extension, 
to  the  easily  eroded  amygdaloids  and  other  soft  beds,  out  of  which  these 
bays  are  worn.  Each  bay  has  a  sea-wall  of  some  of  the  more  resistant 
layers. 

In  the  eastern  part  of  Keweenaw  Point,  east  of  Eagle  River,  there  is 
a  maximum  thickness  of  rock  of  about  25,000  feet  below  the  base  of  the 
outer  conglomerate,  which  horizon  I  have  already  selected^  as  the  dividing 
line  between  the  Upper  and  Lower  Divisions  of  the  series.  This  line  in  the 
eastern  part  of  the  point  lies  near  its  northern  margin,  the  dip  here  being 
northward,  and  the  strata  trending  with  the  curving  course  of  the  point 
itself  As  already  indicated,  neither  the  upper  nor  the  lower  limit  of  the 
series  is  reached  on  Keweenaw  Point,  the  downward  extension  being  buried 
beneath  the  newer  horizontal  sandstone  of  the  southern  lowland,  while  the 
upper  limit  lies  under  the  waters  of  the  lake.  Moreover,  the  southern  limit  of 
the  exposures  on  Keweenaw  Point  is,  as  explained  subsequently,  a  fault  line, 
running  the  entire  length  of  the  point,  and  westward  beyond  the  Ontonagon 
River.     This  fault  does  not  follow  the  strike  exactly,  but  cuts  across  it 

1  Chapter  V,  p.  152. 


UNITED  STATES   GEOLOGICAL  SURVEY 


COPPER-BEARING   ROCKS  CF  LAKE  SUPERIOR    PL, Ml; 


—  N 
LAKE       STPERIon 


Outer         'XfjAf   S/tore    Trti/i  •  TJie  Grrat     C*>it4jhimfmU 

Ccn^lametute ' 


Empire  Mine 
GRKKNSTON'f  H\VI;F  ,      , 

/  ilUe     Alonireal    Uiv. 


M-   R,,M  ;l,t 


-N 

LAKE   StrPSniOlt 
Oata-  Cenijltimrritir 


^^^^^^^^^^?T^^ 


l.aK.'     Sho> 
Trap  • 


'^<i>w4^>y/fif/b-4^.i! 


Group '  Mrlapkyr  uwludmy     a  '  number    nf    f'onqioutfratr    //eth- 


Supposed     /ait  It 
W^M'-7'^^^  _^^     B  J-:  TE   _^0  Ji  I  S  E         BAY 

"'"/•'       <.',iiifi  fjfr      East  nil      S'andstotLt 


^//i/fJ: 


.2. 


Delaware  Mn 


i;|- I-  N^l'iiM      l;  \\i; 


/,f///.'  UonireaX    Rjt, 


H     1)      11     1      M     IAN  K   A  N   t;   1-: 


■•v^^S^^?;-:"^'^^!!' 


W"  /Jf/mm' 


Diahasf    Diabane      amygdaloiti    and     tustie       mottf^d    Jlfltiphy 
\fiCluduig    a    number   of   fiorij^lamivntr     fo-ds 


J//  fjj     U  Jf   ////    :L 

Boltrtninii      Rariijf       (ri-ottp  ' 


EASTERN  LOWLANDS 


Jia^lc     Bi\-pr 


Phoenix  Mine 
GEEENSTONE    RAXGE 


\fine  B  0  H  r    \n  \  N       RANCE 


The      Ea^ttrn     Saiulstone 


Heads   of  Tobacco     Itiver  ^  *"    -^ 
I-  \  S  T  E  R  S     L  0\VL  A  N  D  S 


<^vV^' 


Out^r    (otiifloma-alr 


L       A       Jf       E 


Mamnes  Afar\-iiies     '^sfiAfd 

"Group   f  'OrvtipB'       Group ' 


"Grefnsfonf 
Group ' 


Hiabasr ,  Dtabfi.sr  -  nniy^ditlotd     <ind    ln.i/re  -    niotf/nf    A/rftipli\-r     i/irliiduiif 
ti     niunbfj-    of    (hnalnnir-rtitr     brds 


-Jllnucz  fbn^lomefale 


Sokeniuin        Ratiifg        /rnm/i 


^...J^IZIZ^L 


Calumel      Mine 


SU     PERI       OK 


■jt:''-:-'''-^'^^ 


Tfif    Eastern      Stiiidstone. 


S+2=E.- 


t^„^.±^. 


^^>^^--^^'^--[:?"^::^-  ^^.^^^^:.^':-e'^-'?'y^-'-'J--'.^.-!^'-i!^:^''^'^i^  <--\\ 


Hed    Sandstori4;      aiid    Shnie 


yoru'siic/i     Jifd    A'tuitlstaiie 
Sliilr  bat 


Ktet  Sandstone  J/artmfS  .-Ishbed    t,roufi  Otren\rf>n£  JJiabme  ,   Diabase   -  amyijdaioul     and     lustre  -  nudiled     MeJapftrr    utctudin^ 

and  fnn^U?mtruie  'Group  C  Group  _  a     muifher    of'    (tnialtimeraJr    beds 


(lui/iey   JPimiir  Jiav^daloiil 

A'ortJi  Star  (hn4}toiiienite 
.iU;,i}\  ,ii„l   n.KM.'it    r..ti,,t.>m.-i-„i,- 


iJoai^tas  Ainvifdalotd 


The  Easterti 
Sandttone 


rn„n  S  4«  E 


Med     Sandjtlane      and      Shale.- 


Eed  A'andstime  Marymes  J.\hi>:, 

and    Conqlatnet^itr        '(hvup  f"  Grwif> 


Duibnse,    Diabase     .-.,., 
a     nil  ruber     of   fbiti/itn 


'TJte  Eastern  Sandstone 


CKOI.OCICAI.    Si:(     riONS     of     KKWKKNAW        I'OIX 


MICHIGAN 

Scale  .1ZOUO    or   1  inrh- Jhfili    fiM-1 


THE  EAGLE  EIVEE  SECTION.  167 

somewhat  irregularly,  and  it  results  that  the  thickness  above  mentioned — 
25,000  feet — is  met  with  in  only  one  section. 

How  this  thickness  is  built  up  I  shall  try  to  show  by  first  describing 
somewhat  fully  the  section  measured  so  carefully  by  Marvine  in  a  S.  28°  E. 
direction  from  the  mouth  of  Eagle  River  to  the  Phoenix  mine.  This  section 
I  shall  next  extend  roughly  downwards  to  the  Eastern  Sandstone;  adding 
then  brief  descriptions  of  the  still  higher  and  lower  layers  seen  eastward 
from  Eagle  River,  the  maximum  thickness  will  be  reached.  A  brief  sum- 
mary of  the  results  of  this  study  will  then  serve  to  make  them  clearer  to  the 
reader. 

Marvine's  work  on  the  Eagle  River  section  did  not  include  microscopic 
study,  but  this  has  since  been  added  by  Pumpelly,  besides  which  I  have  my- 
self examined  a  number  of  sections  of  specimens  from  here.  I  have  also 
studied  Marvine's  descriptions  on  the  ground. 

The  northernmost  rock  of  this  section,  seen  in  the  streets  of  the  town  of 
Eagle  River  and  on  the  banks  of  the  stream  near  its  mouth,  is  a  pebble- 
and  bowlder-conglomerate  of  great  thickness.  The  horizontal  width  of 
this  conglomerate  exposed  is  about  1,300  feet,  corresponding  with  a  dip  of 
31°  to  a  thickness  of  some  700  feet.  Higher  layers  are  covered  by  the 
lake  waters,  the  overlying  diabases  and  amygdaloids  forming  a  reef  some 
3,400  feet  out  in  the  lake  from-  the  mouth  of  Eagle  River.  The  whole  _ 
width  of  the  conglomerate  thus  appears  to  be  some  4,500  feet,  and  its  thick- 
ness 2,000  to  2,300  feet. 

The  always  rounded  pebbles  of  this  conglomerate,  which  average  from 
3  to  8  inches,  sometimes  reaching  a  foot,  in  diameter,  and  are  coarser  in 
some  layers  than  in  others,  are  of  four  principal  kinds.  The  greatly  pre- 
dominating sort  is  of  a  chocolate-brown  non-quartziferous  porphyry,  with  a 
very  hard,  aphanitic,  readily  fusible  matrix,  scattered  through  which  are  nu- 
merous small,  pinkish  feldspar  crystals.  Under  the  microscope  the  section 
of  these  pebbles  shows  a  matrix  apparently  wholly  crystalline,  all  portions 
polarizing  in  some  part  of  a  revolution,  and  seemingly  wholly  of  orthoclase 
particles.  Thickly  scattered  through  the  matrix  are  minute  brown,  black 
and  red  particles,  which  must  be  chiefly  iron  oxide.  The  porphyritic  crys- 
tals are  mostly  orthoclase,  but  some  of  the  larger  ones  are  oligoclase. 


168       COPPEE-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 

Next  in  abundance  to  these  pebbles  are  otters  of  a  pink  to  red  granite- 
like rock  which  macroscopically  presents  a  fine-grained,  apparently  crys- 
talline mati-ix,  through  which  are  thickly  scattered  porphyritic  crystals  of 
pink  feldspar.  In  the  thin  sections  good  sized  orthoclases  and  oligoclases 
greatly  predominate.  Augite  occurs  in  crystals  of  some  size,  which  are 
penetrated  by  bands  and  needles  of  opaque  brown  ferrite,  arranged  at  defi- 
nite angles.  There  is  also  a  considerable  portion  of  the  section  in  which 
there  appears  a  radial  or  spherulitic  structure,  which  is  either  produced 
or  emphasized  by  the  process  of  alteration.  In  ordinary  light  caly  a  radial 
arrangement  of  the  minute  brown  particles  can  be  seen,  but  in  polarized 
light  the  structure  is  further  brought  out  by  fine  lines,  which  polarize  difi^er- 
ently,  and  in  which  there  is  a  good  deal  of  secondary  quartz.  This  radial 
arrangement  often  presents  also  the  appearance  of  sheaf-like  aggregations. 
It  appears  in  some  cases  to  affect  what  were  originally  crystals  of  feldspar, 
though  it  may  be  chiefly  an  alteration  of  the  matrix. 

Nearly  as  abundant  as  these  pebbles  are  others  of  a  medium-grained, 
very  highly  crystalline  granite,  which,  macroscopically,  is  seen  to  be  chiefly 
formed  of  fine  orthoclase  and  plagioclase  feldspars  and  gray  quartz.  Soft- 
ened black  crystals  of  hornblende,  besides  chlorite  and  epidote,  are  also 
visible.  The  thin  section  reveals  a  rock  composed  almost  wholly  of  ortho- 
.clase,  less  abundant  oligoclase,  and  original  water-filled  quartz.  The  feld- 
spars are  all  highly  clouded  and  pinkish-tinted.  An  opaque  brownish  sub- 
stance represents  biotite  and  hornblende  in  small  quantity.  Some  particles 
of  augite,  calcite  and  secondary  quartz  are  seen. 

Much  rarer  than  any  of  the  preceding  are  pebbles  of  a  true  quartz- 
porphyry,  showing  a  hard  purplish  matrix,  with  abundant  large  black 
quartzes  and  flesh-red  feldspars.  The  thin  section  shows  an  iron-stained 
matrix,  in  which  there  is  but  feeble  polarization  in  little  flocks  of  particles, 
some  small  areas  occurring  in  which  no  efi'ect  on  the  polarized  light  could  be 
observed.  Throughout  the  matrix  are  numerotis  minute  brownish  to  black 
particles,  varying  in  size  from  those  readily  seen  with  a  low  power  to  those 
which  could  barely  be  defined  with  the  highest  power  at  command.  The 
quartzes  are  very  large,  and  present  the  usual  characteristics  of  rounded 
doubly  terminated  crystals  and  embayments  of  the  matrix.     Particles  of 


EAGLE  EIVEE  SECTIOK  169 

the  matrix,  with  the  same  sort  of  devitrification,  are  besides  entirely  included 
in  the  quartz,  and  do  not  form  any  part  of  a  projecting  neck,  as  was  plainly 
to  be  made  out  with  a  high  power.  The  porphyritic  feldspars  in  the  section 
are  mostly  triclinic,  and  yield  the  angles  for  oligoclase.  Crystals  of  augite 
as  large  as  the  quartzes  occur,  and  with  similar  contours  and  embayments 
of  the  matrix  to  indicate  their  coiTOsion  while  the  rock  was  still  fluid.  These 
augites  are  filled  with  the  brown  ferrite  characterizing  the  augites  of  the 
previously  described  pebbles. 

The  matrix  of  the  conglomerate  consists  of  the  same  material  as  the 
pebbles  in  a  fine  state,  and  more  or  less  altered,  being  largely  impregnated 
with  calcite.  Toward  the  base  of  the  bed  the  matrix  becomes  relatively 
much  more  plentiful,  until  finally  the  conglomerate  merges  into  a  dark-red 
sandstone  of  varying  coarseness  of  grain.  There  are  included  in  this  sand- 
stone some  layers  much  darker  than  the  rest,  the  usual  red  color  being 
darkened  by  the  admixture  of  some  black  particles.  The  thin  section  of 
this  portion  shows  a  quite  loose  aggregation  of  particles  worn  from  the  several 
kinds  of  acid  rocks,  including  the  matrix,  as  well  as  the  porphyritic  feld- 
spars and  quartzes,  which  are  readily  recognizable  as  originally  constituents 
of  one  of  the  porphyries.  The  peculiar  secondary  quartz  so  characteristic 
of  the  augite-syenites  marks  some  particles  as  coming  from  that  class  of 
rocks.  There  is  also  a  little  infiltrated  calcite  and  more  or  less  earthy  red 
iron  oxide.  So  far,  the  rock  is  just  like  the  predominating  red  sandstone  of 
this  bed,  but  there  is  in  this  section,  in  addition,  an  abundance  of  rounded 
particles  of  magnetite,  to  whose  presence  the  dark  color  of  the  rock  is 
undoubtedly  due. 

Below  this  sandstone  comes  a  succession  of  basic  eruptive  rocks,  mostly 
fine-grained  diabases,  with  their  amygdaloidal  portions,  and  occasional 
interstratified  sandstone  and  conglomerate  layers.  Marvine  has  measured 
in  detail  a  total  thickness  of  4,845  feet  to  the  southernmost  rock  in  the 
Phoenix  mine,  from  the  base  of  the  sandstone  layer  just  described.  This 
thickness  gives  a  width  on  the  surface  of  9,510  feet,  the  average  dip  being 
placed  at  30°.  Marvine  has  divided  this  thickness  into  four  subordinate 
groups,  which  are  described  below  in  descending  order.  The  descriptions 
are  condensed  from  the  macroscopic  descriptions  of  Marvine's  detailed  sec- 


170       COPPEE-BEAEING  EOCKS  OP  LAKE  SUPEEIOE. 

tion,^  and  are  made  to  include  some  of  the  results  of  Pumpelly's  micro- 
scopic investigations.^ 

Of  these  four  groups,  the  uppermost  (Marvine's  "c,"  including  beds  1 
to  44,  inclusive,  of  his  detailed  section)  has  a  horizontal  width  of  2,700  feet, 
and  corresponding  thickness  of  1,417  feet,  extending  as  far  southward  as 
the  upper  falls  of  Eagle  Eiver.  It  is  mostly  made  up  of  sharply  separable 
diabase  flows,  neai-ly  every  one  having  its  well-marked  vesicular  or  amyg- 
daloidal  portion,  its  pseud-amygdaloidal  middle  portion,  and  its  compact 
lower  portion.  The  beds,  including  always  these  several  divisions,  run  from 
6  to  80  feet  in  thickness,  eighteen  ranging  from  6  to  20  feet,  nine  from  20 
to  40  feet,  and  three  from  40  to  50  feet,  while  one  is  62  feet  and  one  80  feet 
thick.  The  amygdaloidal  or  true  vesicular  portions  of  these  beds  range 
from  2  to  10  feet  in  thickness,  rarely  exceeding  5  feet.  The  pseud-amyg- 
daloids  run  from  5  to  16  feet  in  thickness,  graduating  imperceptibly  into 
the  compact  rock  below. 

The  predominant  type  of  diabase  of  this  group  is  "a  rather  fine-grained 
and  compactly  textured  rock,  breaking  with  an  irregular  to  semi-con- 
choidal  fracture,  and  somewhat  brittle  and  elastic.  The  color  is  generally 
a  dark  but  dull  green,  vaguely  mottled  with  purple,  the  latter  often  pre- 
dominating, having  in  it  a  mottling  of  green,  and  occasionally  all  is  green, 
a  darker  shade  mottling  a  lighter  background."^  One  light-green  bed  and 
two  greenish  ones  are  of  exceptional  character.  The  green  color  of  these 
beds  is  due  to  the  green  alteration-product  of  the  augitic  ingredient.  The 
thin  section  sometimes  shows  much  of  the  augite  fresh,  but  always  some- 
what altered  to  the  green  substance,  and  in  some  sections  the  alteration  is 
nearly  or  quite  complete.  The  specific  gravities  range  from  2.71  to  2.89. 
The  amygdaloids  carry  predominatingly  calcite  in  the  amygdules,  and  next 
prehnite  and  quartz,  the  quartz-bearing  amygdaloids  having  often  a  much 
indurated  matrix.  This  matrix  is  only  rarely  similar  to  that  of  the  under- 
lying compact  diabases,  "  being  almost  always  very  fine-grained  to  com- 
pact, sometimes  inclined  to  earthy,  sometimes  indurated,  and  generally 

'  Geological  Survey  of  Michigan,  Vol.  I,  Pt.  U,  pp.  95-140. 

^  Metasomatic  Developiiient  of  tlie  Copper-Bearing  Eocks  of  Lake  Superior.  Proc.  Am.  Acad., 
Vol.  XllI,  p.  268  (1878). 

=  Geological  Survey  of  Michigan,  Vol.  I,  Part  II,  p.  102. 


EAGLE  EIVEE  SECTION^.  171 

reddish  brown  in  color,  with  sometimes  red  crystals  of  feldspar  porphy- 
ritically  imbedded  in  it."^  In  some  of  the  harder  amygdaloids  the  base  is 
often  greenish  in  color.  In  the  intermediate  pseud-amygdaloids  a  pinkish 
radiated  prehnite  is  the  chief  pseud-amygdule. 

Interstratified  with  the  diabases  and  amygdaloids  of  this  portion  of  the 
section  are  eleven  beds  of  sandstone,^  running  from  3  to  63  feet  in  thick- 
ness, with  a  total  thickness  of  some  860  feet,  and  increasing  in  thickness 
and  coarseness  towards  the  north.  They  consist  of  the  same  materials  as 
already  described,  and  are  all  of  a  dark-reddish  color.  They  all  contain 
more  or  less  infiltrated  calcite.  The  section  of  one  of  these  sandstones, 
the  first  below  the  Great  Conglomerate,  is  figured  on  Plate  XVI,  Fig.  3. 
It  shows  subangular  particles  worn  from  the  several  kinds  of  porphyry, 
chiefly  from  their  matrices.  A  few  of  the  single  quartzes  and  feldspars  of 
the  porphyry  are  included.  Very  abundant  secondary  calcite  fills  up  the 
spaces  between  the  fragments. 

Next  below  these  layers  there  succeeds  a  group  (Marvine's  "&",  in- 
cluding beds  45  to  66  of  his  detailed  section)  618  feet  in  thickness,  extend- 
ing from  the  upper  falls  of  Eagle  River  southeastward  to  the  so-called  and 
well-known  "Ashbed,"  whose  name  may  not  inappropriately  be  given  to 
the  whole  group.  At  both  summit  and  base  of  this  group  are  diabases  of 
peculiar  character.*  The  lower  portion  of  each  of  these  two  beds  is  a  hard, 
brittle,  elastic  rock,  of  specific  gravity  2.93,  with  a  very  fine-grained  light- 
colored  purplish  matrix,  in  which  are  porphyritically  imbedded  numerous 
minute  plagioclases.  The  upper  portion  of  each  of  these  beds  is  a  scoria- 
ceous  amygdaloid,  separated  from  the  main  part  of  the  bed  by  rather  a  sharp 
line  of  demarkation.     This  peculiar  amygdaloid  is 

composed  of  irregular  bunclies  or  "bomb-like"  masses  of  calcitic  amygdaloid,  1.5 
feet  to  .5  inch  diameter,  filled  in  with  a  very  fine-grained  to  compact  brick-red  mate- 
rial, which  often  shows  fine  but  irregular  bands  or  lines,  apparently  of  stratification. 
This  material  increases  in  amount  toward  the  top  of  the  bed,  where  it  quite  often 
incloses  and  surrounds  the  smaller  irregularly  round  amygdaloidal  balls.  The  strata- 
like  bands  are  more  evident  when  the  material  is  in  larger  amounts,  and  they  often 

1  Geological  Survey  of  Michigan,  Vol.  I,  Part  II,  p.  103. 

*  One  more  than  counted  by  Marvine.    Tliis  additional  bed  was  pointed  out  to  me  by  M.  L.  G. 
Emerson,  who  aided  Marvine  in  preparing  his  section. 
'  Beds  45  and  66  of  Marvine. 


172       OOPPER-BBAEING  EOCKS  OP  LAKE  SUPEEIOE. 

seem  to  separate  or  open  out  to  inclose  the  imbedded  balls.  In  appearance  it  is  nn- 
distinguishable  from  the  finer-grained  sandstones,  though  perhaps  containing  more 
calcite,  which  is  more  often  collected  into  small  generally  lenticular  cavities,  which 
are  sometimes  more  numerous  in  rude  narrow  bands,  parallel  with  the  bedding,  than 
is  the  case  with  the  sandstones.' 

That  one  of  these  beds  which  has  at  the  top  of  the  group  has  a  total 
thickness  of  86  feet,  of  which  12  are  the  scoriaceous  amygdaloid,  6  pseud- 
amygdaloid,  and  68  compact  portion.  The  corresponding  figures  for  the 
basal  bed  of  the  group  are  7  feet,  7  feet,  and  31  feet — total,  45  feet.  The 
porphyritic  diabase  characterizing  the  lower  compact  portions  of  these  two 
beds  is  the  type  of  the  kind  described  in  Chapter  III,  under  Pumpelly's 
name  of  ashbed-diabase.  This  diabase  is  remarkable  for  its  small  amount 
of  augite,  the  plagioclases  making  up  most  of  the  thin  section,  and  also  for 
its  not  having  the  augite  contours  determined  by  the  outlines  of  the  older 
feldspar  crystals. 

Between  these  two  limits  the  Ashbed  Group  displays  a  considerable 
variation  in  its  subordinate  beds.  Immediately  below  the  uppermost  bed 
just  described  (Marvine's  45)  come  ten  thin  layers,  for  the  most  part  imder 
15  feet  in  thickness,  with  very  strongly  marked  and  thin  amygdaloids  and 
pseud-amygdaloids.  The  amj^gdules  of  the  amygdaloids  are  chiefly  cal- 
cite and  prehnite,  less  commonly  laumontite,  while  in  the  pseud-amygda- 
loids the  pseud-amygdules  are  chiefly  a  greenish,  soft,  chloritic  substance. 
Below  these  again  comes  a  thickness  of  "curious,  coarse,  easily  decompos- 
able beds  of  irregular  structure,"  of  a  dull  reddish-brown  color,  mottled  with 
dark  green,  or  vice-versa,  one  bed  being  characterized  by  the  very  unusual 
production,  on  a  large  scale,  of  anal  cite  as  a  pseud-amygdule.  Beneath 
these  layers  again  is  first  a  bed  of  semi-columnar,  hard,  dark-greenish,  very 
fine-grained  rock,  allied  to  the  ashbed  type  of  rocks.  Then  follows  a  thin 
seam  of  sandstone  a  few  inches  in  thickness.  Below  the  sandstone  are  two 
heavy  beds  (64  and  65)  of  the  ashbed  type  of  diabase,  with  strongly  marked 
amygdaloids. 

The  massive- portion  of  bed  64,  according  to  Pumpelly,  is  a 

dark-green,  almost  black,  crypto-crystalline  rock,  which  is  easily  scratched  with  the 
knife.    Under  the  microscope,  it  is  found  to  consist  chieiiy  of  plagioclase  in  very  small 

1  Geol.  Survey  of  Mioli.,  Vol.  I,  Part  II,  p.  125. 


THE  EAGLE  EIVER  SECTION.  173 

crystals,  a  soft,  green  mineral,  probably  pseudomorphous  after  olivine,  minute  grains 
of  angite,  and  occasional  small,  often  wedge-shaped,  occurrences  of  a  green  soft  sub' 
stance  occupying  the  interstices  between  feldspar  crystals.' 

According  to  the  same  authority,  the  amygdaloid  of  this  bed 

has  about  sixty  per  cent,  of  its  volume  occupied  by  amygdules,  sometimes  wholly 
prehnite,  sometimes  an  outer  layer  of  white  prehnite,  and  a  central  filling  of  calcite. 
The  matrix  is  chocolate-browu,  and  has  a  crystalline  texture  wholly  foreign  to  the 
melaphyrs,  and  more  resembling  that  of  a  fine-grained,  somewhat  oxidized  spathic 
iron  ore.^ 

This  matrix  is  seen  under  the  microscope  to  be  almost  wholly  altered 
to  prehnite,  with  abundance  of  particles  of  iron  oxide.  The  amygdaloid  of 
bed  65  is  the  layer  so  well  known  as  the  Ashbed,  though  the  name  is  cer- 
tainly a  misnomer  so  far  as  it  means  to  indicate  an  origin  in  the  condition 
of  volcanic  ash.  The  Ashbed  has  been  much  worked  for  the  copper  it  car- 
ries. As  already  described,  this  bed  is  a  peculiar  mixture  of  sandstone  and 
amygdaloid. 

Below  the  Ashbed  Group  comes  next  a  thickness  of  925  feet  (constitut- 
ing Marvine's  group  "«"  beds  67  to  90,  inclusive).  This  includes  a  series  of 
beds  mostly  of  very  considerable  thickness,  ranging,  for  the  most  part, 
between  40  and  80  feet,  while  several  are  100  to  150  feet  thick.  The  upper 
members  of  the  series  are  made  up  of  "  a  rather  coarse  and  not  closely- 
textured"  rock,  which  is  "tough  but  not  brittle,  breaking  with  a  rough  frac- 
ture. The  colors  are  dirty  light-green,  strongly  mottled  with  quite  well-de- 
fined spots  of  dark-green,  *  *  *  ."  The  specific  gravity  is  2.87.  The 
"capping  amygdaloids  are  not  very  strongly  developed."^  One  of  these  lay- 
ers (69)  has  been  examined  microscopically  by  Pumpelly.*     This  bed 

consists  of  56  feet  of  the  lower  zone,  11  feet  of  pseud-amygdaloid,  and  6  feet  of 
amygdaloid.  The  lower  zone  is  a  fine-grained,  dirty-green  rock  with  uneven  fract- 
ure. It  is  easily  scratched,  has  specific  gravity  2.87-2.95,  and  the  powder  yields 
a  little  magnetite.  The  thin  sections  resemble  those  of  the  lower  zone  of  bed  87. 
The  plagioclase  is  much  altered — containing  in  the  freshest  many  tufts  of  chlo- 
rite— and  is  often  represented  only  by  pseudomorphs  of  chlorite,  and  in  places  these 
are  merged  into  chlorite  pseudo-amygdules.  The  augite  is  in  part  very  fresh, 
in  part  changed  to  its  characteristic  pseudomorph.  The  amygdaloid  is  a  very 
compact,  hard  rock,  with  subconchoidal  fracture.     It  consists  of  very  irregularly 

iProc.  Am.  Acad.  Sci.,  Vol.  XIII,  p.  291. 
2Proe.  Am.  Acad.  Sci.,  Vol.  XIII,  p.  290. 
»  Geo].  Surv.  of  Mich.,  Vol.  I,  Part  II,  p.  101 
■"Proo.  Am.  Acad.  Sci.,  Vol.  XIII,  p.  288. 


174       COPPEE-BEAEING  EOCKS  OP  LAKE  SUPEEIOE. 

mixed  brown  and  green  portions,  both  hard,  the  brown  abounding  in  amygdules,  from 
one-third  inch  in  diameter  down,  chiefly  of  prehnite;  often  of  prehnite  as  an  outer 
member,  and  a  central  filling  of  quartz  in  some,  in  others  calcite.  The  green  contains 
fewer  apparent  amygdules.  Thin  sections  of  the  brown  part  show  the  sharp  outlines 
of  comparatively  large  porphyritic  feldspar  crystals,  and  of  countless  long  slender 
feldspar  microlites  separated  by  an  opaque  brown  substance.  These  feldspar  forms 
are  now  occupied  by  brilliantly  polarizing  aggregates  of  prehnite.  Splinters  of  this 
brown  matrix  fuse  in  the  flame  of  an  alcohol  lamp.  Some  of  the  feldspar  forms  con- 
tain a  large  amount  of  soft,  light-green,  seemingly  amorphous  mineral,  which  is,  prob- 
ably, pseudomorphous  after  prehnite;  the  rest  of  the  pseudomorph  in  these  cases  seems 
to  be  quartz.  The  amygdules  have  very  sharply  defined  contours,  and  form  brilliantly 
polarizing  aggregates  of  prehnite.  Quartz  occurs  in  seams  which  cut  through  the  preh- 
nite of  the  matrix,  and  of  the  amygdules.  An  examination  of  thin  sections  of  the  green 
parts  shows  that  they  are  derived  from  the  brown.  They  consist  still  to  a  great  extent 
of  prehnite,  and  many  pseudomorphs  of  this  after  the  feldspar  are  visible ;  but  it 
is  everywhere  more  or  less  changed  to  the  light-green,  soft  substance  (of  which  some 
was  seen  in  the  brown  variety),  and  considerable  areas  of  the  field  are  wholly  changed 
to  this  substance,  which  is  thoroughly  cut  up  by  curving  cracks  of  irregular  shape  and 
size,  which  are  evidently  due  to  contraction,  aud  are  now  filled  with  quartz.  But 
little  of  the  brown  staining  seen  in  the  brown  variety  is  present  here;  the  iron  oxide 
causing  it  has,  perhaps,  gone  towards  forming  the  green  earth-like  alteration-product 
of  the  prehnite.  Splinters  of  this  variety  show  under  the  loupe  by  transmitted 
light,  nearly  opaque,  light-green  portions,  separated  by  transparent  white.' 

Further  down  in  this  division  of  this  series  the  beds  are  not  well  ex- 
posed, but  finer-grained,  darker,  and  more  evenly  colored  beds  seem  to 
prevail.  Near  the  base  of  the  group  the  beds  are  heavy  and  more  like  those 
near  the  summit.  The  bottom  bed  (90)  is  a  hard,  fine-grained,  bluish- 
black,  typically  luster-mottled  melaphyr. 

Marvine's  next  subdivision  (beds  91  to  108,  inclusive),  in  descending 
order,  which  we  may  appropriately  call  the  Greenstone  Group,  since  its  great 
basal  bed  forms  the  well-known  Greenstone  Ridge,  he  regards  as  a  series 
of  diorites,  but  Pumpelly  has  shown  that  all  are  pyroxenic.  These  beds 
form  a  massive  group,  sharply  contrasted  with  all  the  rocks  above  them, 
and  with  those  immediately  below.  The  thickness  is  1,200  feet,  in  heavy 
beds  from  20  to  150  feet  or  more  in  thickness.  There  are  no  intercalated 
amygdaloidal  bands.  Excepting  a  very  massive  layer  at  the  base  and  a 
thinner  one  upon  the  top,  all  the  interior  two-thirds  of  the  group  have  a 
moderately  coarse-grained  texture,  especially  so  as  compared  with  the 
adjoining  rocks. 

'  Proceedings  American  Academy  Science,  "Vol.  XIII,  p.  280. 


EAGLE  EIVBE  SECTION.  175 

The  specific  gravities  of  these  coarse  rocks  range  from  2.89  to  3.03. 
They  include  kinds  belonging  both  to  the  coarse  orthoclase-free  diabases 
and  gabbros,  and  to  the  orthoclase-bearing  gabbros.  Bed  94  is  of  the  latter 
kind.  It  is  made  of  a  coarsely  crystaHine  rock,  whose  most  prominent  con- 
stituent is  a  feldspar  in  narrow  tabular  crystals.  Magnetite,  a  green  and 
black  substance,  and  apatite  are  also  macroscopically  visible.  The  thin 
section  shows,  as  original  ingredients,  apatite,  orthoclase,  oligoclase,  mag- 
netite and  augite,  with  secondary  quartz,  ferrite,  chlorite  and  magnetite.^ 
The  basal  bed  (108)  is  the  type  of  the  luster-mottled  melaphyrs,  and  has 
been  fully  described  in  Chapter  III. 

These  massive  dark-colored  rocks  occur  again  and  again  about  the  Lake 
Superior  basin  in  just  such  relations  as  here,  always  in  a  considerable  aggre- 
gate thickness  of  relatively  heavy  beds,  always  without  amygdaloids,  and 
always,  in  any  given  district,  less  abundant  than  the  finer-grained  and  more 
easily  softened  diabases  and  amygdaloids.  To  these  last-named  rocks  they 
always  ofier  a  marked  contrast,  not  only  on  account  of  their  dark  color  and 
massive  structure,  but  because  of  the  bold  ridges  they  always  form  by  virtue 
of  their  greater  resistant  power. 

Beneath  the  Greenstone  (bed  108),  and  separated  from  it  by  a  red  clay 
seam,  which  is  known  locally  as  "  The  Slide,"  and  is  a  thin  conglomerate 
bed  further  east  and  west,  a  few  hundred  feet  of  rock  are  exposed  to  view 
in  the  Phoenix  mine,  which  is  one  of  the  numerous  workings  just  beneath  the 
Greenstone  on  north  and  south,  or  crossing  veins.  The  total  thickness  of 
these  beds,  which  I  may  appropriately  call  the  Phoenix  Mine  Group,  is  685 
feet,  in  beds  ranging  from  9  to  160  feet.  In  the  upper  part  of  this  group, 
whose  beds  are  all  inclined  through  their  entire  thickness  to  the  pseud- 
amygdaloidal  alteration,  the  rock  is — 

of  a  rather  coarse-grained,  inclined  to  loose  texture,  being  rather  soft  and  tough,  and 
having  a  very  uneven  fracture.  The  feldspar  is  greenish-white,  and  occurs  in  elon- 
gated crystals,  *  *  *  and  in  the  coarser  varieties  appears  spread  like  a  net- 
work upon  a  background  of  dark  green  [and  pink,  the  greenish  ingredient  a  chloritic 
alteration-product  which  is]  separated  into  frequent  and  rather  well-defined  spots, 
while  magnetic  and  specular  iron  are  present  in  large  quantities.  In  the  finer-grained 
beds  brownish-red  largely  prevails,  with  dirty  white  in  the  base,  the  colors,  though 
dull  as  in  nearly  all  rocks,  being  well  marked  and  in  strong  contrasts.    At  one  or  two 

'  E.  Pumpelly,  op.  cit.,  pp.  5275-280. 


176  COPPEE-BEAiilNG  EOCKS  OP  LAKE  SUPEEIOE. 

points  tlie  whole  rock  becomes  a  nearly  uniform,  decided  brownish-red  color.  [In 
looseness  of  texture,  and  consequent  rough  and  uneven  fracture,  these  rocks  some- 
what resemble  the  typical  kinds  of  the  group  immediately  above  the  greenstone  group 
(Marvine's  "O'")])  but  in  colors  they  are  decidedly  dissimilar.  [The  lower  layers  of 
this  group]  become  finer-grained  and  more  compactly  built,  harder,  more  brittle,  and 
elastic,  and  with  more  even  and  conchoidal  fracture,  while  the  color  darkens,  a  dark- 
green  predominating,  indefinitely  mottled  with  dark  dull  purple.' 

In  the  upper  part  of  the  Phcenix  Mine  Group  the  amygdaloids  are 
poorly  developed,  and  appear  to  be  mere  modifications  of  the  pseud-amyg- 
daloids,  "the  matrix  not  being  compact,  but  fine-grained  and  of  a  dull 
brownish-red  color,  and  with  but  little  calcite  or  prehnite"  accompanying 
the  chloritic  pseud-amygdules.  Further  south  the  amygdaloids  are  better 
developed,  "calcite  predominating  in  some,  prehnite  in  others." 

Southward  from  the  Phoenix  mine  the  rocks  are  but  poorly  exposed  in 
the  valley  between  the  Bohemian  and  Greenstone  ranges,  a  few  ledges  of 
the  typical  fine-grained  diabase  appearing  here  and  there  above  the  surface. 
At  a  point  500  paces  south  of  the  center  of  Sec.  37,  T.  57,  K  31  W.,  4,650 
feet  southeastward  from  the  lowest  rocks  of  the  Phoenix  mine,  the  Kingston 
conglomerate  is  exposed,^  with  a  thickness  of  45  feet,  and  containing  pebbles 
of  an  unusually  compact  brown-red  quartziferous  porphyry.  The  covered 
space  between  the  conglomerate  and  the  Phoenix  mine  corresponds  to  a 
thickness  of  2,325  feet. 

Southeast  from  the  Kingston  conglomerate,  the  line  of  section  passes 
through  a  country  for  the  most  part  drift-covered,  though  showing  a  few 
scattering  ledges.  One  of  these,  in  the  south  part  of  Sec.  4,  T.  56,  R.  31 
W.,  shows  a  typical  luster-mottled  melaphyr,  with  nodular  brown  weather- 
ings, closely  resembling  the  rocks  of  the  Greenstone.  Another  exposure, 
on  the  north  slope  of  the  Bohemian  Range,  near  its  summit,  in  the  middle 
part  of  Sec.  i^,  T.  56,  R.  31  W.,  is  a  typical  brown  and  green  pseud-amyg- 
daloidal  diabase,  with  abundant  pseud-amygdules. 

Still  further  southeast,  near  the  southwest  corner  of  section  10,  on  the 
old  Suffolk  mining  location,  one  of  the  head  streams  of  Tobacco  River 
shows  in  its  bed  large  structureless  exposures  of  a  brown  quartzless  por- 
phyry, which  shows,  in  an  aphanitic,  dark  reddish-brown  to  purple,  easily 

1  Geol.  Survey  Mich.,  Vol.  I,  Part  II,  pp.  103, 104. 
^Marvine,  Atlas  Geol.  Sur.  Mich.,  Plate  XXII. 


SECTION  FEOM  EAGLE  EIVEE  TO  THE  EASTEEN  SANDSTONE.    177 

fusible  matrix,  abundant  porphyritic  red  feldspars,  often  as  much  as  one- 
fourth  inch  in  length,  many  of  which  are  striated.  Minute  dark-green  spots 
and  an  occasional  tendency  in  the  base  to  a  finely  crystalline  texture  are 
brought  out  by  the  lens.  A  vein  carrying  copper  sulphide  was  formerly 
worked  here,  and  the  crystals  of  the  porphyry  occasionally  carry  native 
copper  and  silver  in  minute  flakes  along  the  cleavage  planes.^  This  por- 
phyry contains  59.52  per  cent,  of  sihca.  Its  thin  section  shows  as  porphy- 
ritic  ingredients,  oHgoclase,  less  abundant  orthoclase,  much  altered  augite, 
apatite  in  large  crystals,  and  magnetite.  The  matrix  appears  in  the  thin  sec- 
tion to  be  composed  of  a  translucent  minei-al  much  stained  with  red  iron 
oxide,  abundant  black  and  red  particles,  and  more  minute  and  rare  green 
ones.  The  principal  mineral  appears  to  be  orthoclase,  the  opaque  particles 
magnetite  and  ferrite,  the  green  ones  altered  augite.  From  the  older  mine 
workings  near  these  porphyry  exposures,  much  typical  prehnitized  amyg- 
daloid has  been  thrown  out,  showing  that  we  have  such  rocks  continuing 
close  to  the  porphyry.  A  short  distance  down  stream  from  the  old  mine 
workings  the  Eastern  Sandstone  comes  in. 

From  the  line  of  the  Kingston  conglomerate  to  the  Suffolk  or  Prays- 
ville  porphyry  just  described  is  a  distance  of  about  two  and  a  half  miles. 
At  the  usual  30'^  angle  of  dip,  which  holds  everywhere  north  of  the 
Kingston  conglomerate,  this  would  add  some  6,600  feet  in  thickness  to  the 
section  1  have  been  describing.  At  the  Suffolk  location  the  dip  was  very 
unsatisfactorily  made  out,  but  further  east  the  Bohemian  Range  shows  higher 
dips  than  are  met  with  in  the  more  northern  ridges,  and  we  may  reasonably 
suppose  the  same  to  be  the  case  here.  Allowing  for  this,  the  last-named 
figures  should  probably  be  increased  to  some  8,000  feet.  The  following  is 
a  summary  of  the  section  thus  described,  from  the  northern  trap,  off  the 
mouth  of  Eagle  River,  at  right  angles  to  the  general  trend,  and  in  a  S.  28°  E. 
direction,  to  the  Eastern  Sandstone: 

Thickness  in  feet. 
The  Great  Conglomerate,  from  the  outer  trap  reef,  off  the  mouth  of  Eagle 

Eiver,  and  including  the  sandstone  at  base 2, 200 

Marvine's  Group  "  c" 1>  ^1" 

1  As  to  tills,  see  E.  Pnmpelly,  Geological  Survey  of  MicHgan,  Vol.  I,  Part  II,  p.  39.    The  ex- 
posures are  much  more  satisfactory  now  than  at  the  time  of  Pumpelly's  examination,  owing  to  tho 
draining  of  the  mill  pond  and  destruction  of  the  old  saw  mill  at  this  place. 
12  L  S 


178       COPPEE-BEARING  ROCKS  OP  LAKE  SUPERIOR. 

Thiotaeas  in  feet. 

Marvine's  Oroup  "  ft,"  or  the  Aslibed  Group 618 

Marvine's  Group  "a" 925 

Tlie  Greenstone  Group 1, 200 

The  Plimnix  Mine,  or  Subgreenstone  Group 685 

From  the  base  of  the  Phoenix  Mine  Group,  to  and  including  the  Kingston 

Conglomerate 2, 325 

From  the  Kingston  Conglomerate  to  the  Eastern  Sandstone  (estimated) 8, 000 

17,390 

or,  say,  between  17,000  and  17,500  feet. 

Eastward  from  Eagle  River  the  several  subdivisions  of  the  section  thus 
described  continue  well  marked.  In  this  direction  the  trend  of  the  strata 
changes  a  degree  or  two  in  the  mile,  curving  around  more  and  more  to  the 
eastward,  until,  between  Agate  and  Copper  Harbors,  the  N.  62°  E.  trend  of 
Eagle  River  has  become  east  and  west.  Still  further  east,  this  curving  con- 
tinues, until  at  the  end  of  the  point  it  has  become  some  degrees  south  of  east. 
The  dip,  too,  in  the  more  northerly  ranges  flattens  towards  the  eastward, 
becoming  in  the  vicinity  of  the  Delaware  mine  as  low  as  24°.  A  still  further 
flattening  is  reported  towards  Copper  Harbor,  and  then  between  that  and 
the  end  of  the  point  another  increase  in  the  dip  angle. 

At  Sand  Point  and  thence  eastward  the  Great  Conglomerate  of  the  Eagle 
River  section  is  visibly  overlaid  by  an  upper  series  of  diabases  with  strongly 
marked  amygdaloids.  These  beds  are  well  exposed  at  Eagle  Harbor  and  at 
the  several  similar  peculiarly  formed  harbors  further  east,  all  of  which  are 
worn  in  the  softer  amygdaloids,  or  in  the  underlying  sandstone,  the  more 
compact  beds  forming  the  long  seawalls  of  the  harbors.  This  peculiar  ero- 
sion may  be  often  seen  repeated  on  a  smaller  scale,  as  at  Eagle  Harbor,  where 
each  thin  amygdaloid  is  worn  back  into  a  long  narrow  gorge,  the  harder  rock 
on  the  south  sloping  away  at  the  dip  angle,  that  on  the  north  rising  precip- 
itously. 

yyciter'TXna 

Fig.  2.— Surface  contour  and  arrangement  of  beds  at  Eagle  Harbor.  Represents  a  distance  of 
about  200  feet.    Scale  natural. 

The  beds  seen  at  Eagle  Harbor  and  thence  eastward  are  clearly  like  many 
of  those  already  described  as  occurring  below  the  Great  Conglomerate  in  the 


THE  OUTER  CONGLOMERATE— THE  BOHEMIAN  RANGE.   179 

Eagle  River  section.     They  are  the  typical  fine-grained  diabases,  having  a 
grain  whose  highly  crystalline  nature  can  yet  be  distinctly  seen  with  the 
naked  eye.     In  the  compact  portion,  which  forms  much  the  greater  part  of 
each  bed,  the  color  is  usually  a  dark  brownish-gray  to  black,  withalight  shade 
in  more  altered  layers.     Some  beds  show  numerous  minute  dark- green  spots 
of  chlorite,  dotting  the  surface  of  the  fracture.     These  lower  portions  have 
often  a  strong  tendency  to  a  columnar  structure,  much  more  marked  than 
usually  seen  on  the  South  Shore,  though  not  nearly  so  striking  as  that  often 
to  be  observed  on  the  Minnesota  coast.     The  amygdaloids  are  very  thin, 
and  have  usually  a  dark  reddish-brown,  tough,  aphanitic,  often  very  hard, 
at  times  soft  and  earthy,  matrix,  with  abundant  calcite  and  laumontite.    These 
amygdaloids  show  often  a  sort  of  rude  stratified  appearance  of  the  same 
nature,  though  not  so  pronounced,  as  that  of  the  stratified  amygdaloids  of 
the  vicinity  of  Agate  Bay  on  the  Minnesota  coast.     The  typical  structure 
of  the  diabase  flows  of  the  Keweenaw  Series  is  here  most  beautifully  shown, 
each  bed,  with  its  thick,  massive,  columnar  lower  portion,  and  its  thin,  pecu- 
liar, slaggy  upper  portion,  standing  out  shai-ply  from  the  adjoining  layers. 
Agates  occur  quite  commonly  as  sparsely  scattered  amygdules  in  the  mid- 
dle and  lower  portions  of  the  beds. 

East  of  Agate  Harbor  the  whole  thickness  of  these  upper  diabases 
comes  in— some  1,500  feet— and  continues  to  the  end  of  the  point,  being 
overlaid  in  all  this  distance  by  an  outer  conglomerate,  the  base  of  the  Upper 
Division  of  the  series.  This  Outer  Conglomerate  forms  the  greater  part  of 
Manitou  Island,  to  the  east  of  Keweenaw  Point. 

As  in  the  section  south  from  Eagle  River,  so  in  the  more  easterly  part 
of  Keweenaw  Point,  the  exposures  in  the  median  valley  south  of  the  mines 
under  the  Greenstone  are  infrequent.  Enough  is  seen,  however,  of  these 
covered  belts  where  they  reach  the  shore  of  Bgte  Grise  Bay,  to  prove  that 
they  are  made  up  of  the  usual  fine-grained  diabases  and  amygdaloids  with 
rarer  luster-mottled  melaphyrs  and  porphyry-conglomerates. 

The  Southern  or  Bohemian  Range,  however,  shows  in  the  vicinity  of 
Lac  La  Belle  many  interesting  exposures.  The  rocks  of  which  this  range 
is  made  up  have  been  described  as  wholly  different  from  those  of  the  more 
northern  belts,  and  have  even  been  regarded,  by  more  than  one  authority,  as 


180       COPPER-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 

belonging  to  a  different  rock  series.  Speaking  of  the  Bohemian  Range,  Dr. 
C.  T.  Jackson  says: 

*  *  *  if  the  rock  was  not  connected  with  the  more  Ihornblendic  trap,  and  in  the 
same  line  of  direction,  bursting  through  the  same  kind  of  sandstone  strata,  I  should 
feel  disposed  to  regard  it  as  of  more  ancient  origin.  Indeed,  I  am  far  from  being  sat- 
isfied that  it  is  not  more  ancient,  for  the  limited  exposure  of  the  rocks  does  not  allow 
any  geologist  to  be  too  confident  in  his  opinion  respecting  its  age.^ 

Foster  and  Whitney  say  that  the  Bohemian  Range — 

differs  from  the  northern  both  in  lithological  character  and  in  the  mode  of  its  occur- 
rence. While  the  former,  before  described,  is  comj)osed  of  numerous  beds  of  trap, 
in  the  main  of  the  amygdaloid  and  granular  varieties,  interstratified  with  the  detrital 
rocks,  the  southern  range  consists  of  a  vast  crystalline  mass,  forming  an  anticlinal  axis, 
flanked  on  the  north  by  the  bedded  trap  and  conglomerate,  and  on  the  south  by  con- 
glomerate and  sandstone.  The  contour  of  the  unbedded  trap  is  also  very  different 
from  that  of  the  bedded  trap.  We  nowhere  recognize  the  stair-like  structure  in  the 
hills;  they  are  either  dome-shaped  or  rounded.^ 

Elsewhere  Foster  and  Whitney  speak  of  the  Bohemian  Range  as  formed 
by  a  later  outflow  than  the  more  northern  beds,  and  as  having  forced  the 
more  northern  beds  into  their  present  inclined  position.  Charles  Whittlesey 
speaks  of  the  rocks  of  the  Bohemian  Range  in  the  same  vein,  though  he 
calls  them  Huronian,  and  hence  older  than  the  more  northern  beds;  he  re- 
fers also  to  the  anticlinal  structure  described  by  Foster  and  Whitney.^ 
Still  more  recently  Dr.  T.  S.  Hunt  speaks  in  the  same  way  of  the  rocks  of 
the  Bohemian  Range,  and  quotes  approvingly  the  opinion  of  Mi".  Ernest 
Gaujot  to  the  effect  that  they  belong  to  an  older  series.* 

Before  visiting  the  Bohemian  Range  myself  I  was  rather  inclined  to 
suppose  that  its  rocks  might  possibly  be,  in  part  at  least,  more  ancient  than 
the  typical  Keweenawan,  and  more  especially  to  believe  in  the  existence 
there  of  an  anticlinal  axis,  as  described  and  pictured  by  Foster  and  Whit- 
ney ;  the  existence  of  which  anticlinal  I  conceived  might  help  to  explain 

' ' '  Report  on  tlie  Geological  and  Mineralogical  Survey  of  the  Mineral  Lands  of  the  United  States 
in  the  State  of  Michigan."    Ex.  Doc.  No.  5,  p.  473,  Slst  Congress,  1st  sess. 

^Geological  Eeport  on  the  Copper  Lands  of  the  Lake  Superior  Land  District,  Michigan,  Slst  Con- 
gress, 1st  session,  1849  to  1850,  Ex.  Doc.  No.  69,  p.  64. 

3  "Physical  Geology  of  Lake  Superior.  Proc.  Am.  Assoc.  Adv.  Sci.,  Detroit  meeting,  August, 
1875,  p.  71."  Col.  Whittlesey  here  speaks  of  the  Bohemian  Eauge  rocks  as  "intrusive  Huronian,"  and 
yet  as  having  caused  the  tilting  of  the  more  northern  rocks  of  Keweenaw  Point,  so  that  it  is  difficult 
to  tell  exactly  what  he  does  mean. 

■*  Special  Report  on  Trap  Dykes  and  Azoic  Rocks  of  Southeastern  Pennsylvania,  1878,  p.  230. 


THE  BOHEMIAN  RANGE.  181 

some  of  the  grander  structural  problems  that  present  themselves  to  the  stu- 
dent of  Lake  Superior  geology.  A  week's  study  in  the  field,  however,  of 
the  region  about  Lac  La  Belle,  and  Mount  Houghton,  and  subsequent  ex- 
amination of  a  large  number  of  thin  sections,  have  served  to  convince  me, 
not  only  that  the  rocks  of  the  Bohemian  Range  are  an  integral  part  of  the 
Keweenaw  Series,  but  also  that  they  are — with  the  exception  of  the 
brick-red  augite-syenites,  and  possibly  also  of  the  orthoclase-gabbro  of 
Mount  Bohemia — distinctly  bedded;  that  their  rounded  contours  and  lack 
of  stairlike  structure  are  due  to  a  very  high  northern  dip  as  contrasted  with 
the  flat  dips  of  the  more  northern  belts;  that  no  anticlinal  axis  exists;  that 
the  contrast  between  these  rocks  and  those  of  the  more  northern  belts  has 
been  greatly  exaggerated ;  that  those  rocks  of  this  series  which  appear  pe- 
culiar, viz:  quartzose  porphyry,  and  orthoclase-bearing  gabbro,  are  just 
such  as  are  repeatedly  to  be  seen  at  similarly  low  horizons  throughout  the 
entire  extent  of  the  formation;  and  that  interstratified  with  these,  and  pre- 
dominating over  them,  are  luster-mottled  melaphyrs,  fine-grained  diabases, 
diabase  pseud-amygdaloids,  true  amygdaloids  and  porphyry-conglomerates, 
in  no  way  different  from  the  more  northern  rocks  of  Keweenaw  Point. 

The  opportunities  for  studying  the  structure  of  the  Bohemian  Range  in 
the  vicinity  of  Lac  La  Belle  are  very  good.  By  following  the  south  side 
of  the  range  from  the  head  of  Lac  La  Belle  a  distance  of  seven  or  eight 
miles  to  the  porphyry  bluffs  on  the  north  shore  of  Bgte  Grise  Bay,  Sec.  29, 
T.  58,  R.  28  W.,  one  obtains  gradually  a  nearly  continuous  cross-section 
of  the  range,  while  the  large  exposures  on  Mount  Houghton  and  Mount 
Bohemia,  and  again  on  the  road  from  Lac  La  Belle  to  the  Delaware  mine, 
help  greatly  towards  a  shorter  and  more  direct  cross-section. 

Among  the  most  northern  and  at  the  same  time  most  interesting  of  the 
belts  of  rocks  which  compose  the  Bohemian  Range,  using  that  name  now  to 
cover  all  of  the  line  of  elevations  south  of  the  valley  of  the  Little  Mon- 
treal River,  is  the  red  felsite  which  constitutes  the  bold  point  known  as 
Mount  Houghton.  This  mountain,  whose  summit  is  crossed  by  the  west 
line  of  Sec.  24,  T.  58,  R.  29  W.,  at  6,250  feet  north  of  the  north  shore  of 
B^te  Grise  Bay,  is  one  of  the  most  prominent  topographical  features  on 
Keweenaw  Point.     It  reaches  an  altitude  of  847  feet,  a  height  which  is 


182       COPPEK-BEAEING  BOOKS  OF  LAKE  SUPEEIOE. 

exceeded  by  only  two  or  three  elevations  on  the  point,  viz:  Mount  Bohe- 
mia, which  rises  to  a  height  of  867  feet  from  the  north  shore  of  Lac  La 
Belle;  Gratiot  bluff,  and  another  point  to  the  westward  on  the  Bohemian 
Range,  which  exceeds  900  feet.  Mount  Houghton  is  rendered  especially 
prominent  by  its  isolation,  the  summit,  which  has  a  length  of  only  about  a 
hundred  feet,  being  surrounded  on  the  east  and  south  sides  by  precipitous 
cliffs,  while  all  around  the  ground  falls  off  200  feet  within  less  than  a  quar- 
ter of  a  mile. 

The  rock  of  which  Mount  Houghton  is  composed,  and  which  shows 
in  bold  precipitous  faces  all  about  the  top,  is  a  light-pink  to  brick-red  felsite, 
without  visible  porphyritic  ingredients.  The  thin  sections  of  this  rock  show 
also  no  porphyritic  quartzes,  but  some  orthoclases  which  might  be  called 
porphyritic.  The  sections  vary  from  nearly  colorless  to  a  deep  red,  accord- 
ing to  the  amount  of  iron  oxide  present.  The  matrix  in  some  sections 
appears  to  be  wholly  composed  of  feebly  polarizing,  minute  orthoclases 
arranged  in  a  felt-like  mass,  rarely  with  linear  outlines  to  the  crystals,  and 
is  nearly  or  quite  destitute  of  any  non-crystalline  mattei'.  Other  sections 
show  a  much  larger  proportion  of  n  on -polarizing  matter,  and  the  feldspars  are 
in  thin  tabular  crystals,  with  sharply  linear  outlines.  No  quartz  recogniza- 
ble as  such  could  be  detected  with  the  microscope.  Particles  of  black  mag- 
netite are  often  included,  though  never  abundant.  No  other  accessory 
minerals  were  observed.  Before  the  blowpipe  this  rock  is  fusible  with 
difficulty,  the  lighter  varieties  being  somewhfft  more  difficult  to  fuse  than 
the  darker  colored  ones.  The  siHca  content  is  76.9  per  cent,  or  greater 
than  it  would  be  were  there  no  silica  besides  that  in  the  orthoclase. 

The  general  appearance  of  the  mass  of  rock  forming  the  summit  of 
the  mountain  is  such  as  to  suggest  a  very  high  northern  dip,  some  75°,  and 
this  is  further  indicated  by  the  abundant  irregular  parallel  bandings  in  the 
rock  of  the  top  of  the  mountain.  The  appearance  of  lamination  is  pro- 
duced by  waving  bands  of  lighter  and  darker  shading,  which  are  often 
emphasized  by  minute  quartz  seams  following  their  direction. 

To  the  eastward  the  same  felsite  shows  in  the  Bare  Hills,  in  Sec.  29, 
T.  58,  R.  28  W.,  and  beyond  on  the  lake  shore  in  section  28.  Similar  rocks 
appear  on  the  east  point  of  the  bay  into  which  the  Little  Montreal  River 


THE  BOHEMIAN  RANGE.  183 

empties,  and  again  in  Sec.  30,  T.  58,  R  27  W.,  still  further  east.  All  of 
these  exposures  cannot  well  belong  to  the  same  belt,  and  it  would  appear 
probable  that  the  Mount  Houghton  rock,  along  with  some  smaller  elevations 
leading  off  from  it  in  an  easterly  direction,  belong  with  the  more  easterly 
of  the  exposures  on  the  coast,  while  the  Bare  Hills  belong  to  another  belt. 

These  red  rocks  Foster  and  Whitney  regarded  as  sandstones  baked  by 
the  heat  of  the  intrusive  rock  of  the  Bohemian  Range,  but  they  are  plainly 
enough  but  one  phase  of  the  quartziferous  porphyries  and  felsites,  which  I 
have  heretofore  recognized  as  characterizing  the  Keweenaw  Series  through- 
out its  extent.  They  show  no  trace  of  fragmental  origin,  either  microsco- 
pically or  macroscopically;  and  if  the  other  porphyries  are  eruptive,  these 
are  as  well.^ 

Westward  from  Mount  Houghton  I  have  not  seen  anything  of  this 
poi'phyry  belt,  unless — as  is  very  probable — a  quartziferous  porphyry  on 
the  line  of  the  railroad  from  the  Calumet  mine  to  Torch  Lake,  in  Sec.  36, 
T.  56,  R.  33  W.,  should  belong  here.  On  the  geological  map  of  Keweenaw 
Point,  by  Stevens  and  Hill,  this  porphyry  belt  is  indicated  as  far  west  as 
the  eastern  part  of  range  32. 

South  of  the  Mount  Houghton  porphyry  belt,  for  a  width  of  frou# 
one-half  to  three-fourths  mile,  the  prevailing  rocks  are  typical  fine-grained 
diabases  with  the  usual  pseud-amygdaloids  and  true  amygdaloids,  which 
neither  in  the  specimen  nor  in  the  thin  section  present  any  differences 
from  the  more  northern  diabases.  With  these  prevailing  kinds  are  sev- 
eral belts  of  the  typically  luster-mottled  rocks  which  Pumpelly  has  called 
melaphyrs,  and  which  are  here  very  rich  in  much  altered  olivine,  and  are 
here,  as  always,  sharply  distinguished,  both  macroscopically  and  micro- 
scopically, from  the  associated  olivine-bearing  diabases.  There  are  also  one 
or  more  belts  of  typical  fine-grained  diabase  of  the  ashbed  type,  and  two 
or  more  porphyrj^-conglomerates.  All  of  these  rocks  may  be  seen  well 
exposed  on  the  road  from  Lac  La  Belle  to  the  Delaware  mine,  especially 
in  the  vicinity  of  the  stream  in  the  S.  E.  ^,  Sec.  30,  T.  58,  R.  29  W.,  where 
an  arrangement  into  belts  and  the  usual  division  of  the  diabases  into  massive 
lower  portions  and  amygdaloidal  upper  portions  is  distinctly  to  be  made  out. 

I  See  Foster  and  WMtney,  op-  cit.,  pp.  64,  65. 


184       COPPER-BEARING  BOOKS  OF  LAKE  SUPEEIOE. 

A  northern  dip  of  some  60°  is  also  plain.  The  same  beds  are  exposed  on 
the  north  flank  of  Mount  Bohemia,  in  the  northern  part  of  section  29,  in  the 
vicinity  of  the  old  Mendota  mine,  which  was  worked  on  a  north  and  south 
vein.  Immediately  about  the  old  mine  buildings  are  exposures  of  a  very 
fine-grained  ashbed-diabase.  The  same  beds  show  on  the  sides  of  the 
ravines  immediately  south  of  Mount  Houghton. 

Still  farther  south,  luster-mottled  melaphyrs  and  olivine-diabases  ap- 
pear to  predominate.  Mount  Bohemia — the  high  point  on  the  north  shore 
of  Lac  La  Belle — shows  immense  exposures  of  these  rocks  on  its  southern 
flanks,  where  are  plainly  to  be  seen  all  of  the  characteristics  of  the  mela- 
phyr  of  the  type  belt,  the  Greenstone  of  the  more  northern  part  of  Ke- 
weenaw Point.  There  are  also  large  ledges  of  the  same  rock,  but  crumb- 
ling and  much  altered,  on  the  bluff  rising  behind  the  old  smelting  works  on 
the  north  side  of  Lac  La  Belle.  On  the  north  shore  of  B^te  Grrise  Bay 
again,  in  sections  25  and  26,  where  the  contact  with  the  Eastern  Sandstone 
may  be  seen,  the  rocks  are  prevailingly  luster-mottled  melaphyrs,  though 
much  crumbled,  altered,  and  seamed  with  laumontite  and  calcite.  All  of  these 
melaphyrs  are  exceedingly  rich  in  olivine,  which,  in  the  thin  section,  is 
•chiefly  represented  by  a  brown  or  red  alteration-product. 

Mount  Bohemia  shows  also  large  exposures  of  two  other  rocks,  viz :  a 
brick-red  augite-syenite,  or  granitic  porphyry,  and  a  rather  coarsely  crys- 
talline, uralitic  orthoclase-gabbro.  The  former  of  these  two  is  seen  on  the 
upper  part  of  the  mountain,  and  its  relation  to  the  adjoining  rocks  was  not 
satisfactorily  made  out,  though  it  seems  most  probable  that  it  is  intersecting. 
The  other  rock  shows  lower  down,  on  the  southeast  side  of  the  mountain, 
and  appears  to  constitute  an  interstratified  belt  (not  impossibly  an  intersect- 
ing mass).  It  is  the  rock  on  which  were  chiefly  based  the  statements  of 
Jackson,  Foster  and  Whitney,^  and  others,  that  the  Bohemian  Range  is  al- 
together different  as  to  its  kinds  of  rock  from  the  more  northern  belts.  It 
constitutes,  however,  but  a  very  small  portion  of  the  mass  of  the  range,  and 

'  "The  lower  portion  of  the  elevation  is  here  made  up  of  a  peculiar  rock  composed  of  chlorite  and 
lahrador  in  nearly  equal  proportions.  These  two  minerals  are  each  in  a  distinctly  crystalline  condition, 
and  the  feldspathic  portion  is  of  a  light-reddish  color.  The  mass  is  filled  irregularly  with  crystals  of 
magnetic  iron  ore,  which  occasionally  form  a  large  portion  of  the  rock.  Particles  of  copper  pyrites 
are  also  scattered  through  it.  This  variety  of  rock  seems  to  pass  gradually  into  the  dark-colored,  fine- 
grained greenstone  which  occurs  on  the  summit  of  the  mountain." — Foster  and  Whitney,  op.  cit.,  p.  65. 


THE  BOHEMIAIf  EANGE.  185 

belongs  plainly  enough  to  a  class  of  rocks  which  has  been  recognized  at  a 
number  of  other  points  in  the  extent  of  the  formation,  and  which  includes 
also  some  of  the  beds  immediately  over  the  Greenstone. 

Externally  this  rock  appears  to  be  a  mixture  of  a  greenish  mineral, 
with  red  feldspar  and  rare  magnetite.  The  thin  section  shows  the  principal 
ingredients  to  be  orthoclase,  oligoclase,  augite,  diallage  and  titanic  iron, 
the  augitic  mineral  being  largely  altered  to  a  green  uralite,  and  this  still 
further  to  a  chloritic  substance ;  while  very  abundant  apatite,  a  gray  sub- 
stance secondary  to  titanic  iron,  a  little  secondary  quartz  and  secondary 
magnetite,  are  all  to  be  seen.  The  thin  section  of  this  rock  is  pictured  in 
Fig.  3,  Plate  V. 

All  of  the  exposures  on  the  bluffs  immediately  north  of  Lac  La  Belle, 
and  of  the  shore  cliffs  at  the  head  of  B^te  Grise  Bay,  seem  to  point  to  a 
very  high  northern  dip,  the  strike  in  this  distance  varying  between  north  of 
east  and  north  of  west. 

Foster  and  Whitney  speak  of  a  belt  of  "chlorite"  as  always  occurring 
on  the  south  flank  of  the  Bohemian  Range,  at  the  contact  with  the  Eastern 
Sandstone.  They  also  map  it  as  extending  all  the  way  from  B^te  Grise 
Bay  to  Portage  Lake.^  It  does  not  appear  to  me,  after  seeing  this  contact 
at  Bete  Grise  Bay,  at  Lac  La  Belle  and  on  the  Douglas  Houghton  River 
near  Torch  Lake,  that  the  supposed  chlorite  belt  is  anything  more  than  the 
usual  diabase  or  amygdaloid  greatly  decomposed,  as  it  naturally  would  be 
along  such  a  contact,  and  probably  also  much  broken  up,  as  would  be  the 
case  along  such  a  fault  as  must  have  taken  place  at  this  contact.^  Even  at 
these  places  the  decomposition  varied  greatly  in  extent,  and  there  was  no 
evidence  of  any  continuous  belt,  the  sandstone  being  seen  at  times  exactly 
in  contact  with  a  not  unusually  altered  diabase.  The  contact  line  between 
the  sandstone  and  north-dipping  traps  does  not  follow  exactly  any  one 
horizon,  but  crosses  the  trend  of  the  trappean  belts  back  and  forth  in  the 
more  easterly  part  of  the  point,  and  farther  west  rises  quite  rapidly  in  the 
series. 

It  is  worthy  of  note  in  this  place  that  the  account  of  the  Bohemian 

'Foster  and  Whitney,  op.  cit,  pp.  65-67. 

'The  original  faulting  is  believed  to  have  antedated  the  sandstone,  hut  faulting  has  taken  place 
here  since  this  sandstone  teas  formed. 


186       COPPEE-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 

Eange  rocks  thus  given  shows  that  in  this  range  we  have  the  source  of  all 
the  different  kinds  of  porphyry  pebbles  and  porphyry  detritus  which  char- 
acterize the  conglomerates  and  sandstones  of  the  series,  viz:  felsite  without 
visible  quartz  or  feldspar  (Mount  Houghton  belt),  augite-syenite  (Mount 
Bohemia),  quartzless  porphyry  (old  Suffolk  location),  and  true  quartziferous 
porphyry  with  large  porphyritic  quartzes  and  orthoclases  (Torch  Lake  Rail- 
road). The  points  named  are  of  course  not  the  only  ones  in  which  several 
kinds  of  porphyry  respectively  occur,  but  they  serve  to  show  that  in  these 
lower  belts  we  are  to  find  the  source  of  all  the  porphyry  detritus — a  fact 
which  has  never  been  recognized  before.  It  is  shown  elsewhere  that  these 
original  porphyries  characterize  the  formation  throughout  its  entire  extent 
around  Lake  Superior.  Moreover,  the  variation  of  the  conglomerates  as  to 
predominating  pebbles,  when  followed  along  the  strike,  is  to  be  connected  with 
the  varying  characters  of  the  original  porphyries  when  followed  in  the  same 
direction. 

The  lowest  horizon  reached  east  of  Eagle  River,  or  indeed  anywhere 
on  Keweenaw  Point,  would  appear  to  be  in  the  neighborhood  of  Gratiot 
Lake.  The  highest  horizon  east  of  Eagle  River  is  reached  in  the  outer 
conglomerate  to  the  east  of  Copper  Harbor.  Between  these  two  limits  the 
eastern  part  of  Keweenaw  Point  appears  to  be  made  up  as  indicated  in  the 
following  summary  : 

xjppEK  nrvisioN  oe  the  keweenaw  series. 

Thicloiesti  in  feet. 

The  Outer  Conglomerate;  porphyry-conglomerate  and  sandstone,  about 1,000 

lower  division   op   the   KEWEENAW   SERIES. 

The  Lake  Shore  Trap;  very  plainly  bedded  fine-grained  diabases,   strongly 

marked  amygdaloids,  and  one  or  more  thin  porphyry-conglomerates;  about    1, 500 

The  Great  Conglomerate;  porphyry-conglomerate  and  sandstone 2,  200 

Marvineh  Group  "c";  plainly  bedded  and  separable  fine-grained  diabases,  with 
strongly  marked  amygdaloids,  predominatiagly  calcitic ;  and  some  850  to 
900  feet,  in  all,  of  interstratified  sandstones 1, 417 

Marvine^s  Group  ^^b,"  or  the  Ashbed  Group;  made  up  mostly  of  thin,  fine-grained 
diabases,  which  vary  a  good  deal  in  appearance,  but  are  generally  provided 
with  distinct  amygdaloids ;  including  some  beds  of  the  peculiar  type  known 
as  ashbed-diabase;  also  several  scoriaceous  amj-gdaloids,  being  intermin- 
gled sandstone  and  amygdaloid ;  also  one  thin  sandstone  seam 618 

Marvine's  Group  "a";  made  up  of  relatively  heavy  beds  without  strongly  devel- 
oped amygdaloids ;  including  one  thin  seam  of  sandstone 925 


SUMMARIZED  SECTION  OF  EASTERN  PART  OF  KEWEENAW  POINT.   187 

Thickness  in  feet. 

The  Greenstone  Group;  made  up  of  relatively  heavy  beds,  without  amygda- 
loids,  of  rocks  for  the  most  part  relatively  coarse-grained ;  these  belong 
mostly  to  the  coarse-grained  olivine-free  diabases  and  gabbros  and  to  the 
luster-mottled  melaphyrs,  or  fine-grained  olivine-diabases,  the  Green- 
stone at  the  base  of  the  group  being  of  the  last-named  class 1, 200 

The  Sub- Greenstone  Group,  in  which  all  of  the  fissure-vein  mines  are  working; 
having  at  top  a  thin  conglomerate,  the  equivalent  of  the  "Allouez"  and 
"Albany  and  Boston"  conglomerates  in  the  Portage  Lake  district ;  com- 
posed of  fine-grained  diabases,  with  not  very  strongly  developed  amygda- 
loids;  about 1)600 

The  Central  Valley  Beds;  the  layers  not  well  exposed,  but  evidently  chiefly 
fine-grained  diabases  and  amygdaloids,  with  a  number  of  thin  porphyry- 
conglomerates,  in  all  respects  like  the  overlying  group;  about 5,540 

The  Bohemian  Range  Beds;  made  up  chiefly  of  diabases  and  melaphyrs  in  all  re- 
spects like  the  higher  layers,  and  including  some  of  the  usual  porphyry- 
conglomerates;  but  also  in  part  made  up  of  quartziferous  porphyry,  fel- 
site,  and  non-quartziferous  porphyry,  and  coarse-grained  orthoclase-gab- 
bro;  in  all,  about -- 10,000 

Total.. ---   26,000 

Southwestward  along  the  strike  from  the  Eagle  River  type  section 
above  described,  both  the  topographical  and  geological  characters  of  this 
section  soon  change.  By  the  time  the  Gratiot  Eiver  is  reached  the  several 
ridges  have  all  merged  into  one  broad  swell,  the  median  valley  disappearing. 
The  geological  changes  of  importance  are:  (1)  A  general  thinning  of  the 
series,  due  in  some  considerable  measure  to  the  nearly  complete  disappear- 
ance of  the  group  of  coarse-grained  diabases  and  luster-mottled  melaphyrs 
which,  under  the  name  of  the  Greenstone  Group,  has  been  described  as  so 
prominent  a  feature  in  the  geology  of  the  eastern  part  of  Keweenaw  Point- 
This  thinning,  however,  is  by  no  means  confined  to  the  Greenstone  Group, 
the  rocks  above  and  below  thinning  as  well.  In  the  Eagle  River  section, 
between  the  slide  at  the  base  of  the  Greenstone  and  the  base  of  the  Great 
Conglomerate,  there  is  a  thickness  of  some  4,000  feet,  while  the  equivalent 
beds  at  Portage  Lake  cannot  be  more  than  2,300  feet  thick.  (2)  A  consid- 
erable increase  in  the  amount  of  lakeward  dip,  which  at  the  Allouez  mine, 
Sec.  31,  T.  57,  R.  32  W.,  reaches  46°;  at  the  Albany  and  Boston,  Sec.  8, 
T.  55,  R.  33  W.,  52°;  and  at  the  mines  about  Portage  Lake,  65°.  (3)  The 
fault  line,  or  contact  with  the  Eastern  Sandstone,  is  now  at  a  much  higher 
horizon  in  the  trappean  series  than  further  east;  so  much  so  that  in  all  of 


188       COPPEE-BEAEING  EOCKS  OP  LAKE  SUPEEIOE. 

the  region  southwest  of  T.  57,  E.  31  W.,  all  save  the  very  uppermost  of 
the  rock  belts  of  the  Bohemian  Eange,  using  that  term  in  the  wide  sense 
of  the  summary  of  page  187,  are  beneath  the  Eastern  Sandstone.  (4)  On  the 
other  hand,  west  of  Eagle  River  the  rock  belts  soon  diverge  in  strike  from 
the  lake  coast,  striking  more  to  the  southward,  and,  as  a  result,  a  rapidly 
increasing  amount  of  the  Upper  Division  of  the  series  is  brought  between 
the  trap  belt,  or  Lower  Division,  and  the  lake  coast.  At  Portage  Lake 
there  must  be  upwards  of  8,000  feet  of  this  Upper  Division  between  the 
trap  belt  and  the  west  coast  of  the  point. 

Southwest  of  the  Gratiot  River,  the  outcrops  and  openings  are  chiefly 
on  those  belts  whose  equivalents  in  the  eastern  extension  of  Keweenaw 
Point  are  the  ill-exposed  and  comparatively  little  known  belts  of  the  median 
valley.  Beginning  on  the  south  side  of  Portage  Lake,  mining  openings 
extend  nearly  continuously  with  the  course  of  the  layers  some  seven  or 
eight  miles  to  the  northeast,  and  in  the  vicinity  of  that  lake  are  pretty  well 
distributed  across  the  formation,  so  that  Pumpelly's  descriptive  sections 
cover  nearly  all  of  the  Lower  Division  here  at  surface.  Northeastward  from 
the  Albany  and  Boston  mine,  as  far  as  the  Allouez,  the  openings  are  much 
rarer — one  being  the  famous  Calumet  and  Hecla  mine — and  the  exposures 
very  sparse,  the  only  ones  of  consequence  being  at  or  near  the  contact  with 
the  Eastern  Sandstone  in  the  tributary  streams  of  the  west  side  of  Torch 
Lake.  This  gap  Marvine  has  bridged  in  his  masterly  correlation  of  the 
rocks  of  Houghton  and  Keweenaw  Counties.^ 

The  Portage  Lake  section  will,  then,  be  best  next  described,  after 
which  the  rarer  exposures  to  the  northeast  may  be  more  briefly  alluded 
to.  This  section,  between  the  fault  line  and  the  conglomerate  num- 
bered by  him  22,  Pumpelly  has  worked  out  in  great  detail.  The  total 
length  of  his  measurement  is  about  14,400  feet,  corresponding,  with  the  dip 
of  55°,  to  a  thickness  of  about  12,000  feet.  With  the  exception  of  a  few 
relatively  thin  conglomerates,  this  section  is  made  up  of  beds  of  diabase  and 
amygdaloid,  with  some  melaphyr,  in  no  respect  diff"erent  from  the  prevail- 
ing beds  of  the  Eagle  River  section.     The  lower  layers  are  heavier  than  the 

'  Geological  Survey  of  Michigan,  Vol.  I.,  Part  II,  Cap.  IV. 


PORTAGE  LAKE  SECTION.  189 

higher.  Certain  portions  of  the  thickness  are  marked  by  certain  distinguish- 
ing Hthological  characteristics, 

which,  without  in  any  instance  being  peculiar  to  a  given  horizon,  still  serve  to 
mark  decidedly  those  parts  of  the  series  where  they  are,  respectively,  most  frequent- 
Thus,  to  begin  towards  the  eastern  part  of  the  field,  from  the  neighborhood  of  "  Mabb's 
vein" '  to  within,  say,  1,000  feet  east  of  the  Isle  Eoyale  "  vein," '  there  is  a  tendency, 
among  the  different  traps,  to  a  compact  or  fine-grained  texture  with  a  dark-green, 
almost  black  color,  sometimes  slightly  mottled,  especially  on  the  weathered  surface. 

*  *  *  From  this  region  till  1,500  feet  or  more  west  of  the  Isle  Eoyale  copper-bearing 
bed,  the  upper  portions  of  very  many  of  the  beds  have  the  amygdaloidal  cavities  filled 
with  a  light-greenish  white  or  pale  pink  prehnite,  which  sometimes,  for  a  width  of  2  to  6 
feet,  form  10  to  40  per  cent,  of  the  rock,  and  lend  it  a  very  characteristic  spotted 
appearance.  During  the  next  2,000  feet  or  more,  the  traps  have  frequent  seams  3  to 
20  inches  thick,  consisting  of  distinctly  in dividualizedtriclinic  feldspar,  delessite,  preh- 
nite and  specular  iron ;  these  occur  both  parallel  to  the  plane  of  the  bedding  and 
oblique  to  i  t.  The  traps  through  a  portion  of  this  distance  are  frequently  impregnated 
with  epidote,  as  is  also  the  cement  of  the  conglomerate  beds.  On  the  "  Dacotah"  prop- 
erty we  come  to  a  belt  of  the  formation  in  which  many  beds  have  a  tendency  to  a 
coarse-grained,  crystalline  texture,  and  in  some,  the  character  is  highly  developed- 

*  *  *  *  Still  further  west,  on  the  "South  Side"  property,  the  brown  amygdaloids 
often  present  a  scoriaceous  appearance  which  is  quite  characteristic.  Some,  at  least, 
of  these  features  are  traceable  for  miles  in  the  longitudinal  extension  of  the  zones  in 
which  they  occur." 

Belovs^  the  upper  hmit  of  Pumpelly's  Portage  Lake  section — conglomer- 
ate 22 — he  recognizes  twenty  interstratified  porphyry  conglomerates,  which 
he  numbers  from  below  upwards;  the  twenty-first,  or  No.  13,  not  hav- 
ing been  met  with  on  Portage  Lake.  It  is  the  well-known  Calumet  con- 
glomerate. Of  the  Portage  Lake  conglomerates,  three — Nos.  1,  2,  and 
3 — about  50  feet  thick  each,  lie  in  the  lower  2,900  feet.  Then  follows  a 
zone  of  about  1,300  feet,  in  which  there  are  five  conglomerates — Nos.  4, 
6,  6,  7,  and  8 — ^from  2  to  20  feet  thick.  Next  follow  2,250  feet  without  con- 
glomerates. Then  for  5,000  feet  in  thickness  are  eight  conglomerates  from 
200  to  1,000  feet  apart,  and  from  mere  seams  to  30  feet  in  thickness.  These 
are  Nos.  9,  10, 11,  18,  14, 15,  16,  and  17.  Then  four  conglomerates— Nos.  18, 
19,  20,  and  21 — aggregating  200  feet  in  thickness,  in  a  zone  of  only  550  feet. 
In  the  total  thickness  of  12,000  feet  measured,  the  conglomerates  aggregate 
about  580  feet,  in  twenty-one  different  layers.'     According  to  Pumpelly, 

'  Copper-bearing  amygdaloids. 

^  Geological  Survey  of  Michigan,  Vol.  I,  Part  II,  pp.  17,  18. 

'The  figures  here  given  are  taken  from  Pumpelly's  sections  in  the  Atlas  of  the  Geological  Survey 
of  Michigau,  and  do  not  in  all  respects  conform  with  those  giveuby  Marvine  inVol.  II,  Geol.Surv. 
Mich.,  p.  61. 


190       COPPEE-BEARING  EOCKS  OF  LAKE  SUPEEtOR. 

The  conglomerates  of  Portage  Lake  differ  from  each  other  bat  little,  if  at  all,  in  litho- 
logical  characteristics.  The  pebbles  vary  from  the  size  of  a  pea  to  one  foot  or  more  in 
diameter,  being  coarser  in  some  beds  than  in  others.  *  *  #  *  The  pebbles,  in 
most  of  the  beds  on  Portage  Lake,  consist  almost  exclusively  of  varieties  of  non- 
quartziferous"  felsitic  porphyry;  two  kinds  predominate;  one  of  these  has  a  chocolate- 
brown  to  liver-brown,  subcrystalline  to  compact  almost  vitreous  matrix  containing 
very  scattered  minute  crystals  of  triclinic  feldspar  of  the  same  color  as  the  base. 
The  other  and  rarer  variety,  also  non-quartziferous,  has  a  chocolate-brown,  compact  to 
minutely  crystalline  matrix,  in  which  lie  crystals,  J  to-J  inch  long,  of  a  flesh-colored 
triclinic  feldspar.  In  some  beds  there  appear  pebbles  of  a  flesh-red  rock,^  composed 
almost  entirely  of  granular  feldspar,  containing  small  specks  of  a  black  undetermined 
mineral.  In  some  instances  the  feldspar  is  wholly  triclinic,  in  others  the  twin-striation 
is  frequently  absent.  This  variety  of  pebble  is  altogether  absent  in  some  beds,  at  least 
where  they  are  opened,  while  in  others  they  predominate,  as  in  the  Albany  and  Boston 
Conglomerate.  Pebbles  of  compact  melaphyr^  and  of  melaphyr-amygdaloid^  also 
occur,  but  are  quite  subordinate  in  number  to  those  already  enumerated.  The  normal 
form  of  cement  is  a  fine-grained  sandstone,  composed  apparently  of  the  same  material 
as  the  pebbles.  Often  the  cement  is  very  subordinate  in  volume,  the  pebbles  touching 
each  other.  Frequently,  however,  the  reverse  is  the  case,  and  often,  the  sandstone 
forms  layers  from  less  than  an  inch  to  many  feet  in  thickness.^ 

The  large  dump  at  the  old  shaft  of  the  Albany  and  Boston  mine  is  a  good 
place  to  study  these  conglomerates.  The  pebbles  are  usually  large,  some- 
times six  inches  or  a  foot  in  greatest  diameter.  They  are  commonly  oblong, 
or  flattened.  A  pink  to  brick-red,  medium-grained,  highly-crystalline  augite- 
syenite  predominates,  but  dark-red  to  chocolate-brown,  aphanitic,  felsitic 
porphyry  is  abundant.  I  have  studied  four  typical  samples  of  the  peb- 
bles of  this  conglomerate,  both  macroscopically  and  in  the  thin  section. 

Of  these  the  first  is  a  finer-grained  but  distinctly  crystalline,  dark  red- 
dish-brown augite-syenite,  the  predominant  red  color  being  thickly  dotted 
with  small  black  points.  In  the  thin  section  the  chief  ingredient  is  seen 
to  be  ohgoclase  in  quite  good-sized  and  fresh  crystals.  Orthoclase 
follows  next,  and  there  is  some  quartz,  most  of  which  appears  to  be  of  a  sec- 
ondary origin.  The  black  particles  seen  macroscopically  resolve  themselves 
into  open  aggregations  of  brown  ferrite  and  black  opacite  (magnetite  and 
ferrite),  commonly  of  very  irregular  outhne,  but  not  unfrequently  having 
linear  boundaries.     From  the  same  appearances  seen  in  many  others  of 

'  Evidently  meaning  without  -visible  porphyritic  quartz. 

^Granitic  porphyry  or  augite-syenite  of  this  volume. 

2  Diabase  of  this  report. 

■•  Diabase-amygdaloid. 

f^K.  Pumpelly,  in  Geological  Survey  of  Michigan,  Vol.  I,  Part  II,  pp.  16,  17. 


POET  AGE  LAKE  SECTION.  191 

these  augite-syenites,  I  take  these  aggregations  to  be  altered  augite,  of 
which  mineral  a  brightly  polarizing  core  is  now  and  then  visible.  Magnet- 
ite and  apatite  also  occur  in  the  section. 

The  second  pebble  shows  a  much  lighter  red  and  coarser  rock  than 
the  last,  with  much  rarer  black  particles.  The  thin  section  of  this  is  com- 
posed chiefly  of  orthoclase,  oligoclase— the  latter  less  abundant  than  in  the 
preceding  pebble— and  secondary  quartz.  A  marked  appearance  of  sphe- 
rulitic  structure  runs  through  the  shce,  being  produced  by  the  radiating 
arrangement  of  the  secondary  quartz.  Unmistakable  augite  is  seen  in  this 
section,  and  is  largely  replaced  by  ferrite  and  magnetite.  These  aggrega- 
tions are,  however,  much  rarer  than  in  the  previous  pebble. 

The  third  pebble  is  close  to  the  last,  but  deeper  red  in  color.  It  shows 
in  the  section  orthoclase  and  oligoclase  predominating,  and  some  secondary 
quartz,  but  no  spherulitic  structure.  There  is  much  red  oxide  of  iron, 
plentiful  needles  of  brown  ferrite  arranged  in  definite  directions,  and  a 
little  quite  fresh  augite. 

The  fourth  pebble  shows  an  aphanitic,  hard,  chocolate-brown  matrix, 
fusible  with  difficulty,  with  a  few  minute  porphyritic  feldspar  crystals.  In 
the  section  of  this  pebble  the  matrix  is  made  up  chiefly  of  a  translucent 
substance  much  stained  with  red  iron  oxide,  and  thickly  studded  with 
minute  black,  opaque  particles.  The  porphyritic  feldspars  are  triclinic,  and 
there  are  also  a  few  good-sized  augites  with  the  usual  ferritic  alteration. 

Conglomerate  22  of  Pumpelly's  section^  on  the  south  side  of  Portage 
Lake  corresponds  either  to  the  base  of  the  main  or  Great  Conglomerate  of 
the  Eagle  River  section,  or  to  one  of  the  sandstone  layers  immediately  below 
that  horizon.  On  the  north  side  of  Portage  Lake,  both  on  the  road  running 
westward  from  Houghton,  and  in  the  ravine  just  west  of  the  Mineral  Range 
Railroad,  in  the  southern  part  of  Sec.  22,  T.  55,  R.  34  W.,  are  black  shales, 
which  are  unquestionably  the  same  that  are  found  all  the  way  to  Bad 
River,  in  Wisconsin.  These  shales  belong  over  the  outer  conglomerate  of 
Keweenaw  Point.  Between  these  two  horizons  is  a  horizontal  width  of  some 
4,800  feet.  In  the  southeastern  part  of  this  space  the  layers  dip  at  a  high 
angle 40°  to  50°— northward,  but  this  angle  must  rapidly  decrease  to  the 

1  Atlas. Geological  Survey  of  MicMgan,  Plate  XIV-XIX. 


192       COPPER-BBAEING  EOCKS  OF  LAKE  SUPEEIOE. 

northwest,  for  at  the  Atlantic  mill,  on  the  south  side  of  Portage  Lake — N. 
W.  ^  Sec.  34 — sandstone  is  observed  dipping  northwest  28°  to  30°,  while 
the  shales  in  the  ravines  on  section  22,  near  the  railroad,  dip  at  26°  only,  so 
that  the  entire  rock  thickness  in  this  space  cannot  be  more  than  2,500  to 
2,600  feet. 

Within  this  thickness  must  be  crowded  the  equivalents  of  the  Great 
Conglomerate,  of  the  outer  or  Lake  Shore  Trap,  and  of  the  Outer  Con- 
glomerate of  the  district  east  of  Eagle  River,  a  total  thickness  of  some 
4,700  to  4,800  feet.  Within  this  space  must  come  also  the  dividing 
line  between  the  Upper  and  Lower  Divisions  of  the  series.  The  only  expo- 
sures in  this  interval,  however,  are  those  of  the  sandstone  near  the  Atlantic 
mill,  on  the  south  shore  of  Portage  Lake,  and  one  conglomerate  near  the 
southeastern  side  of  the  gap.  Between  this  sandstone  and  the  base  of  con- 
glomerate 22  is  a  thickness  of  some  1,400  feet,  and  it  would  appear  prob- 
able that  the  sandstone  belongs  to  the  top  of  the  Grreat  Conglomerate.  This 
sandstone  is  a  reddish  kind,  much  of  the  usual  character,  and  is  in  its  lower 
layers  very  conglomeratic,  becoming  even  a  moderately  coarse  conglome- 
rate. This  character  may  be  very  well  seen  on  the  road  from  the  Atlantic 
mill  to  the  Atlantic  mine.  The  finer-grained  sandstone  was  examined  under 
the  microscope,  and  showed  a  fine  reddish  matrix  of  subangular  to  angular 
fragments  of  the  usual  porphyry  detritus,  but  with  an  abundance  of  angular 
quartz  particles.  Scattered  through  this  finer  matrix  are  large  particles, 
many  of  which  are  single  quartzes.  There  are  also  among  them  single 
feldspars,  chiefly  orthoclase,  particles  of  porphyry  matrix  and  here  and 
there  diabase  fragments.  The  abundance  of  angular  quartz  fragments, 
both  in  the  finer  matrix  and  in  the  larger  particles,  is  a  matter  of  interest, 
since  these  fragments  are  unusually  full  of  large  cavities,  filled  with  a 
bubble-bearing  fluid,  in  many  of  which  there  is  to  be  seen  a  little  cube  of 
salt.  These  characters  seem  to  render  it  improbable  that  these  quartz  frag- 
ments should  have  come  from  a  quartziferous  porphyry.  Their  origin  is 
very  doubtful. 

Of  the  Upper  Division  in  the  Portage  Lake  region  the  exposures  are 
not  plenty.  The  shales  of  the  ravine  near  the  Mineral  Range  Railway; 
the  same  shales  with  sandstones  on  the  wagon-road  west  from  Hancock;  the 


POKTAGE  LAKE  SECTION.  193 

dark  sandstone  belonging  near  the  same  horizon  in  the  quarry  on  the  north 
side  of  Portage  Lake,  two  miles  below  Hancock;  the  sandstone  on  the  Lake 
Superior  coast,  near  the  mouth  of  the  ship-canal,  and  a  few  other  small  and 
indefinite  sandstone  exposures,  are  all  that  have  come  to  my  knowledge  as 
occurring  anywhere  near  the  line  of  section  now  under  description.  That 
the  whole  distance  between  the  upper  limit  of  the  Lower  Division  and  the 
main  lake  coast  is  occupied  by  sandstone,  at  all  events  with  not  more  than 
a  very  few  subordinate  layers  of  diabase  near  the  base  of  the  Upper  Divis- 
ion, is  not  only  inferred  from  the  topography,  and  from  all  that  can  be 
learned  of  the  Upper  Division  of  the  series  farther  west,  where  sections 
across  most  of  its  thickness  can  be  seen,  but  also  seems  to  be  proved  by 
the  exposures  along  the  coast  of  Keweenaw  Point,  north  of  the  ship-canal 
entry. 

The  black  shale  and  associated  sandstone  occurring  west  of  Hancock 
are  of  peculiar  interest  because  they  unquestionably  mark  the  same  horizon 
as  that  of  the  black  shale  and  sandstone  of  the  Porcupine  Mountains,  and 
thence  many  miles  both  to  the  east  and  west.  They  vary  from  light-gray, 
rather  fine-grained  sandstones,  to  nearly  black  shales  and  shaly  sandstones. 
They  are  closely  associated  with  and  even  interstratified  with  coarser  and 
redder  kinds,  which  are  yet  of  a  darker  shade  than  the  usual  red  sandstone. 
All  appear  to  consist  chiefly  of  the  usual  porphyry  detritus,  but  the  par- 
ticles are  predominatingly  either  the  originally  porphyritic  feldspar  or  quartz, 
particles  of  matrix  being  subordinate.  There  is  also  in  every  section  more 
or  less  basic  detritus,  in  the  shape  of  rounded  particles  of  magnetite  and 
augite,  the  latter  often  much  altered  to  a  greenish  substance,  and  particles 
of  the  basic  rocks  showing  the  several  ingredients  together. 

On  the  lake  shore,  a  mile  below  the  entrance  to  the  ship-canal,  Sec.  29, 
T.  56,  R  34  W.,  begins  a  long  line  of  low  cliffs  of  sandstone,  which  ex- 
tend westward  for  some  miles.  These  sandstone  layers  dip  lakevyard  about 
8°,  and  strike  more  to  the  southward  than  the  lake  coast,  so -that  higher 
layers  come  in  to  the  westward,  in  which  direction  also  the  dip  continues  to 
flatten.  The  rock  seen  on  section  29  is  mostly  red  shale,  but  there  are 
heavier  layers  of  relatively  coarse  sandstone,  sometimes  light-gray,  some- 

13  L  s 


194       COPPEE-BEARING  EOCKS  OP  LAKE  SUPEEIOE. 

times    brownish-red,  and   again  bright-red,    irregularly  mottled  with  the 
light-gray  kind. 

Briefly  summarized,  then,  the  Portage  Lake  section  is  as  follows : 

XJPPEB  DTTISION  OP  THE  KEWEENAW  SERIES. 

Thickness  in  feet. 

Largely  covered,  but  apparently  for  the  most  part  red  shales  and  sandstone; 
towards  the  base  there  is  a  considerable  thickness  (upwards  of  200  feet) 
of  dark-colored,  fine-grained  sandstone  and  black  shale,  in  which  the 
usual  porphyry  detritus  is  mingled  with  more  or  less  basic  detritus ,  the 
lowest  layers  are  also  conglomeratic ;  in  all  about 9,000 

LOWER  DIVISION   OP   THE   KEWEENAW  SERIES. 

Covered  space  of  some  1,200  feet,  in  which  must  be  the  equivalents  of  the 
outer  trap  of  the  eastern  part  of  Keweenaw  Point,  corresponding  to  a 

thickness  of  about 500 

The  Great  Conglomerate,  including  the  sandstone  and  conglomerate  at  the 
Atlantic  mill  and  conglomerate  22  on  the  south  side  of  Portage  Lake, 

with  some  intervening  exposures;  about 1,600 

Diabase  66 

Conglomerate  21 15 

Diabase  and  amygdaloid 51 

Conglomerate  20 19 

Diabase 100 

Conglomerate  19 13 

Diabase 94 

Conglomerate  18 155 

Diabases  and  amygdaloids * 340 

Conglomerate  17  (Hancock  West) 32 

Diabases  and  amygdaloids ;  including  the  South  Pewabic  cupriferous  amygda- 
loid at  50  feet  below  17 650 

Conglomerate  16  (not  seen  on  south  side  Portage  Lake) 10 

Diabases  and  amygdaloids;  including,  at  400  feet  above  conglomerate  15,  the 
Pewabic  cupriferous  amygdaloid  or  "lode"  so  largely  worked  for  cop- 
per on  the  west  side  of  Portage  Lake 900 

Conglomerate  15  (Albany  and  Boston  conglomerate  on  the  north  side  of 

Portage  Lake) 33 

Diabases  and  amygdaloids 330 

Conglomerate  14  (the  Houghton  conglomerate  of  the  north  shore) 2 

Diabases  and  amygdaloids 1, 460 

Conglomerate  12  (north  side  of  Portage  Lake) 3 

Diabases  and  amygdaloids ■■■■  680 

Conglomerate  11 20 

Diabases  and  amygdaloids 200 

Conglomerate  10  . , 60 

Diabases  and  amygdaloids 460 

Conglomerate  9  (sandstone  seam) -- 


EOCKS  BETWEEN  PORTAGE  LAKE  AND  GEATIOT  EIVER.      195 

Thickness  in  feet. 

Diabases  and  amygdaloids ;  including,  at  670  feet  above  conglomerate  8,  the 
Grand  Portage  cupriferous  amygdaloid,  and  at  510  feet  the  Isle  Royale 
cupriferous  amygdaloid,  largely  worked  on  the  south  shore  of  Portage 

Lake 2,050 

Conglomerate  8 ■. 12 

Diabases  and  amygdaloids ^20 

Conglomerate  7.    24 

Diabases  and  amygdaloids 260 

Conglomerate  6 --   3 

Diabases  and  amygdaloids 181 

Conglomerate  5 24 

Diabases  and  amygdaloids 240 

Conglomerate  4 12 

Diabases  and  amygdaloids Ij  1^9 

Conglomerate  3 ^^ 

Diabases  and  amygdaloids •  ■  370 

Conglomerate  2 35 

Diabases  and  amygdaloids 1;  l^^O 

Conglomerate! ■    ^^ 

Amygdaloid 1^ 

Total 22,680 

The  comparatively  rare  exposures  to  the  northeast  of  Portage  Lake 
and  to  the  southwest  of  Gratiot  River  exhibit  only  a  few  points  of  particu- 
lar interest.  One  of  the  most  striking  of  these  is  the  great  change,  already 
alluded  to,  that  takes  place  in  the  nature  of  the  conglomerate  about  six  miles 
from  Portage  Lake.  About  Portage  Lake  the  greatly  predominant  pebbles 
are  compact  felsites  or  porphyries  with  only  feldspars,  and  no  visible  quartz, 
while  augite-syenites  are  found  in  some  beds.  Porphyries  with  visible 
free  quartz  are  unknown.  Towards  Calumet,  however,  all  of  the  conglom- 
erates show  true  quartz-porphyry  predominating  among  the  pebbles,  while 
still  further  to  the  northeast  the  quartzless  porphyries  again  predominate, 
although  not  to  the  exclusion  of  those  with  visible  quartzes.  This  change, 
as  pointed  out  by  Pumpelly^and  Marvine,^  must  be  due  to  a  variation  in 
the  nature  of  the  source,  in  a  direction  parallel  to  the  general  trend  of 
Keweenaw  Point.  This  source  Marvine  and  others  would  place  north  of 
the  point  where  now  lie  the  waters  of  Lake  Superior.  I  have  already 
shown  several  times  that  the  porphyry  conglomerates  derived  their  pebbles 

»Geol.  Survey  of  Micb.,  Vol.  I,  part  II,  p.  17.     ''Geol.  Survey  of  Mich.,  Vol.  I,  part  II,  p.  60. 


196       COPPER-BE AEING  EOCKS  OF  LAKE  SUPERIOR. 

from  belts  of  rock  within  the  same  formation,  and  have  also  shown  the  ex- 
istence of  such  a  belt  in  that  of  which  the  Mount  Houghton  rock  forms  a 
part. 

I  was  also  fortunate  enough  to  find  in  position,  south  of  Calumet,  on  the 
Torch  Lake  Railroad,  a  quartz-porphyr}?-  precisely  like  the  pebbles  of  the 
conglomerate  about  Calumet.  The  rock  is  poorly  exposed,  but  unquestion- 
ably in  place  and  unquestionably  original.  It  lies  in  the  southern  part  of 
Sec.  36,  T.  56,  R.  33  W.,  near  the  southern  border  of  the  Trap  Range, 
and  is  just  about  the  horizon  of  the  Mount  Houghton  felsite  belt,  of  which, 
or  of  a  closely  parallel  one,  it  may  be  reasonably  regarded  as  the  continua- 
tion. The  rock  exposed  here  shows  a  dark-red  aphanitic  matrix,  through 
which  are  scattered  very  abundant  and  large  porphyritic  quartzes  and  feld- 
spars. The  quartzes  are  black,  and  reach  two-tenths  inch  in  diameter;  the 
feldspars  red,  partly  quite  fresh,  partly  much  altered  and  whitened.  They 
run  from  two-tenths  to  one-fourth  inch  in  length.  Some  show  striations 
plainly.  Under  the  microscope  the  very  large  quartzes  are  seen  to  exhibit 
all  the  usual  characters  of  the  quartzes  of  such  rocks.  The  grains  are  sin- 
gle crystals,  or  parts  of  single  crystals,  often  showing  plainly  the  rhombo- 
hedral  planes,  more  rarely  the  prismatic.  Commonly  the  grains  are  much 
rounded  and  penetrated  by  bays  and  lines  of  the  matrix.  The  feldspars  are 
chiefly  oligoclase,  but  orthoclases  are  also  frequent.  Augite  crystals,  largely 
replaced  by  magnetite,  appear  porphyritically.  The  matrix  presents,  for 
the  most  part,  a  faint  pinkish  color  and  a  cloudy  appearance.  Thickly 
scattered  in  it  are  minute  brown  particles  and  needles  of  ferrite,  arranged 
in  lines  whose  directions  show  most  beautifully  the  so-called  flowage  struct- 
ure. This  matrix  affects  the  polarized  light  but  very  little,  if  at  all.  Other 
portions  of  the  matrix,  of  a  white  color,  appear  to  consist  of  individualized 
quartz  and  feldspar,  the  former  perhaps  secondary.  Calcite  is  also  seen  in 
small  particles  in  these  places.  This  rock  is  pictured  at  Fig.  2,  Plate  XII, 
where  is  also  figured  a  section  of  one  of  the  common  porphyry  pebbles  of 
the  Calumet  conglomerate.     The  identity  of  the  two  is  readily  apparent. 

The  map  and  sections  of  Keweenaw  Point  accompanying  this  descrip- 
tion (Plates  XVII  and  XVIII),  along  with  the  general  map  illustrating  the 
structure  of  the  Lake  Superior  basin,  will  help  greatly  to  convey  a  correct 


STANNAED'S  EOCK.  197 

idea  of  the  structure  of  this  district.  The  map  and  sections  of  Keweenaw 
Point  are  compiled  from  all  available  sources,  including  original  observa- 
tions and  original  deductions  from  the  work  of  others.  In  this  connection 
should  be  studied,  by  any  one  desiring  to  understand  the  stratigraphy  of 
Keweenaw  Point  more  in  detail,  the  correlation  chart  by  Marvine,  with  his 
admirable  explanations,  given  in  Vol.  I  of  the  Geological  Survey  of  Mich- 
igan.^ The  Eastern  Sandstone,  and  the  relations  which  subsist  between  the 
stratigraphy  of  Keweenaw  Point  and  that  of  other  parts  of  the  extent  of 
the  formation  are  considered  hereafter. 

Stannardi's  Rock. — Twenty-nine  miles  S.  51°  E.  from  the  eastern  extrem- 
ity of  Keweenaw  Point,  and  43  miles  N.  10°  E.  from  Marquette,  there  is 
an  isolated  reef  rising  abruptly  from  very  deep  water.  Above  water  the 
rock  is  only  a  few  feet  square,  but  the  entire  reef  is  some  3,500  feet  in 
length,  as  I  learn  from  a  large  scale  chart  furnished  by  the  United  States 
Lake  Survey.  This  length  is  divided  into  two  portions  by  a  gap  some  600 
feet  wide,  in  which  the  water  is  some  40  feet  deep,  the  depth  in  the  open 
lake,  just  outside  the  reef,  exceeding  600  feet.  The  northern  portion  of  the 
divided  reef  trends  northwest,  and  has  above  the  20-foot  contour  a  length 
of  some  1,300  feet  and  width  of  800  feet.  The  southern  part  of  the  reef 
trends  slightly  east  of  north,  and  is  1,700  feet  long  by  50  to  300  feet  wide 
above  the  20-foot  contour.  To  judge  from  the  soundings  of  the  Lake 
Survey  chart,  this  reef  and  another  more  deeply  buried  ridge  to  the  west 
of  it,  might  be  the  outcropping  edges  of  layers  trending  north  and  north- 
west, and  dipping  eastward  at  rather  a  high  angle. 

Foster  and  Whitney,  quoting  from  Mather,  speak  of  the  rock  of  Stan- 
nard's  Rock  as  a  sandstone,^  but  the  specimen  furnished  me  by  the  kindness 
of  Mr.  John  Chassells,  of  Houghton,  Mich.,  is  a  dark-brown  felsite.  In  the 
thin  section  this  felsite  shows  a  base  composed  of  a  mixture  of  quartz, 
which  in  part  appears  to  be  secondary,  a  considerable  proportion  of  pink- 
stained  non-polarizing  matter,  and  many  nearly  to  quite  opaque  dark-brown, 
black,  occasionally  (in  transmitted  light)  red  ferrite  particles,  of  wholly 
irregular  shape.  There  are  also  included  numerous  larger  brown  and  black 
porphyritic  particles,  with  crystalline  outlines,  which  now  and  then  appear 

» Vol.  I,  Part  II,  Chapter  IV.  ^  Qp.  cit. ,  p.  20. 


198       COPPEE-BEARING  EOCKS  OP  LAKE  SUPEEIOE. 

as  if  rounded  or  eaten  into  by  the  matrix.  Cores  of  brightly  polarizing 
augite  remain  in  many  of  these  crystals,  and  where  the  black  material  is 
very  thin  it  transmits  a  red  light.  The  crystals  are  thus  seen  to  be  augite, 
with  a  ferritic  alteration.  The  silica  content  is  65.8  per  cent.  Evidently  we 
have,  then,  in  the  Stannard's  Rock  reef,  one  of  the  felsite  belts  of  the  Ke- 
weenaw Series  which  have  furnished  so  many  of  the  interbedded  conglom- 
erates with  pebbles,  the  sections  of  many  of  these  pebbles  appearing  pre- 
cisely like  that  of  the  Stannard's  reef  rock.  The  shortness  of  the  reef  coin- 
cides perfectly  with  what  is  known  of  these  felsite  belts  elsewhere.  The 
belts  themselves  are  often  relatively  short,  and,  moreover,  tend  to  stand  up 
in  just  such  summits,  some  parts  of  the  belt  resisting  erosion  more  than 
the  rest.  Mount  Houghton  is  just  such  a  summit,  and  it  would  not  be  a 
bold  speculation  to  consider  Stannard's  Rock  and  Mount  Houghton  as  parts 
of  the  same  belt,  or  rather  as  belonging  near  the  same  general  horizon. 
However  that  may  be,  Stannard's  Rock  certainly  marks  for  us  the  course 
of  the  Keweenaw  Point  Range.  At  the  end  of  the  point  the  beds  are  trend- 
ing directly  towards  Stannard's  Rock,  whose  own  curving  trend  coincides 
perfectly,  as  subsequently  shown,  with  the  general  structure  that  I  have 
worked  out  for  the  whole  Keweenaw  basin. 


Section  II.— THE  EEGION"  BETWEEN  POETAGE  LAKE  AND  THE  ONTON- 

AGON  EIVEE. 

In  the  40  miles  that  intervene  between  Portage  Lake  and  the  Onton- 
agon River  the  exposures  are  not  abundant  until  the  latter  stream  is  nearly 
reached.  Evidently  enough,  however,  the  conditions  observed  at  the  for- 
mer place  continue  throughout  most,  if  not  all,  of  the  distance.  There  is 
the  same  southeastern  lowland,  with  its  horizontal  sandstone;  the  same  single 
Trap  Range,  with  abrupt  southern  and  gradual  northern  slopes,  composed 
of  the  same  strata,  dipping  at  the  same  high  angle  towards  Lake  Superior, 
and  having  about  the  same  thickness;  and  the  same  northwestern  lowland 
bordering  the  lake  and  occupied  by  the  sandstone  of  the  Upper  Division  of 
the  series.     Near  the  Ontonagon  River  the  main  mass  of  sandstone,  black 


POEPHYEY  AND  BLACK  SHALE  OP  THE  ONTONAGON  EIVER.  199 

shale,  and  conglomerate  of  the  Upper  Division,  and  the  Outer  Trap  and 
Grreat  Conglomerate  of  the  Lower  Division  are  all  distinctly  recognizable. 
No  detailed  measurements  have  ever  been  made  in  the  Ontonagon  country, 
but  when  there  I  saw  enough  to  convince  me  that  some  satisfactory  corre- 
lations could  also  be  reached  between  the  remainder  of  the  Lower  Division 
as  here  developed  and  as  developed  on  Portage  Lake,  especially  by  the 
use  of  the  interleaved  sandstone  and  conglomerate  bands,  which  here,  as 
further  east,  continue  down  to  quite  low  horizons. 

There  are,  however,  some  important  differences  between  the  Portage 
Lake  series  and  that  of  the  Ontonagon.  One  of  these  is  the  characteristic 
epidote-quartz  alteration  of  the  amygdaloids,  the  so-called  "veins"  which 
carry  the  Ontonagon  copper.  A  more  important  difference,  however,  is 
the  occurrence  in  the  Ontonagon  region  of  massive  quartziferous  porphy- 
ries at  what  appear  to  be  quite  high  horizons,  an  occurrence  which  con- 
tinues to  prevail  for  nearly  a  hundred  miles  to  the  westward,  the  porphyry 
occurring  in  irregular  belts  of  greatly  varying  lateral  extent.  Large  expos- 
ures of  a  dark  reddish-brown  porphyry,  holding  large  crystals  of  orthoclase 
and  oligoclase,  occur  north  of  the  village  of  Rockland,  in  the  NE.  ^  Sec. 
9,  T.  50,  R.  39  W.,  but  a  very  short  distance  south  of  a  broad  conglomerate 
that  appears  to  be  the  equivalent  of  the  Great  Conglomerate  of  Keweenaw 
Point. 

The  black  shale  and  gray  sandstone  belt  of  the  Upper  Division  of  the 
series,  already  recognized  as  occurring  near  Portage  Lake,  is  strongly 
marked  in  the  vicinity  of  the  Ontonagon  River,  having  been  recognized 
as  far  east  as  Sec.  14,  T.  51,  R.  38  W.,  10  miles  to  the  northeast,  along  the 
strike,  from  the  Ontonagon.  Near  the  Ontonagon  it  has  been  traced  across 
sections  22,  21,  28,  29,  32,  and  31  of  T.  51,  R.  38  W.,  and  sections  4,  5, 
and  7  of  T.  50,  R.  39  W.,  to  the  Ontonagon  in  Sec.  12,  T.  5(»,  R.  40  W. 
The  exposure  on  the  SE.  ^  Sec.  21,  T.  51,  R.  38  W.,  is  on  the  banks  and 
in  the  bed  of  a  small  stream  just  to  the  west  of  the  Grreenland  and  Ontonagon 
road.  There  are  to  be  seen  here  several  hundred  feet  in  thickness  of  alter- 
nating dark-gray  sandstone  and  black  shales,  overlaid  by  a  considerable 
thickness  of  red  sandstone,  and  underlaid  by  a  broad  porphyry-conglom- 
erate, evidently  the  equivalent  of  the  Outer  Conglomerate  of  Keweenaw 


200       COPPEE-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 

Point  and  the  Porcupine  Mountains.  All  of  these  rocks  trend  N.  30°  E., 
and  dip  40°  to  the  northwest,  or  at  a  lower  angle  than  the  beds  of  the 
Trap  Range  just  to  the  southward.  The  exposures  in  the  streams  on  sections 
4,  5,  and  7  of  T.  50,  R.  39  W.  are  closely  similar  to  the  last  described,  but 
much  less  extensive;  In  the  NE.  ^  Sec.  4  the  shale  dips  48°  NW.  and  trends 
N.  65°  E.,  while  in  sections  5  and  7  the  corresponding  figures  are  35°  NW. 
and  K  50°  E. 

The  shale  seen  at  these  several  points  is  dark  purplish-gray  to  nearly 
black,  and  from  excessively  fine-grained  to  aphanitic,  the  finest  kinds  being 
quite  soft  and  highly  argillaceous,  the  interbedded  sandstone  also  varying 
in  color  from  dark-gray  to  heavy  black.  Thin  sections  of  it  were  ex- 
amined both  from  the  exposures  on  Sec.  21,  T.  51,  R.  38  (3009-3011), 
and  from  those  on  Sec.  4,  T.  50,  R.  39  (3008).  All  of  the  sections  show 
the  usual  porphyry  detritus,  viz,  matrix  and  single  quartzes.  With  these, 
in  all  the  sections,  is  mingled  more  or  less  basic  detritus.  This  seems  to 
be  most  abundant  in  the  blackest  kinds,  in  which  there  is  also  an  abundant 
calcite  cement,  filling  all  interstices  between  the  fragments.  The  basic 
detritus  appears  in  the  shape  of  particles  of  the  basic  rocks,  showing  more 
or  less  plainly  the  several  ingredients,  always  much  altered,  and  of  par- 
ticles of  the  single  minerals,  viz :  augite,  almost  wholly  altered  to  a  greenish 
substance,  triclinic  feldspar  and  magnetite. 

Everywhere  north  of  the  line  of  the  black  shale,  between  Portage 
Lake  and  the  Ontonagon,  the  country  is  underlain  by  the  sandstones  of  the 
Upper  Division,  which  are  now  and  then  exposed  in  the  stream  beds,  and  form 
large  cliffs  on  the  lake  shore.  The  inclination  in  these  beds  is  everywhere 
lakeward,  but  the  amount  of  dip  lessens  rapidly  northward  from  the  Trap 
Range,  and  on  the  lake  shore  becomes  very  slight. 

From  the  Portage  to  the  point  between  the  Fire-Steel  and  Flint-Steel 
rivers  the  general  trend  of  the  layers  is  more  to  the  south  than  that  of  the 
coast,  which  thus  constantly  ascends  in  the  series,  the  sandstones  on  the 
point  alluded  to  belonging  near  the  top  of  the  Upper  Division.  Further 
west  the  reverse  is  the  case,  the  coast  trending  more  to  the  south  than  the 
rock  layers,  and  from  the  Flint-Steel  to  the  Union  River  there  is  a  steady 


THE  SOUTH  EANGB.  201 

descent,  the  base  of  the  Upper  Division  lying  not  more  than  half  a  mile 
inland  at  the  latter  point. 

The  total  thickness  of  the  Keweenaw  Series  between  the  Eastern  Sand- 
stone and  the  lake  coast  on  the  line  of  the  Ontonagon  River  is  some  28,000 
feet,  of  which  about  12,500  feet  belong  to  the  Upper  Division  and  the  re- 
mainder to  the  Lower  Division.  This  estimate  is  based  upon  the  breadth  of 
country  occupied,  and  the  trends  and  dips  of  the  strata.  The  Lower  Divis- 
ion, constituting  the  Trap  Range,  occupies  much  the  narrower  belt,  on  ac- 
count of  the  higher  northern  dip  of  its  beds,  while  the  Upper  Division  has 
a  broad  spread,  on  account  of  the  flat  dips  of  the  sandstones  composing  it, 
this  dip  lessening  rapidly  as  the  Trap  Range  is  receded  from,  until  at  the 
lake  it  varies  only  a  very  few  degrees  from  horizontality.  The  greater 
thickness  of  the  Upper  Division  present  in  the  Ontonagon  as  compared  with 
the  Portage  Lake  section  is  of  course  due  to  the  rising  of  the  coast  west- 
ward in  geological  horizon,  and  not  to  any  actual  increase  or  decrease  in 
thickness. 

Section  III.— THE  SOUTH  EANGE. 

The  low  area  lying  south  of  the  Keweenaw  Point,  or  Main  Trap 
Range,  and  underlain  by  horizontal  sandstone,  extends  from  B^te  Grise  Bay 
for  over  a  hundred  miles  in  a  southwesterly  and  westerly  direction,  as  far 
as  the  western  part  of  T.  48,  R.  44  W.,  west  of  Lake  Agogebic.  Keweenaw 
Bay  occupies  much  of  the  eastern  part  of  this  depression,  everywhere  south 
of  which  is  again  a  belt  of  high  country.  This  southern  margin,  in  the 
vicinity  of  Keweenaw  Bay,  is  composed  of  the  iron-bearing  schists  gen- 
erally referred  to  the  Huronian  system.  Farther  west,  however,  these 
iron-bearing  schists  have  between  them  and  the  horizontal  sandstone  on 
the  north  a  belt  of  rocks  closely  similar  to  those  of  the  Main  Range,  forming 
the  northern  border  of  the  sandstone-filled  depressiori;  i.  e.,  they  are  bedded 
diabases  and  amygdaloids,  including  porphyry-conglomerates.  This  is  the 
belt  to  which  the  name  of  South  Trap  Range  was  given  by  early  ex- 
plorers.-* 

'  The  information  obtainable  with  regard  to  the  South  Range  was  very  scanty  previous  to  the  be- 
ginning of  the  special  -work  on  which  the  report  is  based. 


202  COPPEE-BBARESTG  EOCKS  OF  LAKE  SUPEEIOE, 

From  T.  48,  R.  44  W.,  where  it  unites  with  tlie  Main  or  Keweenaw 
Range,  eastward  for  50  miles,  the  South  Range  shows  typical  Keweenawan 
rocks.  Further  east  and  north,  in  T.  48,  R.  36  "W.,  between  the  east  branch 
of  the  Ontonagon  and  Sturgeon  River,  I  am  informed  -^  of  the  existence  of 
exposures  of  diabase  and  porphyry-conglomerate,  which  from  their  posi- 
tion and  character  must  belong  to  the  same  belt.  Still  further  east  and 
north,  on  the  S.  E.  |  of  Sec.  1,  T.  49,  R.  36  W.,  a  bold,  rocky,  isolated  bluff, 
known  as  Silver  Mountain,  is  composed  of  diabase,  dipping  northwestward 
30°,  and  apparently  belonging  to  the  same  belt,  which  is  thus  carried  nearly 
to  Keweenaw  Bay.  It  is  not  meant  to  assert  that  these  rocks  form  a  con- 
tinuous belt  at  surface,  because  the  horizontal  sandstone  undoubtedly  over- 
lies them  very  often,  extending  quite  over  them  to  the  Huronian  or  even 
Laurentian  on  the  south.  Silver  Mountain  appears  to  be  surrounded  by 
sandstone,  and  west  from  here  for  many  miles  sandstone  appears  to  cover 
the  Keweenawan  rocks.  On  the  west  branch  of  the  Ontonagon,  in  Sec.  13, 
T.  46,  R.  41  W.,  sandstone  is  to  be  seen  cutting  quite  across  the  Kewee- 
nawan and  lying  against  the  older  slates. 

Two  or  three  of  the  points  examined  on  this  range  deserve  more 
detailed  description.  Beginning  on  the  northeast.  Silver  Mountain  first 
merits  attention.  This  is  a  bold  hill,  three-fourths  of  a  mile  in  length, 
trending  northeast.  It  is  on  the  S.  E.  ^  of  Sec.  17,  T.  49,  R.  36.  Its  north- 
western face  is  a  long  gradual  slope,  its  southeastern  a  bold  precipice,  400 
to  500  feet  high,  this  contour  being  evidently  due  to  the  northwestern  dip. 
The  rock  is  very  fine-grained,  varying  from  nearly  black  to  light  greenish- 
gi-ay  in  much  altered  portions.  Scattered  through  the  fine-grained  mass 
are  numerous  red  and  white  patches,  or  pseud-amygdules,  containing  quartz 
and  orthoclase.  Some  specimens  show  also  porph3'ntic  feldspar.  Under 
the  microscope  the  rock  is  seen  to  be  very  much  altered,  and  to  consist 
chiefly  of  tabular  feldspars,  and  fibrous,  faintly  dichroic  hornblende,  often 
showing  the  characteristic  prismatic  cleavage.  Augite  also  occurs,  and  in 
such  a  way  as  to  suggest  very  strongly  that  all  of  the  greenish  hornblende 
constituent  has  come  from  it,  i.  e.,  is  uralite,  as  some  undoubtedly  is. 
Chlorite,  titaniferous  magnetite,  and  its  usual  grayish  product  of  decompo- 

'By  L.  Q.  Emerson  and  B.  N.  White,  both  well  acquainted  with  the  foTmation. 


UNCONFORMITY  OF  THE  SOUTH  EANGE.  203 

sition  are  present  also.  Some  of  the  feldspars  show  no  banding,  and  may- 
be orthoclase.  The  red  porphyritic  and  pseud-amygdaloidal  feldspars  are 
orthoclase.  The  rock  appears  to  be  a  uralitic  diabase,  somewhat  allied  to 
the  rock  found  in  sections  26  and  27  of  T.  37,  R.  17  W".,  and  sections  23  and 
28  of  T.  37,  R.  16  W.,  in  the  Saint  Croix  River  district  of  Wisconsin,  though 
without  the  epidote.  It  should  probably  be  placed  with  the  uralitic  ortho- 
clase-bearing  gabbros.  It  cannot  be  regarded  as  certain  that  the  Silver 
Mountain  rock  is  Keweenawan.  It  may  possibly  be  Huronian,  though  the 
probabilities  are  greatly  in  favor  of  the  former  supposition. 

In  the  S.  E.  i  of  Sec.  1,  T.  46,  R.  39  W.,  the  Ontonagon  River  makes 
heavy  falls — 90  feet  in  six  leaps — over  diabase  and  diabase-amygdaloid, 
■which  rocks  appear  also  largely  in  a  low  ridge  east  and  west  of  the  stream. 
Both  in  the  hand  specimen  and  under  the  microscope  these  rocks  are 
indistinguishable  from  many  of  the  finer-grained  diabases  and  amygdaloids 
of  Keweenaw  Point.  The  massive  portions  are  dark-gray  and  minutely 
crystalline;  the  amygdaloids  have  a  brownish  matrix,  and  in  the  specimens 
brought  away  show  epidote,  orthoclase,  prehnite  and  chlorite  in  the  amyg- 
dules. 

The  exposures  on  the  vrest  branch  of  the  Ontonagon,  in  the  northwest 
part  of  T.  46,  R.  41  W.,  are  of  especial  interest  on  account  of  the  evidence 
they  offer  in  proof  of  unconformity  between  the  Eastern  Sandstone  and 
the  Keweenawan.  The  exposures  at  this  place  are  shown  in  the  accompany- 
ing figure  (Fig.  3.)  The  sandstone  is  horizontally  bedded,  showing  in  a 
south-facing  cliff  60  feet  high  and  350  feet  long.  It  is  reddish,  very  coarse, 
and  composed  almost  entirely  of  rounded  grains  of  quartz.  One  hundred 
paces  from  the  foot  of  this  cliff  are  reddish  schists,  trending  northeast  and 
dipping  45°  to  60°  SE.  Seven  hundred  paces  northeast,  near  the  south- 
east corner  of  section  11,  is  a  small  ledge  of  a  dark-brown,  weathered, 
medium-grained  diabase,  of  a  Keweenawan  type,  while  beginning  in  the 
northeast  part  of  the  same  section,  and  running  thence  westward  through 
sections  9  and  10,  and  terminating  in  the  S.  E.J,  Sec.  5,  is  a  series  of  exposures 
of  a  fine-grained,  greenish-gray  diabase  pseud-amygdaloid,  with  chlorite 
pseud-amygdules. 

The  relation  of  the  rocks  of  the  South  Range  to  those  of  the  Ke- 


204 


COPPEE-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 


weenaw  Point  Range  is  one  of  the  greatest  interest.  A  moment's  inspection 
of  the  map  of  Plate  I  will  serve  to  show  that  towards  the  east  the  two 
ranges  are  widely  separated,  the  distance  between  them,  even  west  of  the 
Ontonagon,  being  as  much  as  18  miles,  while  still  further  west  they  rap- 


Rg.  3.— Map  of  Exposures  in  Vicinity  of  'West  Brancli  of  Ontonagon  Eiver,  T.  46.  E.  41  W.,  Michigan. 


idly  approach,  and  finally  join.  The  beds  of  both  ranges  dip  northward. 
Should  we  suppose  a  continuous  series  beneath  the  intervening  horizontal 
sandstone,  we  should  obtain  an  incredible  thickness,  and  one  which  west- 
ward must  diminish  with  an  incredible  rapidity,  for  after  the  two  ranges 
have  joined  the  total  apparent  thickness  of  the  Lower  Division  of  the  series 
does  not  exceed  33,000  feet,  or  only  8,000  feet  more  than  on  Keweenaw 
Point.  That  there  is  a  fold  beneath  the  sandstone-filled  area  seems  im- 
probable. There  is  no  sign  of  a  southern  dip  along  the  south  side  of  the 
Keweenaw  Range  during  all  its  course  from  Bgte  Grise  Bay  to  its  junction 
with  the  South  Range.  I  had  at  one  time  the  idea  that  such  a  fold  might 
exist,^  Foster  and  Whitney  in  their  report  indicating  the  existence  of  a 

^See  Geology  of  Wisconsin,  Vol.  Ill,  Part  I,  p.  19,  foot-note. 


THE  KEWEENAW  FAULT.  205 

southern  dip  along  the  south  side  of  the  Keweenaw  Eange/  but  I  have  since 
convinced  myself  by  examination  that  no  southern  dip  exists. 

To  explain  the  sudden  break  on  the  south  side  of  the  Keweenaw  Range 
between  the  Keweenawan  beds  and  the  Eastern  Sandstone,  Foster  and 
Whitney  long  since^  supposed  this  line  to  be  one  of  fault,  and  the  Eastern 
Sandstone  to  be  the  equivalent  of  that  on  the  west  side  of  the  range,  the  two 
separated  only  by  the  faulting-.  The  latter  position  I  shall  show  subsequently 
to  be  untenable;^  and  yet  that  some  faulting  has  taken  place  on  this  line,  even 
after  the  deposition  of  the  sandstone,  is  proven  plainly  enough  by  the  fact 
that  at  the  contact  the  sandstones  commonly  rise  in  a  remarkable  manner, 
presenting  for  short  distances  from  the  junction  high  southern  dips.  On 
B§te  Grise  Bay  these  dips  reach  50°  at  the  contact,  lessening  to  40°  and 
30°  within  200  feet,  and  to  horizontality  within  a  mile  or  less.  Farther 
west  the  dips  at  the  contact  lessen  in  amount,  becoming  scarcely  perceptible 
at  Portage  Lake,  beyond  which  to  the  west  they  again  become  high.  These 
phenomena  are  beautifully  displayed  at  a  number  of  points  along  the  west 
branch  of  the  Ontonagon,  east  of  Lake  Agogebic. 

These  facts  render  it  plain  enough  that  some  faulting  took  place  on 
this  line  after  the  deposition  of  the  sandstone,  but  the  main  faulting,  I  con- 
ceive, took  place  before.  By  this  fault  the  Keweenaw  Range  escarpment 
and  the  valley  south  of  it  were  first  made,  the  width  of  the  valley  depend- 
ing on  the  amount  of  throw  of  the  fault,  which  was  thus  greatest  to  the 
eastward.  Subsequently  the  newer  sandstone  was  deposited  in  this  valley, 
and,  after  its  deposition,  a  comparatively  insignificant  amount  of  faulting 
took  place  on  the  same  line.  On  this  view  the  South  Range  beds  are 
the  basal  beds  of  the  series,  while  the  underlying  basement  of  the  interme- 
diate sandstone-filled  valley  is  composed,  in  a  measure,  of  the  same  beds  as 
those  forming  the  Keweenaw  Range. 

»  Op.  cif.,  pp.  65-66.  «0p,  ci«.,  p.  68.  =  chap.  VII. 


206  COPPEE-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 


Section  IV.— THE  EBGION  BETWEEN  THE  ONTONAGON  EIVEE  AND 
NUMAKAGON  LAKE  OF  WISCONSIN ;  INCLUDING  THE  PORCUPINE 
MOUNTAINS. 

This  region  is  one  of  especial  interest.  Within  it,  as  indicated  on  the 
map  and  sections  of  Plates  XXII  and  XXIII,  is  included  the  entire  thick- 
ness of  the  Keweenaw  Series,  both  of  the  Upper  and  Lower  Divisions. 
Within  it  the  Eastern  Sandstone  reaches  its  western  termination,  and  the 
ranges  of  Keweenawan  rocks  south  and  north  of  the  valley  occupied  by 
the  sandstone  unite.  Both  the  underlying  formations — the  iron-bearing 
Huronian  and  the  gneissic  Laurentian — are  here  largely  developed,  and 
in  close  proximity  to  the  Keweenawan.  Midway  the  district,  in  the  vicinity 
of  the  Montreal  River,  a  line  drawn  southeasterly  crosses,  in  a  distance  of 
some  twelve  miles,  the  entire  thickness  of  the  Huronian  and  Keweenawan. 
Within  this  region  we  have  also  the  eastern  termination  of  the  horizontal 
Western  Sandstone.  Here  we  have  again  far  more  extensive  developments 
of  quartziferous  and  felsitic  porphyries  than  on  Keweenaw  Point.  Augite- 
syenites  also  occur.  Here,  too,  we  have,  in  the  Bad  River  country,  a  great 
showing  of  the  coarse  gabbro,  which  is  developed  in  only  one  other  district 
about  Lake  Superior,  viz:  about  Duluth,  in  Minnesota. 

In  this  area,  moreover,  occurs  the  peculiar  fold  producing  the  Porcupine 
Mountains,  and  here  we  have  the  axis  of  the  great  trough  in  which  most 
of  Lake  Superior  lies  running  on  to  the  land.  Here  are  found  all  degrees 
of  inclination,  from  near  horizontality  to  complete  verticality.  With  all  of 
this  there  is  also  a  perfect  development  of  the  usual  bedded  amygdaloids, 
melaphyrs,  conglomerates,  etc.  Thus,  save  that  the  structural  relations  of 
the  acid  porphyries  are  not  to  be  so  distinctly  made  out  as  on  the  Min- 
nesota coast,  had  this  restricted  region  been  worked  in  such  detail  as  those 
portions  of  Keweenaw  Point  examined  by  Pumpelly  and  Marvine,  the 
Keweenaw  Series  would  be  nearly  as  well  understood  as  from  a  study  of 
the  entire  basin  of  Lake  Superior,  and  what  is  to  be  seen  elsewhere  about 
the  lake  would  be  of  little  more  than  confirmatory  value. 

Unfortunately  the  whole  district  is  wilderness,  and  with  the  exception 


SOURCES  OF  INFOEMATION  AS  TO  THIS  EBGIOK  207 

of  the  workings  on  the  gray  sandstone  of  the  Upper  Division  of  the  series, 
at  the  Nonesuch  mine,  in  the  Porcupine  Mountains,  there  are  no  mining 
operations  to  assist  in  a  detailed  study.  Foster  and  Whitney's  brief  ac- 
count of  the  region  between  the  Ontonagon  and  the  Montreal  rivers,^  a 
short  account  of  the  Porcupine  Mountains,  by  Col.  Chas.  Whittlesey,^  and 
an  equally  brief  description,  by  the  same  geologist,  of  the  region  between 
the  Montreal  and  Bad  rivers,'  embody  all  the  information  obtainable 
previous  to  my  own  acquaintance  with  this  district. 

In  the  summers  of  1873,  1876  and  1877  I  made  examinations  of  the 
regions  drained  b}'-  the  Montreal  and  Bad  rivers,  and  the  results  were  pub- 
lished in  1880  in  the  Geology  of  Wisconsin.*  In  the  same  volume  appears 
an  account  by  Professor  Pumpelly  of  the  microscopic  characters  of  a  num- 
ber of  the  specimens  collected  by  me  in  this  region.  In  addition  to  what 
is  given  in  the  above-named  publications  I  have  to  base  the  following  de- 
scriptions upon  the  results  of  a  detailed  examination  of  the  Porcupine 
Mountains,  by  my  assistants,  Messrs.  W.  M.  Chauvenet,  A.  C.  Campbell,  B. 
N.  White  and  R.  McKinlay,  of  a  cursory  examination  by  myself  of  the 
rocks  in  the  vicinity  of  the  Ontonagon,  and  of  my  own  detailed  micro- 
scopic study  of  all  specimens  gathered. 

The  conditions  already  described  as  obtaining  at  the  crossing  of  the 
Ontonagon  remain  the  same  all  the  way  to  the  Montreal  River,  so  far  as 
general  stratigraphy  and  kinds  of  rocks  are  concerned,  save  that  there  is  a 
very  considerable  expansion  downwards,  due  chiefly  to  the  appearance  at 
the  surface  of  layers  which  farther  east  are  buried  beneath  the  Eastern 
Sandstone.  There  are,  however,  noteworthy  changes  in  structural  features, 
in  the  courses  of  the  rock  belts,  and  in  the  width  of  country  occupied  by 
them.  Save  in  the  Porcupine  Moimtains,  which  lie  to  the  north  of  the 
Main  Trap  Range,  the  usual  northern  dip  everywhere  prevails,  but,  after 
two  townships  are  crossed  west  from  the  Ontonagon,  it  has  flattened  enough 
to  increase  very  materially  the  width  of  country  occupied  by  the  Lower 
Division  of  the  series.     The  rising  from  beneath  the  Eastern  Sandstone  of 

'Op.  cit.,  pp.  74-80,  1850. 

^  Engineering  and  Mining  Journal,  Vol.  23,  p.  254,  April  21,  1877. 

'Proc.  Boston  Soc.  Nat.  Hist.,  July,  1863;  also  Geology  of  Wisconsin,  Vol.  Ill,  Part  III,  Ap- 
pendix A,  pp.  216-223. 

*  Geology  of  Wisconsin,  Vol.  Ill,  Parts  I  and  III. 


208       COPPEE-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 

layers  previously  concealed  aids  in  increasing  this  width,  which  on  the 
west  line  of  range  41  is  as  much  as  five  miles,  as  against  only  three  on  the 
Ontonagon. 

This  increase  in  surface  width  becomes  very  rapid  in  the  next  ranges 
westward,  the  width  in  the  east  part  of  range  42  being  as  much  as  eight 
miles.  Beyond  range  44  the  Eastern  Sandstone  and  the  depression  it  oc- 
cupies come  to  an  end,  the  South  and  Main  Eanges  coming  together.  As 
already  explained,  the  South  Range  is  made  up  of  belts  of  the  usual  basic 
eruptive  rocks  dipping  northward  and  constituting  the  lowermost  part  of  the 
Keweenaw  Series;  their  separation  from  the  beds  of  the  Main  Range  is 
due  to  faulting.  It  has  also  been  shown  already  that  this  fault  dies  out  to 
the  westward,  the  South  and  Main  Ranges  coming  together.  As  a  result  of 
this  the  width  of  country  occupied  by  the  Lower  Division  of  the  series 
becomes  on  the  Black  and  Presqu'  Isle  rivers  as  much  as  ten  miles. 

East  of  the  Ontonagon  River  the  course  of  the  rock  belts  is  southwest, 
or  even  a  little  south  of  southwest.  After  crossing  the  Ontonagon  there  is  a 
change  to  a  direction  only  slightly  south  of  west,  and  for  the  rest  of  the  dis- 
tance to  the  Montreal  the  general  trend  of  the  belts  constituting  most  of  the 
Lower  Division  oscillates  between  5°  and  20°  south  from  west.  On  the 
Montreal  a  rapid  change  takes  place  to  a  direction  only  30°  to  40°  west  of 
south.  With  regard  to  these  lower  belts  in  this  distance  I  have  nothing 
new  to  offer,  and  can  only  say  that  so  far  as  known  they  present  the  usual 
diabases,  melaphyrs  and  amygdaloids,  with  some  porphyries,  and  in  the 
lowest  portions  coarser  rocks  belonging  to  both  the  olivine-  and  orthoclase- 
gabbros. 

The  upper  belts  of  the  series  and  all  of  the  Upper  Division  are  not  in- 
cluded in  the  statement  just  made  as  to  general  trend.  On  following  them 
westward  they  are  found  bending  out  to  the  north  in  a  curious  loop,  return- 
ing on  the  western  side  of  the  loop  to  their  original  positions.  The  neck  of 
the  loop  trends  nearly  north,  or  at  right  angles  to  the  Main  Range,  but  near 
the  lake  its  end  bends  well  around  to  the  eastward.  Within  this  loop  are 
the  Porcupine  Mountains,  whose  interior  portions  are  made  up  of  felsitic 
and  quartziferous  porphyries,  without  any  definitely  traceable  structure. 
These  porphyries  plainly  enough  belong  to  the  same  belt  or  belts  which 


FPEP  r-i;AR;''r-  f-'..'k_  -.^  iaye  _jcep 


TIIK  KASTKIIX  SANDSTONE 

KKWKKNAW  SP;RIES. 


I'PPKH  DIVISION' 

Itpft  Saiiilsloiic 

T^ 

DnHfrjfi-wSmKi.slimc 

uiiinilark  ShfiJe 

^^H 

Sjiiiilstoiicu'ilJilUm 

hiiuijfi  ut'Ciiii^lniitPi'ale 

.  1 

LOWEH  UIMSION 

Diabusp.Diabasf  anij-gdnloid . 

te^Hrt^j 

and  .MnIaiih\T 

■H 

SniKlstoiieniifU'tdi^Ioiiieinlf- 

Ihnliasf  Dial)iisp  atm'^^dalniij 
.\lrlapit\T  aird  Dmliasp-poqdivrv 
intlMnieRn7ilinTiyrii»lomfnirc 

liiiartzpnrpiim'aiicIFelsile 


Uifihasp  and  Diabasp  amygdaloid 
naiTow  Coiigldiiu'rate  Bell^ 


Scale    133 ooo.  or  1  inch- l.!)7."j  miles 

P«inuneiit  PXituMirt-s  uf  * 

iiiatisiviTnrks  I 

Kxj)()siires  oldplritalroclts 

sll<^w^ng  dip  and  stnlw  -^ 


r.    XLIV    \N 


K  imi  w. 


OKOIAH'tICM.  M.VPOKTHE  PORCrPIXE   MOIiNTAIXS. 

iirCHUlAN. 


THE  PORCUPINE  MOUNTAINS.  209 

are  first  found  to  the  east  on  the  Ontonagon  River,  and  thence  extend  west- 
ward all  the  way  to  and  beyond  Bad  River,  in  Wisconsin.  From  this  cen- 
tral mass  of  porphyry  the  remaining  beds  of  the  Lower  Division,  and  those 
of  the  Upper,  dip  away  on  all  sides  in  a  manner  that  will  be  best  under- 
stood from  a  study  of  the  map  and  sections  of  Plates  XIX,  XX  and  XXI. 

Above  the  porphyry  comes  first  a  succession  of  beds  of  diabase,  mela- 
phyr,  amygdaloid  and  diabase-porphyry,  with  one  included  bed  of  porphyry- 
conglomerate,  in  all  400  feet  in  thickness.  To  this  succeed  1,900  feet  of 
sandstone  and  conglomerate;  300  to  400  feet  of  beds  of  diabase  and  diabase- 
amygdaloid;  upwards  of  3,000  feet  of  sandstone  and  conglomerate;  600 
feet  of  dark  sandstone  and  black  shale;  and  then  the  main  mass  of  sandstone 
of  the  Upper  Division.  This  succession  is  plainly  enough  the  same  as  that 
which  obtains  on  the  Ontonagon,  while  the  black  slate,  outer  conglomerate, 
trap,  and  lower  conglomerate,  are  as  plainly  the  equivalents  of  the  shales, 
Outer  Conglomerate,  Lake-Shore  Trap,  and  main  or  Great  Conglomerate  of 
Keweenaw  Point. 

The  structure  of  the  Porcupine  Mountains  having  never  before  been 
worked  out,  nor  indeed  any  of  the  constituent  rock  beds  described,  save  in 
the  roughest  way,  I  may  properly  include  here  somewhat  detailed  de- 
scriptions of  each  of  the  members  of  the  succession  above  laid  down.  Plates 
XIX,  XX  and  XXI  should  be  studied  in  connection  with  these  descriptions. 

The  porphyry  which  constitutes  the  central  mass  of  the  mountains  ap- 
pears in  numerous  bold,  structureless  exposures  through  the  middle,  from 
south  to  north,  of  T.  50,  R.  44  W.  ;  in  the  northern  parts  of  the  north- 
western sections  of  T.  50,  R.  43  W. ;  in  the  southwestern  sections  of  T.  51, 
R.  42  W.,  especially  on  the  upper  Carp  River ;  and  over  a  large  area  in 
the  southern  and  southwestern  sections  of  T.  61,  R.  43  W.  The  ridges  in 
this  area  rise  often  1,000  to  1,200  feet  above  Lake  Superior,  while  the  lower 
portions  rarely  sink  to  800  feet  above  the  same  level. 

The  rock  seen  on  the  various  exposures  within  this  area  varies  in  color 
through  various  shades  of  red,  lilac  and  j^urple.  It  shows  everywhere  a 
distinct  tendency  towai'ds  becoming  a  non-porphyritic  felsite,  porphyritic 
quartz  and  feldspar  being  sometimes  altogether  absent,  though  more  com- 
monly present  in  small  and  sparsely  scattered  particles.  There  is  occa- 
14  L  s 


210       COPPEE-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 

sionally  visible  a  very  distinct,  but  more  or  less  irregularly  curving,  fine 
banding,  such  as  is  often  found  in  similar  porphyries  from  other  regions, 
and  which  must  be  attributed  to  flowage  in  a  molten  state.  Occasionally 
the  matrix  shows  a  tendency  to  a  more  distinctly  crystalline  texture,  the 
rock  tending  towards  granitic  porphyry. 

1  have  examined  a  number  of  thin  sections  of  the  Porcupine  porphy- 
ries, with  the  following  results.  The  first  is  pale-lilac  felsite  (2,614)^  from 
the  large  vertical  walls  on  the  north  slope  of  the  hill  on  the  north  line  of 
Sec.  5,  T.  50,  R.  43  W.,  at  700  paces  west  of  the  northeast  corner.  It 
shows  a  colorless  base  without  porphyritic  ingredients ;  and  is  apparently  a 
minutely  crystalline  admixture  of  quartz  and  feldspar.  Some  of  the  quartz 
is  secondary.  Sparsely  scattered  through  the  matrix  are  black  and  brown 
opaque  particles.  The  bright-red,  blotched,  rough-textured  felsite  (251.5) 
from  the  same  hill  in  the  S.  E.  i  Sec.  35,  T.  51,  R  43  W.,  (250  N.,  900' 
W.),  shows  in  the  section  very  much  the  same  appearance  as  the  last  de- 
scribed, but  contains  also  some  non-polarizing  base.  The  lilac  felsite  (2516) 
brought  from  the  continuation  of  the  same  great  ledge  in  the  S.  W.  ^,  Sec. 
35,  (500  N.,  1100  W.)  gives  the  same  section  as  the  last.  The  rock 
(2517)  fi-'om  the  hundred-foot  cliff,  near  the  middle  of  the  same  section  (720 
N.,  1000  W.)  is  somewhat  different.  Macroscopically  it  is  an  aphanitic 
bright-red  rock  blotched  with  white,  and  without  porphyritic  ingredients. 
In  the  thin  section  the  red  portions  are  excessively  fine-grained  and  even 
non-polarizing,  with  abundant,  minute,  brownish,  opaque  particles  and  parti- 
cles of  red  oxide  of  iron.  The  white  portions  consist  of  comparatively  large 
particles  of  orthoclase  and  quartz.  The  dark  purplish-red  rock  (2551) 
from  the  bed  of  Carp  River,  in  the  N.  E.  |,  Sec.  35  (1420  N.,  1400  W.),  is  very 
much  the  same  felsite  as  those  first  described,  being  without  porphyritic 
ingredients.  The  rock  on  the  south  side  of  the  Carp  Lake  road  in  the  S.  E. 
i,  Sec.  24,  T.  51,  R.  43  W.,  (800  N.,  300  W.),  is  dark  purpKsh-red  with  a 
few  small  red  feldspars  and  minute  quartzes.  In  the  thin  section  the  matrix 
presents  an  interesting  appearance.     Much  of  it  is  of  a  faint  reddish  tint 

'  These  numbers  refer  to  specimens,  many  of  which  will  be  found  described  in  the  tables  of  Chap. 
III. 

2  These  numbers  give  the  distances  north  and  west  from  the  southeast  corner  of  the  section,  in 
each  case. 


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FBLSITIC  POEPHYEIES  OF  THE  POECUPINES.  211 

and  non-polarizing,  while  thickly  studded  through  this  are  minute  particles 
of  quartz,  a  number  of  which  polarize  together,  thus  showing  that  they  are 
parts  of  one  crystal.     The  section  thus  has  a  pecuHar  mossy  appearance  in 
polarized  light.     This  quartz  is  doubtless  secondary,  but  the  particles  are 
larger  and  more  nearly  like  those  of  the  original  quartz  than  usual.     The 
few  porphyritic  quartzes  and  feldspars  present  the  usual  characters.      Much 
black  dust  is  present  in  blotches  and  lines,  and  is  in  part  undoubtedly  mag- 
netite.    The  dark  purphsh  quartz-p?)rphyry  (2574)  from  Sec.  20,  T.  51,  R. 
42  W.  (500  N.,  1150  W.),  holds  a  few  minute  original  quartzes,  and  shows 
under  the  microscope  a  matrix  which  in  the  ordinary  light  appears  nearly 
homogeneous  and  thickly  dotted  with  minute  brownish  and  blackish  part- 
icles, but  in  the  polarized  light  presents  an  appearance  analogous  to  that  of 
the  last  rock  described,  except  that  the  secondary  quartz  is  in  much  more 
minute  particles,  large  numbers  of  which  polarize  together.     Orthoclases 
occur  among  the  porphyritic  ingredients,  and  a  very  interesting  relation  was 
noted  between  one  of  the  orthoclase  crystals  and  one  of  the  original  quartzes, 
the  latter  having  grown  around  the  end  of  the  former.     A  few  minute  augites 
occur  in  the  matrix.    A  closely  similar  rock  (2586)  appears  in  large  exposures 
on  the  upper  part  of  the  stream  which  enters  the  southeast  end  of  Carp  Lake, 
in  the  S.  W.  ^,  Sec.  23,  T.  51,  R.  43  W.     The  sections  of  the  dark  purphsh- 
red  rock  (1247  and  1248)  from  the  bed  of  Little  Carp  River  in  the  north- 
east corner  of  Sec.  20,  T.  50,  R.  44  W.,  appear  much  hke  the  last  described, 
having  not  very  rare  original  quartzes  and  orthoclases,  and  abundant  second- 
ary quartz,  some  of  which  is  arranged  in  curving   concentric  lines,  appa- 
rently emphasizing  a  structure  in  the  original  rock.     These  sections  show 
much  fine  ferrite,  and  now  and  then  a  quite  perfectly  developed  augite  crystal. 
The  ferrite  particles  in  places  appear  to  indicate  flowage.     The  ledge  on  the 
south  hne  of  Sec.  9,  T.  50,  R.  44  W.,  270  paces  west  of  the  southeast  corner, 
shows  a  very  plainly  banded  felsitic  porphyry  (1259)  with  not  rare,  white 
orthoclase  crystals  which  often  He  entirely  across  two  or  three  bands.     The 
banding  is  produced  by  hghter  and  darker  shades,  and  is  quite  irregular  and 
wavy.     Under  the  microscope  the  banding  is  seen  to  be  produced  by  the 
presence  of  much  red  iron  oxide  in  some  bands,  and  its  absence  or  relative 
scarcity  in  others.     The  bands  are  quite  without  regularity  or  continuity, 


212       COPPER-BE AEING  EOCKS  OF  LAKE  SUPEEIOE. 

the  white  bands  being  merely  irregular  rows  of  light  blotches.  The  indi- 
vidual blotches  do  not  average  more  than  a  small  fraction  of  an  inch  in 
length,  the  red  material  closing  together  between  them.  Under  the  micro- 
scope both  light  and  dark  portions  show  little  sheaf-like  aggregations  of 
orthoclase,  which  mineral  in  the  whiter  portions  is  often  larger  than  in  the 
red.  A  good  deal  of  the  usual  secondary  quartz  and  ferrite  are  present. 
The  porphyritic  orthoclases  are  much  decomposed  and  reddened,  and  are 
not  very  abundant. 

Another  banded  rock  (1263)  is  shown  in  one  of  the  numerous  north- 
easterly trending  ledges  of  the  southern  part  of  T.  51,  K  43  W.,  near  the 
southeast  corner  of  section  32.  This  is  a  dark-red  felsite  without  porphyritic 
ingredients,  looking  much  like  a  dark-red  quartzite,  with  close  and  quite 
regular  bands  produced  by  variation  in  depth  of  color.  Under  the  mi- 
croscope the  thin  section  shows  but  little  coloring  matter.  The  lighter  bands 
are  seen  to  be  composed  of  rather  coarser  particles  than  the  others,  while 
the  banding  is  further  emphasized  by  fine  lines  of  thickly  crowded  ferrite 
particles.  The  base  appears  to  be  composed  of  individualized  orthoclase 
and  quartz,  and  some  little  quartz  that  appears  as  if  secondary,  but  there 
is  none  of  the  irregular  mossy  or  radiating  or  concentric  structure  produced 
by  secondary  quartz,  such  as  noticed  in  other  sections,  the  base  presenting  a 
remarkably  homogeneous,  finely  crystalline  appearance  The  numerous 
samples  brought  from  other  ledges  within  the  porphyry  area  of  the  Porcu- 
pines do  not  indicate  the  existence  of  any  other  phase  of  the  porj)hyry  than 
those  described  above. 

The  limits  of  the  Porcupine  porphyry  area  are  pretty  well  determined 
by  exposures,  more  especially  on  the  north  and  northwest,  where  the  ex- 
posures of  the  different  rocks  often  lie  quite  close  together.  Thus  in  sections 
30  and  31,  T.  49,  11.  44  W.,  the  Presqu'  Isle  River  exposes  diabases  and 
amygdaloids  of  the  ordinary  type,  and  the  latter  rock  is  seen  again  in"  the 
southwest  of  section  19.  In  the  northeast  of  section  1 9  and  the  northwest  of 
section  20  are  large  exposures  of  red  felsite  and  felsitic  porphyry,  while  in  the 
middle  of  the  west  side  of  section  18  are  seen  the  amygdaloids  and  diabase 
belonging  to  the  belt  north  of  the  porphyry,  the  north  and  south  limits  of 
which  on  the  Presqu'  Isle  are  thus  indicated.     Again,  at  the  rapids  of  Little 


POEPHYET  AEEA  OF  THE  POECUPINBS.  213 

Carp  River,  in  the  northeast  corner  of  Sec.  20,  T.  50,  R.  44  W.,  the  contact 
of  the  porphyry  with  the  overlying  rock  is  in  sight.  The  high  ridge  of 
porphyry  in  the  east  part  of  section  31  and  the  southeast  of  section  30  of  the 
same  township  proves  that  the  boundary  of  the  overlying  belt  of  diabase 
and  diabase-amygdaloid  makes  here  a  considerable  deflection  to  the  west- 
ward from  a  straight  line  drawn  between  its  positions  on  the  Little  Carp 
River  in  Sec.  20,  T.  50,  R.  44  W.,  and  in  the  west  part  of  Sec.  18,  T.  49, 
R.  44  W.  Sec.  32,  T.  51,  R.  44  W.,  exposes  porphyry  largely  in  the  south- 
eastern and  southern  portions,  and  diabase  in  the  western  and  northern, 
while  in  the  stream  at  the  northeast  corner  of  this  section,  lilac  felsite  and 
amygdaloids  are  largely  exposed  at  only  200  paces  from  each  other.  South 
of  Carp  Lake,  in  sections  23  and  22,  the  porphyry  and  overlying  rock  are 
largely  exposed  at  points  not  far  apart,  and  several  exposures  of  the  two 
rocks  between  here  and  the  junction  in  the  N.  E.  J,  Sec.  32,  serve  to  fix 
this  part  of  the  boundary  quite  closely.  The  road  running  east  and  south 
from  the  head  of  Carp  Lake,  through  sections  23  and  24,  crosses  ledges 
both  of  the  porphyry  and  its  overlying  rock.  In  Sec.  30,  T.  51,  R.  42  W.,  the 
limit  is  again  found  in  the  northeast  quarter  of  the  section.  Further  south- 
east, in  T.  51,  R.  42  W.,  there  is  some  doubt  as  to  the  exact  limit  of  the 
porphyry  to  the  eastward,  or  rather,  the  position  of  its  overlying  rock  is 
left  in  doubt  by  lack  of  exposures ;  besides  which  some  doubt  is  introduced 
by  the  uncertainty  as  to  where  the  fault  lying  south  of  the  porphyry  ends. 
Closely  approximated  exposures  of  the  porphyry  and  amygdaloid  on 
the  south  fix  the  southern  limit  very  closely  in  sections  3,  4,  5,  and  6, 
T.  50,  R.  43  W.  In  the  eastern  part  of  T.  50,  R.  44  W.,  a  gap  without 
exposures  leaves  the  exact  position  of  the  eastern  limit  in  some  doubt, 
as  indicated  on  the  map.  The  porphyry  and  felsite  ledges  are  so  generally 
distributed  over  the  area  colored  for  those  rocks  as  to  indicate  that  they 
underlie  most  of  its  extent,  but  in  one  place  a  large  exposure  of  a  rather 
coarse-grained  very  highly  crystalline  gabbro,  such  as  is  frequently  asso- 
ciated with  porphyry  and  felsite  on  the  north  shore  of  Lake  Superior,  was 
noted.  This  is  on  the  south  line  of  Sec.  3,  T.  50,  R.  44  "W.,  1,150  paces  west 
of  the  southeast  corner.  The  ledge  is  100  feet  high  and  200  feet  long  in  a 
southwest  direction. 


214       COPPEE-BEAEmG  BOOKS  OF  LAKE  SUPEEIOE. 


N 


In  attempting  to  trace  the  porphyry  of  the  Porcupine  Mountains  east- 
ward we  find  that  across  T.  49,  R.  43  W.,  the  region  where  it  would  lie  is 
mostly  drift-covered,  but  that  in  the  northern  part  of  T.  49,  R.  42  W. 
there  are  large  exposures  of  quartzose  porphyry  occupying  a  belt  one  to 
two  miles  in  width,  and  trending  east-north-east  across  the  township  to  the 
northeast  corner.  T.  50,  R.  41  W.  is  next  crossed  by  the  belt  in  its  south- 
ern portions,  without  many  exposures.  One  is  reported  by  Mr.  B.  N. 
White  at  the  center  of  Sec.  21,  T.  50,  R.  41  "W.  Across  the  middle  sec- 
tions of  T.  50,  R.  40  W.  the  government  surveyors  have  written  the 
words,  "red  slaty  trap,"  which  are  evidently  meant  for  the  red  porphyry. 
The  same  words  are  used  in  the  plat  of  T.  50,  R.  39,  for  the  quartzose 
porphyry  already  described.  Westward  from  the  Presqu'  Isle  River  to  the 
Montreal  I  have  no  knowledge  of  any  porphyry  exposures.  The  country 
is  mostly  low  and  the  rocks  covered  where  the  porphyry  would  be  expected 
to  appear.     Beyond  the  Montreal  it  reappears,  as  subsequently  described. 

North  and  west  of  the  porphyry  area  of  the  Porcupine  Mountains 
there  is  everywhere  found  a  belt  of  basic  rocks  having  a  surface  width  of 
from  about  one-fourth  to  one-third  mile,  and  a  thickness  varying  from 
300  to  500  feet.  Toward  the  middle  of  the  thickness  a  porphyry-con- 
glomerate is  included,  with  a  thickness  of  over  60  feet.  The  basic  rocks  of 
the  belt  are  in  very  regular  and  rather  thick  flows,  some  half  dozen  beds 
appearing  to  make  up  the  whole  thickness.  Among  the  massive  portions 
of  the  beds  a  very  fine-grained,  dark-red,  compact  and  semi-conchoidal 
rock  is  abundant,  occurring  both  above  and  below  the  intermediate  con- 
glomerate. Under  the  microscope  the  slices  of  this  rock,  from  points  far 
apart  along  the  length  of  the  belt,  show  the  characteristic  arrangement  be- 
longing to  Pumpelly's  melaphyrs.  Olivine,  plagioclase,  magnetite  and 
augite  are  the  ingredients  in  order  of  age.  The  augite  occurs  in  crystals 
including  numbers  of  the  small  plagioclases,  while  the  magnetite  and  the 
olivine  are  crowded  into  the  spaces  between  the  augites,  the  latter  mineral 
being  usually  much  altered  to  both  red  (hematite)  and  green  (serpentine)  sub- 
stances. Another  rock  of  this  belt — occurring  in  some  places  in  two  layers, 
one  above  and  one  below  the  intermediate  conglomerate,  and  in  others  only 
above  that  horizon — is  an  aphanitic  dark-gray  to  black  or  reddish-brown  rock 


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INNER  TRAP  BELT  OF  THE  PORCUPINES. 


215 


with  few  or  many  porpliyritic  feldspars.  The  thin  sections  show  this  rock  in 
some  cases  as  a  typical  ashbed-diabase,  with  a  few  porphyritic  orthoclases, 
and  in  others  as  a  true  diabase-porphyrite.  The  latter  presents  very 
numerous  porphyritic  orthoclases  in  an  excessively  fine  base,  in  which 
there  are  portions  which  polarize  only  very  feebly  or  not  at  all,  and  in 
which  minute  tabular  plagioclases  with  rare  augite,  magnetite  and  ferrite 
particles  are  the  only  recognizable  ingredients.  Some  of  the  beds  are  of 
the  ordinary  type  of  fine-grained  diabase,  and  the  amygdaloids  present  no 
unusual  characters.  The  rocks  of  this  belt  can  be  seen  to  best  advantage 
on  the  upper  Carp  River,  in  sections  19  and  30,  T.  51,  R  42  W.;  on  the  road 
from  Union  mine  to  Carp  Lake,  in  Sec.  24,  T.  51,  R.  43.  W.;  along  the  course 
of  the  stream  south  from  the  same  road  in  the  NE.  ^,  Sec.  23,  T.  51,  R. 
43  W.;  in  the  NW.  i  of  the  same  section  along  the  larger  stream  which 
runs  into  Carp  Lake  near  its  southeast  corner ;  and  on  the  Little  Carp 
River  in  sections  17  and  20,  T.  50,  R.  44  W. 

On  the  upper  Carp  the  junction  with  the  overlying  conglomerate  is 
seen  at  450  paces  north  and 
1,630  west  of  the  southeast 
corner  of  Sec.  19,  T.  51,  R. 
43  W.,  where  the  strike  is 
northwest,  and  the  dip  north- 
east 25°.  Hence  up  stream 
to  the  junction  with  the  in- 
cluded conglomerate  at  282 
paces  north  and  1,630  west 
of  the  same  corner  the  ex- 
posures are  nearly  continu- 
ous and  show  much  of  the 
fine-grained  reddish  mela- 
phyr,  besides  several  bands 
of  amygdaloid  and  diabase  of . .    

—,,.■■     Fig.  4.— Map  of  exposures  ou  upper  Carp  Eiver,  Porcupine 
the  Ordmary  types,    i  he  thick-  Mountains,     one  inch  =  90  paces. 

ness  between  the  two  conglomerates  is  about  200  feet.     The  upward  course 
of  the  stream  follows  the  junction  with  the  lower  conglomerate  to  a  point 


Soiiih.  Zgyate_  3'gc.yg  71^^^  '*^  ^' 


216 


COPPEE-BEAEING  EOCKS  OF  LAKE  SUPBEIOR. 


100  paces  nortli  and  1,520  paces  west  of  the  section  corner,  the  conglomer- 
ate forming  the  southwest  bank,  and  the  diabase  the  northeast,  as  indi- 
cated in  the  accompanying  sketches.  Figs.  4  and  5. 


CbnoZomeral/i 


CI;mu^cUi.loijd. 


Fig.  5. — Cross-section  on  line  CD  of  Fig.  4. 

At  the  last  point  indicated,  B  of  Fig.  4,  the  stream  makes  a  sharp  turn 
westward,  and  the  diabase  is  faulted  against  a  reddish  diabase-porphyry, 
carrying  orthoclase  and  triclinic  feldspars  as  porphyritic  ingredients,  and 
having-  a  confused,  much  altered  base  in  which  the  minute  tabular  plagio- 
clases  are,  however,  very  abundant.  The  whole  rock  is  permeated  by  ht- 
tle  strings  and  patches  of  calcite. 

Further  up  the  stream,  in  the  northwest  part  of  section  30,  and  as  far 
as  a  point  1,675  paces  north  and  1,590  west  of  the  southeast  corner  of  the 

Fig.  6. — Section  on  line  AB  of  Fig.  4. 

latter  section,  other  basic  beds  are  seen  in  place,  including  two  or  three 
amygdaloids,  some  reddish  melaphyr  and  a  heavy  bed  of  diabase-porphyry. 
All  of  these  beds,  from  the  junction  of  the  upper  conglomerate,  trend  north- 
west and  dip  northeast  25°.  Still  further  up  the  stream,  in'section  30  (1,420 
N.,  1,400  W),  the  true  felsitic  porphyry  begins  to  show.  Judging  by  the 
surface  width  and  dip  angle,  the  thickness  on  this  stream  between  the  up- 
per conglomerate  and  the  last  amygdaloid  is  as  much  as  600  feet,  but  some 
of  the  apparent  thickness  may  be  due  to  the  faultings  spoken  of 

The  two  streams  in  Sec.  23,  T.  51,  R.  43  W.  do  not  expose  the  upper 
conglomerate,  but  present  above  the  lower  conglomerate  some  400  feet  in 
thickness  of  very  regularly-bedded  melaphyrs  and  amygdaloids,  including 
much  of  an  exceedingly  fine-grained  diabase  of  the  ashbed  type,  which  at 
times  carries  numerous  porphyritic  orthoclases,  and  runs  then  into  a  diabase- 


INKER  SANDSTONE  OF  THE  POECUPINES.  217 

porphyry.  The  exposures  on  the  Little  Carp  River  in  sections  17  and  20,  T. 
50,  R.  44  W.  extend  nearly  continuously  between  the  upper  conglomerate  and 
the  red  quartziferous  porphyry  below.  The  strike  here  is  north  and  south, 
and  the  dip  some  15°  westward.  The  entire  thickness  between  the  con- 
glomerate above  and  the  quartz-porphyry  below  is  some  280  to  300  feet,  a 
considerable  reduction  on  what  is  seen  further  east.  The  junction  with  the 
upper  conglomerate  is  seen  in  section  17  (160  N.,  1,000  W.).  Between  this 
point  and  the  lower  conglomerate  there  is  a  thickness  of  only  some  75  to  100 
feet,  much  of  which  is  made  up  of  a  dark-brownish  diabase-porphyry.  The 
lower  conglomerate  is  seventy-five  feet  thick,  and  below  it  comes  a  suc- 
cession of  amygdaloids  and  fine  diabases  to  the  junction  with  the  quartz- 
porphyry  below,  which  occurs  at  1,850  paces  north  and  600  west  of  the 
southeast  corner  of  section  20. 

Southeast  of  the  exposures  on  the  upper  Carp  River  in  sections  19  and 
30  of  T.  50,  R.  42  W.,  the  belt  of  country  in  which  the  continuation  of 
these  basic  rocks  would  be  expected  is  without  exposures,  so  far  as  known, 
except  two  of  conglomerate,  which  from  their  position  and  character  may 
very  well  belong  with  the  median  conglomerate  of  the  belt  I  am  now  de- 
scribing. One  of  these  is  in  the  creek-bed  in  Sec.  32,  T.  51,  R.  42  W. 
(840  N.  50  W.).  A  thickness  of  sixty  feet  is  here  seen,  striking  northwest, 
and  dipping  northeast  20°  to  25°.  The  pebbles  are  large,  usually  from  ten 
to  one  hundred  pounds  in  weight,  and  are  composed  almost  entirely  of 
purple  quartz-porphyry  and  felsite.  The  other  point  is  in  Sec.  29,  T.  51, 
R.  42  W.  (950  N.  and  1,450  W.),  where  there  is  a  large  show  of  a  conglom- 
erate similar  to  the  last,  striking  N.  10°-15°  E.,  and  dipping  S.  E.  30°. 

The  broad  sandstone  and  conglomerate  next  succeeding  in  the  Porcu- 
pine Mountains  region,  which  I  regard  as  the  equivalent  of  the  main  or 
Great  Conglomerate  of  Keweenaw  Point,  has  not  been  seen  in  any  one 
section  across  its  width.  Its  existence  as  a  single  great  layer  is  inferred 
from  scattering  exposures  and  from  the  topography.  Its  upper  and  lower 
limits  are  well  seen  in  a  number  of  places,  but  its  middle  portions  underlie 
lowland  and  are  but  little  exposed.  The  layer  is  largely  sandstone,  the 
conglomeratic  portions  occurring  at  the  base,  near  its  union  with  the  under- 
lying amygdaloid.     The  rock  is  made  up  of  the  usual  reddish  porphyry 


218       COPPEE-BEAEING  EOCKS  OP  LAKE  SUPEEIOE, 

detritus,  often  permeated  by  secondary  calcite.  The  upper  portions  of  the 
layer  are  best  seen  in  the  face  of  the  great  cliff  which  forms  the  north  side 
of  the  valley  in  which  lie  Carp  Lake  and  Carp  River.  This  cliff  extends 
nearly  continuously  across  T.  51,  R  43  W.,  a  distance  of  over  six  miles. 
The  crown  of  the  cliff  is  from  800  to  1,000  feet  above  Lake  Superior,  and 
from  400  to  600  feet  above  the  valley  of  Carp  Lake.  •  The  base  of  the  cliff 
is  inarked  by  a  long  slope  of  fragments  fallen  from  the  diabase  and  amyg- 
daloid that  form  its  upper  portions,  but  through  the  greater  part  of  its 
length  there  is  a  perpendicular  face  of  about  400  feet  above  the  talus.  The 
base  and  lower  portion  of  the  cliff  are  composed  of  sandstone  and  conglom- 
erate, but  toward  the  east  the  sandstone  rises  nearly  to  the  top  of  the  cliff, 
while  on  the  west  side  of  the  township  it  is  found  only  at  its  base.  At  Carp 
Lake,  which  lies  at  about  400  feet  altitude  above  Lake  Superior,  the  cliff 
reaches  1,000  feet  above  the  same  level.  Here  the  sandstone  rises  to  about  400 
feet  above  Carp  Lake,  while  in  the  eastern  part  of  section  13  it  rises  almost 
to  the  top  of  the  hill.  Other  places  where  the  top  of  the  layer  may  be  seen 
are  on  the  tram-road  from  the  Nonesuch  mine  to  Lake  Superior,  in  Sec.  27, 
T.  51,  R.  42  W".,  at  1,050  paces  north  and  1,500  west,  and  again  in  the  same 
section  at  1,850  north,  1,600  west.  At  the  latter  point  a  creek  follows  the 
junction  between  the  sandstone  and  overlying  diabase  for  some  distance. 
The  lower  portions  are  exposed  on  the  upper  Carp  River,  S.  W.  ^,  Sec.  19, 
T.  51,  R.  42  W.;  on  Little  Carp  River,  S.  W.  J,  Sec.  17,  T.  50,  R.  44  W.;  on 
the  stream  in  the  N.  E.  ^,  Sec.  13,  T.  49,  R.  45  W.,  and  on  the  stream  in  the 
S.  W.  ^,  Sec.  15,  of  the  same  township. 

As  marked  on  the  map  and  sections  of  the  Porcupine  Mountains,  this 
layer  has  a  thickness  of  some  1,800  or  1,900  feet.  It  is  possible  that  in  the 
middle  of  the  thickness  there  may  be  some  interstratified  diabases  and  amyg- 
daloids,  but  the  existence  of  the  long  low  valley  of  Carp  River,  under  which 
lies  all  of  the  thickness  about  which  there  is  any  doubt,  and  the  entire  ab- 
sence of  exposures  of  any  such  rock  on  the  Carp,  Little  Carp,  and  Presqu'  Isle 
rivers,  where  they  cross  this  belt,  seem  to  forbid  such  a  supposition.  In 
the  region  of  the  Montreal  River  a  number  of  interstratified  diabase  flows 
make  their  appearance,  but  they  are  to  be  seen  on  every  crossing  river. 

Next  in  order  to  this  conglomerate  in  the  Porcupine  Mountains  comes 


OUTER  TEAP  BELT  OF  THE  PORCUPINES.         219 

the  second  belt  of  basic  rocks,  with  a  thickness  of  some  300  to  400  feet. 
The  rocks  of  this  belt  are  finely  exposed  in  the  great  cliff  which  stretches 
in  a  curving  direction  entirely  across  T.  51,  E.  43  W.,  on  the  north  side  of 
the  Carp  Lake  Valley.  As  already  explained,  to  the  eastward  these  rocks 
are  found  just  capping  the  cliff,  and  then  extending  to  a  relatively  long  dis- 
tance down  the  north  slope  of  the  mountain;  as,  for  instance,  in  Sec.  13,  T. 
61,  R.  42  W.,  where  this  belt  has  a  surface  width  of  over  half  a  mile. 
Further  west  the  junction  with  the  underlying  sandstone  is  lower  down  in  the 
face  of  the  great  cliff,  the  belt  descending  but  a  short  distance  down  the 
north  slope  of  the  mountain,  and  as  a  consequence  presenting  but  a  narrow 
spread  on  the  map.  Beyond  T.  61,  R.  43  W.,  to  the  westward,  this  belt 
pursues  a  course  at  first  southwest  and  then  more  nearly  south  through  T. 
51,  R.  44  W.;  T.  60,  R.  44  W.;  T.  60,  R.  45  W.,  and  T.  49,  R.  45  W.,  to  the 
Presqu'  Isle  River,  where  there  is  a  sharp  bend  to  the  westward,  as  indicated 
on  the  map.  The  best  exposures  are  on  the  Little  Carp,  in  Sec.  18,  T.  50, 
R.  44  W.,  and  on  the  Presqu'  Isle,  in  sections  14  and  15,  T.  49,  R.  45  W.  On 
both  streams  the  junction  with  the  overlying  sandstone  is  exposed,  and  both 
streams  make  falls  over  the  basaltic  rocks,  those  on  the  Presqu'  Isle  reaching 
a  height  of  twenty -five  feet. 

Eastward  from  T.  51,  R.  43  W.,  this  belt  continues  on  an  easterly 
course  for  some  three  miles,  and  then,  turning,  pursues  a  tortuous  southerly 
course  to  the  south  side  of  T.  51,  R.  42  W.,  whence,  turning  again  ab- 
ruptly, it  runs  westward  through  the  northern  sections  of  T.  50,  R.  43  W. 
Here  it  is  found  to  come  directly  in  contact  with  the  central  porphyry  of 
the  Porcupine  Mountains,  some  2,500  feet  of  rock  that  appears  on  the  north 
side  of  the  mountains  being  now  absent,  at  least  at  the  surface.  This 
peculiar  behavior  I  regard  as  the  result  of  a  fault,  as  indicated  on  the  sec- 
tions of  Plates  XX  and  XXI.  The  peculiar  serpentine  course  of  the  belt 
through  T.  51,  R.  42  W.  is  not  conjectural,  having  been  very  satisfacto- 
rily made  out  from  the  locations  and  inclinations  of  a  number  of  expo- 
sures. Moreover,  it  corresponds  entirely  with  the  courses  marked  out  for 
the  overlying  belts.  In  the  vicinity  of  the  Union  mine,  on  sections  27  and 
22  of  T.  51,  R.  42  W.,  both  upper  and  lower  limits  of  the  belt  are  well 
exposed. 


220       COPPEE-BEAEING  EOOKS  OF  LAKE  SUPEEIOE. 

The  beds  of  which  this  belt  is  composed  are  very  sharply  defined 
and  regular,  with  strongly  marked  vesicular  or  amygdaloidal  portions,  and 
massive,  often  semi-columnar,  lower  portions.  The  massive  rock  is  very 
fine-grained  and  commonly  of  a  chocolate-brown  to  reddish-brown  hue. 
Very  often  it  carries  mottlings,  or  pseud-amygdules,  of  dark-green  chlorite, 
epidote  and  calcite.  Nearly  all  appears  to  carry  olivine  in  a  much  altered 
condition,  and  many  of  the  sections  show  the  characteristic  arrangement  of 
relatively  large  augites  free  from  magnetite  or  olivine,  but  including  num- 
bers of  plagioclases,  while  the  interspaces  are  thickly  studded  with  mag- 
netite and  altered  olivine.  One  of  these  is  figured  on  Plate  X,  at  Fig. 
1.  The  amygdaloids  do  not  present  any  peculiar  points.  Dark-green 
chlorite,  epidote,  calcite,  laumontite  and  prehnite  appear  to  be  the  com- 
mon fillings.  At  several  points  these  amygdaloids  have  been  worked  for 
their  copper,  but  unsuccessfully. 

Next  in  order  in  the  Porcupine  section  comes  a  thickness  of  over  3,000 
feet  of  red  sandstone  with  imbedded  conglomerates.  The  sandstone,  which 
is  of  the  usual  porphyry  detritus,  very  greatly  predominates  over  the  con- 
glomerate, the  latter  being  disposed  in  the  sandstone  in  very  thin  and  ir- 
regular bands.  The  pebbles  are,  as  usual,  of  porphyry  and  felsite,  but 
pebbles  of  the  basic  rocks  are  not  excluded.  The  exposures  of  this  sand- 
stone in  the  Porcupine  country  are  too  numerous  for  description.  It  is  the 
best  exposed  belt  of  rock  in  the  region,  being  seen  in  its  entii-e  width.  It 
evidently  includes  no  belts  of  crystalline  rocks.  Iron  River  exposes  it 
largely,  especially  in  the  east  half  of  Sec.  24,  T.  51,  R.  42  W.  Little  Iron 
River  and  Union  River  make  large  exposures  in  the  same  township.  The 
whole  thickness  may  be  seen  by  following  the  coast  line  in  T.  51,  R.  42 
W.,  and  T.  51,  R.  43  W.;  and  again  from  Lone  Rock  to  the  Presqu'  Isle 
River.  Numerous  exposures  are  met  with  in  the  woods  north  of  the  Carp 
River  Valley  cliffs,  while  Carp  and  Little  Carp  Rivers,  in  T.  50,  R.  45  W., 
and  the  Presqu'  Isle,  in  Sec.  9,  T.  49,  R.  45  W.,  expose  the  entire  width. 
The  same  sandstone  is  seen  at  a  number  of  points  in  the  eastern  part  of 
T.  50,  R.  44  W.,  east  of  the  north  and  south  porphyry  ridges  of  the  cen- 
tral and  western  parts  of  that  township. 

Next  in  order  comes  the  belt  of  dark-colored  sandstone  and  shale 


OUTEE  SANDSTONES  OP  THE  POECUPINES.  221 

which  has  been  traced  ah-eady  from  the  vicinity  of  Portage  Lake  to  the 
Ontonagon  River.  As  further  east,  so  also  in  the  Porcupine  region,  this 
layer  consists  of  a  series  of  alternating  dark-gray  to  black  clayey  shales 
and  dark-gray  sandstones  ;  all  dark-colored  because  of  their  comparatively 
large  content  of  basic  detritus.  Calcite  is  very  often  found  in  the  matrix, 
and  one  phase  consists  almost  exclusively  of  basic  detritus  with  calcite 
cement.  The  total  thickness  averages  about  600  feet,  varying  somewhat 
from  this  figure.  On  the  map  of  Plate  XIX  the  belt  is  marked  as  very 
irregular  in  the  surface  width,  the  irregularity  arising  from  the  numerous 
changes  it  undergoes  in  direction  and  amount  of  inclination. 

At  the  Nonesuch  mine,  S.  E.  |,  Sec.  1,  T.  51,  R  43  W.,  the  shale  is 
seen  with  a  thickness  of  over  200  feet,  trending  N.  45°-50°  E.,  and  dip- 
ping S.  E.  28°.  Near  the  base  of  the  shale  is  the  sandstone  seam,  4  feet 
thick,  worked  at  the  Nonesuch  mine  for  its  copper.  This  rock  is  a  dark 
greenish-gray,  fine-grained  sandstone,  which  in  the  thin  section  is  seen  to 
be  composed  in  some  measure  of  basic  detritus,  along  with  porphyry 
detritus,  and  numerous  single  quartzes.  The  basic  detritus  appears  in 
the  shape  of  much  decomposed  red-  and  green-stained  particles,  showing 
both  feldspathic  and  augitic  ingredients.  A  good  deal  of  magnetite  is  pres- 
ent, and  native  copper  is  very  abundant,  for  the  most  part  clustered  around 
the  magnetite  particles.  In  a  few  places  secondary  calcite  lies  between 
the  grains.  The  thin  section  of  this  rock  is  figured  at  Fig.  2,  Plate  XVI. 
Beneath  the  copper-bearing  sandstone  there  come  a  few  feet  of  shale,  also 
carrying  some  copper,  and  beneath  this  some  two  and  a  half  feet  of  a  light- 
gray  sandstone,  harder  than  usual,  beneath  which  again  is  the  great  sand- 
stone layer  previously  described. 

The  thin  seam  just  mentioned  as  occurring  at  this  junction  has  been 
traced  for  a  number  of  miles  in  the  valley  of  Iron  River,  and  is  the  one 
which  has  attracted  so  much  attention  in  this  region  for  the  silver  it  con- 
tains. Under  the  microscope  it  is  seen  to  differ  from  the  Nonesuch  copper 
sandstone,  and,  indeed,  from  the  rock  of  the  shale  belt  generally,  in  that  it 
contains  some  water-deposited  (secondary)  quartz  between  the  grains, 
which  consist,  as  usual,  of  mingled  porphyry  and  basic  detritus  and 
quartz  particles.     Calcite  has  also  been  deposited  more  or  less  plentifully 


222       COPPER-BEAEING  EOOKS  OF  LAKE  SUPERIOR. 

in  the  interstices  of  the  grains,  and  in  some  sections  is  the  only  indurating 
matei'ial. 

From  the  Nonesuch  mine  eastward  the  shale  belt  runs  through  the 
southern  parts  of  sections  6  and  5  of  T.  50,  R.  42  W.  In  the  latter  section 
the  lower  layers,  including  the  Nonesuch  bed,  which  is  here  also  well 
charged  with  copper,  are  exposed  on  Mineral  River  at  250  paces  north 
and  450  west  of  the  southeast  corner,  with  a  southerly  dip  of  40°.  From 
here  the  course  of  the  belt  makes  a  wide  bow  through  sections  4  and  3,  T. 

50,  R.  42  W.,  and  sections  HI  and  30  of  T.  50,  R.  41  W.,  to  Mineral  River,  in 
Sec.  25,  T.  51,  R.  42  W.,  where  it  is  seen  exposed  (1,200  N.,  200  W.),  with 
a  northeasterly  dip.  In  the  same  section,  on  Iron  River  (1,750  N.,800  W.), 
the  junction  with  the  underlying  sandstone  is  again  seen,  and  both  rocks 
make  here  a  sharp  turn  from  a  northwesterly  to  a  northeasterly  course,  the 
dip  at  the  same  time  being  changed  from  a  northeasterly  direction  to  a 
southeasterly  one. 

The  course  of  the  belt  is  next  on  a  bow  to  the  eastward  through  Sec. 
1 9,  T.  51,  R.  41  W.,  returning  to  Iron  River  in  the  S.  E.  4,  Sec.  13,  T.  51,  R. 
42  W.  Here  Iron  River  crosses  the  whole  width  of  the  belt,  exposing  both 
the  underlying  and  overlying  sandstones.  The  dip  here  is  35°  east  of  north, 
and  the  thickness  of  the  belt  600  feet.  From  here  the  course  is  westward 
to  Little  Iron  River,  in  the  S.  W.  ^,  Sec.  1 3.  At  this  place  there  is  a  sharp 
synclinal,  the  belt  turning  abruptly  and  showing  southerly  dips.  Return- 
ing now  eastward  to  Iron  River,  the  junction  with  the  overlying  sandstone 
is  found  in  the  northeast  quarter  of  the  same  section  (1,150  N.,  250  W.). 
Thence  to  its  mouth,  Iron  River  is  constantly  on  the  layers  of  this  belt,  an 
anticlinal  occuri'ing  near  the  N.  E.  ^  of  Sec.  13.  The  turns  of  the  shale  and 
of  its  overlying  and  underlying  sandstones  will  be  best  understood  by  an 
inspection  of  the  sections  of  Plates  XX  and  XXI,  and  of  the  strike  and  dip 
notes  of  Plate  XIX.  At  the  mouth  of  Iron  River  the  shale  belt  passes  into 
the  lake  with  a  northwesterly  trend,  the  junction  with  the  overlying  sand- 
stone appearing  just  at  the  mouth  of  Iron  River,  and  with  the  underlying 
on  the  lake  shore  on  the  west  side  of  the  N.  E.  ^  of  the  N.W.  4,  Sec.  12,  T. 

51,  R.  42  W.  In  all  of  these  exposures  between  the  Nonesuch  mine  and  the 
mouth  of  Iron  River  the  subordinate  bedding  of  the  shale  stratum  remains 


KONESUCH  SHALE  BELT  IF  THE  POECUPINES.       223 

very  much  the  same  as  at  the  former  place,  viz :  a  main  body  of  dark-gray 
shale,  varying  a  good  deal  in  fineness  in  different  layers,  and  occasionally 
becoming  a  sandstone ;  near  the  base  a  bed  of  sandstone,  recognizable  as  the 
"Nonesuch  vein,"  and  below  this  always  the  light-gray  calcitic  sandstone 
known  as  the  "silver  belt." 

On  the  lake  shore,  just  southwest  of  Lone  Rock,  in  the  northern  part 
of  Sec.  26,  T.  51,  R.  44  W.,  the  shaly  belt  appears  again,  with  the  usual 
characters  and  a  considerable  thickness.  It  is  terminated  eastward  by  a 
heavy  dike  of  olivine-diabase,  on  the  east  side  of  which  the  sandstone  is  filled 
with  calcite,  the  calcite  being  a  vein  following  the  course  of  the  dike,  which 
is  N.  40°  W.  Thin  sections  of  the  shale  and  sandstone  exposed  here  show 
them  to  be  just  the  same  as  those  exposed  to  the  eastward.  West  of  this 
point  the  shale  belt  remains  under  the  lake  as  far  as  the  mouth  of  Presqu'  Isle 
River.  Here  it  reappears,  at  first  trending  only  a  little  west  of  north,  with 
a  dip  slightly  south  of  west,  but  changing  rapidly  as  the  river  is  ascended 
to  a  more  nearly  westerly  direction,  with  a  flat  southern  dip,  the  amount 
lessening  to  5°  in  the  southern  part  of  Sec.  30,  T.  50,  R.  45  W.,  between 
which  point  and  the  lake  the  river  makes  a  series  of  falls  over  the  shale  and 
interstratified  sandstone. 

From  here  the  course  of  the  belt  is  east  and  south,  the  shale  and  its 
underlying  sandstone  appearing  in  large  exposures  along  a  small  creek  in 
Sec.  34,  T.  50,  R.  45  W.,  with  a  N.  10°  W.  strike  and  a  westward  dip  of 
10°.  Hence  the  course  continues  southerly  for  about  a  mile,  when  an 
abrupt  turn  westward  is  made,  the  belt  crossing  the  Presqu'  Isle  in  the 
southern  part  of  Sec.  4,  T.  49,  R.  45  W.  The  exposures  are  nearly  con- 
tinuous along  the  river  during  its  course  through  this  section.  At  the  line 
between  sections  4  and  5  the  overlying  sandstone  is  horizontal — being  at  the 
bottom  of  the  synclinal  here  crossed  by  the  Presqu'  Isle.  A  little  further  up 
stream  a  perceptible  northerly  dip  begins,  and  at  the  junction  with  the 
shale,  somewhat  north  of  the  middle  line  of  the  section,  this  has  increased 
to  7°,  with  a  due  east  and  west  strike  continuing.  Further  up  stream  the 
dip  steadily  increases,  and  at  the  section  hne,  where  is  the  junction  with 
the  underlying  sandstone,  is  as  much  as  20° 

Returning  now  to  the  Nonesuch  location,  the  shale  belt  may  be  traced 


224  COPPEE-BEAEIISrG  EOCKS  OF  LAKE  SUPEEIOE. 

by  large  exposures  westward  through  sections  12  and  11  of  T.  50,  R  42  "W". 
Further  west,  in  the  N.  E.  ^  of  Sec.  8,  it  is  found  trending  still  more  to  the 
south  of  west.  South  of  here,  in  this  township,  the  shale  has  not  been 
observed,  but  a  series  of  observations  on  the  underlying  sandstone,  as 
recorded  on  Plate  XIX,  serve  to  fix  the  course  of  the  belt  very  accurately. 
The  uppermost  rock  in  the  Porcupine  region  is  the  main  mass  of  sand- 
stone of  the  Upper  Division  of  the  series.  It  is  seen  at  the  mouth  of  Iron 
River,  overlying  the  shale;  again,  in  a  similar  position,  on  Iron  River  in  the 
S.  E.  ^  of  Sec.  13,  T.  51,  R.  41  W. ;  and  at  several  points  in  the  low  ground 
south  of  the  Porcupines — for  instance,  on  the  upper  part  of  Mineral  River, 
near  the  south  line  of  Sec.  ^,  T.  50,  R.  42  W.,  and  on  the  upper  Iron  River, 
near  the  south  line  of  section  18,  in  the  same  township.  On  the  west  side 
of  the  Porcupine  Mountains  the  same  rock  appears  on  the  Presqu'  Isle  in 
Sec.  32,  T.  50,  R.  45  W.,  and  Sec.  4,  T.  40,  R.  45  W. 

In  tracing  the  several  rock  belts  which  overlie  the  central  porphyry  in 
the  Porcupine  region  east  to  the  Ontonagon  and  west  to  the  Montreal,  I 
have  to  depend  upon  the  geological  notes  of  the  township  surveyors,  upon 
the  few  facts  given  by  Foster  and  Whitney,  and  upon  the  data  communi- 
cated to  me  by  Mr.  B.  N.  White,  of  Ontonagon,  who  has  a  wide  familiarity 
with  the  Ontonagon  region.  Moving  eastward,  then  from  the  Porcupine 
area,  I  note  first  an  exposure  of  the  black  shale  on  the  headwaters  of  Min- 
eral River,  in  the  center  of  the  N.  E.  J  of  Sec.  34,  T.  50,  R.  43  W.,  where  the 
dip  is  40°  to  the  north.  Eastward  from  here  the  course  of  the  belts  makes 
a  good  deal  of  northing,  the  "trap"  ledges  marked  on  the  township  plats  as 
occurring  near  the  N.  E.  corner  of  Sec.  5,  T.  49,  R.  42  W.,  at  the  falls  near 
the  N.  W.  corner  of  Sec.  34,  T.  50,  R.  42  W.,  and  again  near  the  middle  of 
the  north  line  of  the  same  section,  marking  the  course  of  the  upper  trap 
band. 

In  T.  50,  R.  41  W.,  Mr.  White  finds  the  Nonesuch  shale  as  far  north  as 
the  center  of  section  14.  The  same  explorer  reports  red  felsitic  porphyry  at 
the  center  of  section  21;  so  that  we  have  here  a  width  of  just  one  mile  for  all 
the  belts  between  the  porphyry  and  the  black  shale.  This  narrowing  may 
be  due  in  some  measure  to  an  increase  in  the  dip  angle,  but  must  be  chiefly 
caused  by  the  thinning  of  the  two  intermediate  conglomerates  to  the  com- 


OPPER-BEARINr 


Thn  Wtntfrn  Horiiontal  S 


The  KsBiem  Honiontal  Sandsionr 


KEWEENAW       SERIES 
g  I       Tiif  Mam  BoAy  of  StuidGtom 


(,.      H!,iik  -S)iale5 


Tltr  Oulrr   ron^omerate 


SnitilMonc  and  fonilptiineralp 


Cuorfl^'-grauu'd  (JBbliro 


FpIbiIp   onA  Qu«rlz    porjiliji 


LL 


HL-RONIAN 
Mica-ftchUls,  Slule».M«gti«tti.*  racks  vtcl 


LAlJRENTI.«s' 
Homblimdc    ^nrisB,  Mica-  ^ni 


Scale    mSlin  or  linrJc  473S  nvilvs 


(ii;()L()(.i(  Ai,    MAP   di'    THI-;    Hi:(;i()x    i;i-:iavi;i;n   tiiI':    ontonacon   i;i\i:i.'   miciiioax  and   nm'mak.\(H)n  i.akk  Wisconsin 


EAST  AND  WEST  CONTINUATIONS  OF  THE  POECUPINE  BELTS.   225 

paratively  small  thickness  which  they  are  observed  to  possess  east  of  the 
Ontonagon.  In  T.  50,  R.  40  W.,  the  exposures  indicate  the  continuance  of 
the  same  conditions.  The  shale  shows  in  the  N.  W.  J  of  the  S.  W.  J  of  Sec. 
18,  again  in  the  the  N.  W.  J  of  the  N.  E.  ^  of  Sec.  15,  and  again  near  the 
south  quarter-post  of  section  11.  These  exposures,  with  those  already  noted 
on  the  other  side  of  the  Ontonagon,  in  Sec.  7,  T.  50,  R.  39  W.,  serve  to  fix 
the  course  of  the  shale  belt  across  this  township  at  a  little  north  of  east  for 
the  first  two  miles,  then  about' N.  15°  E.  for  two  and  a  half  miles,  and  then 
N.  30°  E.  to  the  Ontonagon  River.  "Red  slaty  trap  and  conglomerate"  are 
marked  on  the  township  plat  as  occurring  within  a  mile  of  the  shale,  and  the 
succession  is  evidently  the  same  as  already  noted  to  the  east  of  the  Ontona- 
gon, viz:  black  shale,  sandstone  and  conglomerate,  traps,  conglomerate, 
traps,  porphyry.  North  of  the  black  shale  the  main  body  of  sandstone  is 
everywhere  the  surface  rock  to  the  lake  shore. 

Westward  from  the  Porcupine  Mountains  the  upper  belts  are  more 
easily  traced.     Following  the  coast  line  westward  from  the  mouth  of  the 
Presqu'  Isle,  the  main  sandstone  of  the  Upper  Division  forms  the  coast  as 
far  as  the  west  line  of  T.  49,  R.  46  W.,  just  beyond  which  the  black  shale, 
which  may  also  be  seen  a  short  distance  up  Black  River  from  the  mouth, 
appears  at  the  coast  for  a  short  time,  soon  retreating  from  it  again.     From 
this  point  the  main  body  of  sandstone  forms  the  coast  continuously  to  the 
Montreal  River.     In  this  distance  the  layers  trend  more  to  the  south  than 
the  coast  line,  which  thus  is  constantly  reaching  a  higher  horizon.     At  the 
same  time  the  dip  angle  rapidly  steepens,  being  45°  about  midway  in  range 
47,  and  nearly  90°  at  the  mouth  of  the  Montreal  River.     It  thus  results 
that  on  the  Montreal  as  much  as  10,000  feet  of  the  main  sandstone  is  crossed 
by  the  lower  reaches  of  the  river,  for  some  ten  miles  to  the  east  of  which  stream 
it  often  forms  bold  clifi"s  forty  to  seventy  feet  high.     On  Black  River  there 
are  falls  in  the  N.  W.  i  of  Sec.  15,  T.  49,  R.  46  W.,  over  conglomerate  and 
sandstone,  which  belong  with  the  broad  conglomerate  beneath  the  shale. 
The  conglomerate  at  the  falls  near  the  S.  W.  i  of  the  S  W.  J  of  the  same 
section  belongs  with  the  Carp  Lake  band,  and  the  "trap"  at  the  falls  just 
above  in  the  N.  E.  |  of  Sec.  21  with  the  underlying  diabases  of  the  band  fol- 
lowing just  above  the  porphyry  of  the  Porcupine  region.     The  "  trap-rock  " 

15  L  s 


226       COPPEE-BEAEING  EOCKS  OP  LAKE  SUPEEIOE. 

of  the  old  Chippewa  location,  S.  W.  ^,  Sec.  22  of  the  same  township,  evi- 
dently belongs  with  the  main  mass  underlying  the  porphyry. 

When  the  Montreal  is  reached,  instead  of  finding  one  broad  conglom- 
erate between  the  outer  or  Carp  Lake  trap,  and  the  belt  next  the  por- 
phyry, we  meet  with  a  succession  of  alternations  of  sandstone  or  red  shale 
and  diabase.  It  has  already  been  indicated  that  even  in  the  Porcupine 
Mountains  the  inner  conglomerate  may  hold  an  interstratified  diabase  band, 
and  a  few  miles  west  of  the  Presqu'  Isle  there  certainly  is  one.  Further 
west  the  sandstone  and  beds  of  basic  crystalline  rocks  must  dovetail  into 
one  another  in  some  such  manner  as  indicated  on  the  map  of  Plate  XXII. 
Exposures  belonging  to  these  trap  bands  are  met  with  on  the  west  line  of 
the  N.  W.  J  of  Sec.  34,  T.  49,  R.  47  W.;  on  the  west  hne  of  the  S.  W.  i  of 
Sec.  31  of  the  same  township;  and  on  the  south  line  of  Sec.  34  of  T.  49,  R. 
48  W.  Further  back  from  the  lake  shore  there  are  other  exposures  belong- 
ing to  the  Main  Trap  Range,  as  far  south  as  and  nearly  in  contact  with  the 
underlying  iron-bearing  slates. 

The  section  displayed  on  the  Montreal  River  is  next  to  be  described. 
Ascending  the  Montreal  River  from  the  mouth,  vertically  placed  ledges  of 
red  sandstone  and  red  sandy  shale  belonging  to  the  main  body  of  the  Upper 
Division,  are  the  only  rocks  seen  up  to  the  S.  E.  J  of  the  N.  E.  ^  of  Sec.  20, 
T.  47,  R.  1.  E.  (Wisconsin),  two  and  a  half  miles  above  the  mouth.  Here 
begin  exposures  of  the  black  shale  and  its  interstratified  sandstones.  The 
shale  of  these  alternations  is  dark-purple  to  black,  very  soft  and  clayey, 
and  quite  regularly  laminated.  The  shale  layers  run  from  ten  to  fifty  feet 
or  more  in  thickness,  and  are  quite  subordinate  in  abundance  and  thickness 
to  the  associated  sandstones  into  which  they  grade.  The  sandstone  is  dark- 
gray  to  brown,  very  close-grained  and  compact,  and  often  appears  macro- 
scopically  like  a  fine-grained,  crystalline  rock.  This  rock  forms  massive 
ledges  in  the  bed  and  on  the  sides  of  the  stream  Under  the  microscope  it 
appears  much  the  same  as  the  sandstones  at  the  same  horizon  in  the  Porcu- 
pine and  Ontonagon  regions,  but  some  sections  show  an  unusually  small 
amount  of  porphyry  detritus,  being  almost  wholly  made  up  of  basic  frag- 
ments, with  the  calcite  cement. 


THE  MONTREAL  RIVEE  SECTION.  227 

After  crossing  a  thickness  of  these  beds  of  some  350  to  400  feet — a 
much  smaller  thickness  than  was  noted  farther  east — a  broad  belt  of  coarse 
conglomerate  is  crossed.  The  constituent  bowlders  are  largely  of  the  sev- 
eral kinds  of  acid  porphyries,  but  there  are  many  of  the  basic  igneous  rocks 
of  the  system.  Much  calcite  is  present  between  the  bowlders  and  sand 
grains,  and  also  occuirs  in  veins  of  some  size.  The  vertical  bedding  is  well 
brought  out  by  the  few  intercalcated  sandstone  seams.  This  conglomerate 
is  exposed  in  cliffs  several  hundred  feet  high,  forming  the  walls  of  the  nar- 
row tortuous  gorge  through  which  the  Montreal  River  passes  at  the  middle 
of  Sec.  20,  T.  47,  R  1  E.  It  is  plainly  enough  the  equivalent  of  the  outer 
sandstone  and  conglomerate  of  the  Porcupine  Mountains.  In  the  latter  dis- 
trict this  stratum  has  a  thickness  of  some  3,000  feet,  but  at  the  Montreal  it 
is  not  more  than  1,200  feet. 

Next  in  descending  order  on  the  Montreal  is  met  an  alternating  series 
of  thin  and  very  regular  beds  of  fine-grained  diabase  and  red  sandstone  and 
shale.  The  beds  of  the  crystalline  rocks  are  sharply  defined  from  each 
other  by  very  strongly  developed  amygdaloids  or  vesicular  portions.  The 
lower  portions  of  the  diabase  flows  are  characterized  by  a  very  distinct 
columnar  structure  at  right  angles  to  the  bedding.  The  finest  exhibition  of 
both  amygdaloids  and  columnar  structure  is  to  be  seen  at  the  head  of  the 
upper  falls,  where  at  low  water  there  is  a  very  large  surface  of  bare  rock. 
The  following  tabulation,  copied  from  my  report  on  the  Greology  of  the 
Eastern  Lake  Superior  District,^  serves  to  show  the  main  facts  with  regard 
to  these  alternations.  It  should  be  said  that  detailed  microscopic  examina- 
tions would  possibly  prove  some  of  the  layers  to  be  melaphyrs : 

Thickness  in  feet. 

I.  Diabase 20 

II.  Red  shaly  sandstone 10 

III,  Diabase 4 

.IV.  Sandstone  and  shale,  including  the  following  subdivisions:  (1)  thin 
laminated  red  shale,  4  feet;  (2)  purplish, lumpy,  fine-grained  sand- 
stone, 8  feet ;  (3)  like  (1),  2^  feet;  (4)  like  (2),  6  feet ;  (5)  like  (1), 
very  bright  red,  IJ  feet;  (6)  like  (2),  5  feet;  (7)  bright  red  clay 
shale,  14  feet;  total 41 

>  Geology  of  Wisconsin,  Vol.  Ill,  p.  191. 


228  COPPEE-BEAEING  EOCKS  OF  LAKfE  SUPEEIOE. 

Thickness  in  feet. 

Y.  Diabase,  iacluding  the  followiug  subdivisions :  (1)  amygdaloid,  with 
abundant  amygdules  of  prehnite,  laumontite,  calcite  and  chlorite, 
and  with  seams  of  laumontite  and  large  patches  of  calcite,  in  the 
more  thoroughly  altered  portions,  having  at  base  a  heavy  laumon- 
tite and  calcite  seam  carrying  copper,  on  which  some  mining  has 
been  done,  25  feet;  (2)  pseud-amygdaloid,  5  feet;  (3)  compact 
portion  with  distinct  columnar  structure,  the  rock  having  on  fresh 
fracture   a  grayish  color   and  distinctly  crystalline  appearance, 

though  very  fine  grained,  70  feet ;  in  all 100 

VI.  Covered 185 

VII.  Diabase,  with  subdivisions  as  in  V 92 

VIII.  Eed  sandstone  and  shale 20 

IX.  Diabase,  with  subdivisions  as  in  V;  compact  portion  dark  chocolate- 
brown 50 

X.  Eeddish  conglomeratic  sandstone 60 

XI.  Diabase,  including  :  amygdaloid,  10  feet;  compact  portion,  20  feet ;  in  all        30 
XII.  Dtaftase,  including  :  amygdaloid,  5  feet;  compact  portion,  20  feet;  in  all        25 

XIII.  Diabase,  with  usual  subdivisions 10 

XIV.  Conglomerate 33 

XV.  Eed  sandstone  and  shale 60 

XVI.  Diabase,  with  usual  subdivisions 60 

XVII.  Eed  sandstone  and  shale 20 

XVIII.  Compact,  dark   greenish-gray  diabase,  with  amygdaloid  at  top,  and 

showing  a  tendencj'^  to  a  cross-columnar  structure 21 

XIX.  Diabase,  including:  (1)  amygdaloid,  in  many  places  showing  a  tendency 
to  cross-columnar  structure,  some  bands  almost  completely  made 
of  amygdules,  and  others  with  but  few,  15  feet ;  (2)  compact  por- 
tion, highly  columnar,  8  feet ;  in  all 23 

XX.  Diabase,  including :  (1)  amygdaloid  in  distinct  bands,  as  in  XIX,  some 
of  the  bauds  showing  a  change  to  laumontite  and  calcite,  the  amyg- 
dules, in  order  ot  abundance,  being  prehnite,  pink  orthoclase, 
orthoclase  and  calcite,  orthoclase,  prehnite  and  calcite,  11  feet; 
(2)  comjjact  portion,  with  columnar  structure,  10  feet ;  in  all  ... .  21 

XXI.  Diabase,  including:  (1)  amygdaloid,  in  many  places  altered  into  laumon- 
tite seams,  2  feet ;  (2)  compact  portion,  30  feet ;  in  all 32 

XXII.  Eed  clay  s/i«ie ■- 5 

XXIII.  Diabase,  without  amygdaloid 10 

XXIV.  I>m6ase,  including :  amygdaloid,  mostly  covered,  15  feet;  compact  por- 

tion, 40  feet ;  in  all 55 

XXV.  Covered 185 

XXVI.  Eed  sMZe 40 


Total 1,212 

These  layers  are  exposed  along  the  stream  as  far  as  the  crossing  of  the 
Lac  Flambeau  trail,  near  the  east  line  of  Sec.  21,  T.  47,  R.  1  E.      They 


THE  MONTREAL  RIVER  SECTION.  229 

undoubtedly  include  the  equivalents  of  the  outer  trap,  the  Carp  Lake 
conglomerate  and  sandstone,  and  more  or  less  of  the  inner  trap,  with  its 
conglomerate,  of  the  Porcupine  Mountains,  but  with  a  greatly  decreased 
total  thickness,  which  effect  is  wholly  produced  by  the  thinning  of  the 
sandstone  and  conglomerate.  On  the  Montreal  River  the  included  detrital 
beds  are  nine  in  number,  with  a  total  thickness  of  290  feet,  as  against  some 
1,900  or  2,000  in  two  (or  three?)  layers  in  the  Porcupines.  The  diabase 
flows,  however,  evidently  increase  in  number  and  total  thickness  between 
the  Porcupine  Mountains  and  the  Montreal  River. 

Through  the  eastern  part  of  T.  47,  R.  1  E.,  and  southern  part  of  T. 
47,  R.  2  E.,  the  Montreal  is  unfortunately  without  exposure.  In  this  in- 
terval one  would  look  for  the  extension  of  the  Porcupine  Mountains  por- 
phyry, and  such  a  rock  has  been  seen  west  of  the  river  in  the  N.  W.  part  of 
T.  46,  R.  1  E.  In  this  interval  must  also  lie  a  lai'ge  projDortion  of  the  beds 
which  make  up  the  Main  Trap  Range,  further  east. 

On  the  Upper  Montreal,  and  its  tributary,  the  Grogogashugun,  in  T.  46, 
R.  2  E.,  thei'e  are  again  quite  large  exposures,  continuing  with  but  little 
break  to  the  junction  with  the  underlying  Htironian  schists.  These  layers 
must  in  lai'ge  measure  correspond  to  those  of  the  Soixth  Range  of  the 
region  east  of  the  Monti-eal,  and  in  a  measure  to  those  which  in  the  Ontona- 
gon region  and  farther  east  are  buried  beneath  the  Eastern  Sandstone. 
One  of  the  northernmost  of  these  beds  is  a  narrow  one  of  red  felsite-por- 
phyry,  exposed  on  the  Gogogashugun,  near  the  south  line  of  section  8. 
Immediately  beneath  it  comes  a  succession  of  beds,  many  of  which  are 
composed  of  a  black  conchoidal-fracturing  aphanitic  rock  of  the  ashbed 
type,  while  still  further  south  to  the  junction  with  the  Htironian  a  peculiar 
dark-greenish  diabase  is  the  prevailing  rock.  The  former  of  these  two 
kinds  of  rock  may  be  seen  largely  exposed  along  the  Gogogashugun  in  its 
passage  through  Sec.  8,  T.  46,  R.  2  E.,  along  the  Montreal  in  sections  2  and 
11  of  the  same  township,  and  in  a  number  of  large  ledges -between  the  two 
rivers.  The  green  diabases  show  on  the  Flambeau  trail,  in  section  19,  on  the 
Gogogashugun,  in  section  16,  and  very  largely  at  the  falls  on  the  Montreal, 
in  sections  11  and  14  of  T.  46,  R.  2  E.  These  greenish  to  greenish-gray 
diabases  are  quite  peculiar,  and  resemble  closely  rocks  seen  in  the  South 


230       COPPER-BEAEINa  EOCKS  OP  LAKE  SUPERIOE. 

Range  of  the  Ontonagon  country,  at  what  must  be  nearly  the  same  horizon. 
They  are  furnished  with  imperfectly  developed  amygdaloids,  quite  char- 
acteristic of  which  are  elongated  (spike)  amygdules,  composed  of  quartz, 
epidote  and  chlorite  in  combination.  The  pronounced  green  color  is  due 
to  the  large  amount  of  chlorite  which  is  present,  both  as  an  alteration-pro- 
duct of  the  feldspars  and  in  large  green  pseud-amygdules.  A  section  of 
one  of  these  rocks  is  represented  at  Fig.  3,  Plate  VIII. 

The  entire  horizontal  width  of  the  Keweenaw  Series  on  the  Montreal 
is  some  50,000  feet.  Allowing  for  supposed  flatter  dips  in  the  lower  por- 
tions, we  may  estimate  the  thickness  at  33,000  to  35,000  feet  for  the  Lower 
Division,  and  about  12,000  for  the  Upper  Division.  As  already  indicated, 
this  estimate  for  the  Lower  Division  will  be  too  great  if  the  influence  of  the 
Keweenaw  fault  should  extend  so  far  as  this,  but  it  does  not  seem  that  the 
discrepancy  can  be  more  than  two  or  three  thousand  feet  at  the  outside. 

In  the  vicinity  of  the  Potato  River,  which  crosses  the  series  some  ten 
miles  west  of  the  Montreal,  along  the  strike,  several  important  changes  are 
to  be  noted.  In  the  first  place,  the  lower  strata  of  the  Upper  Division  and 
the  upper  strata  of  the  Lower  Division  are  greatly  thinned.  The  black  shale, 
with  its  accompanying  sandstone,  has  thinned  from  400  to  250  feet,  and  its 
underlying  conglomerate  from  1,200  to  800  feet,  while  all  that  is  left  of  the 
290  feet  of  sandstone  and  shale,  which  on  the  Montreal  River  are  inter- 
stratified  with  the  uppermost  diabases,  is  one  little  seam  of  shale  ten  feet  in 
width.  This  thinning  of  the  sandstones  is  of  course  simply  a  continuation 
of  the  thinning  process  already  described  as  obtaining  between  the  Porcu- 
pine Mountains  and  Montreal  River. 

Now,  however,  a  thinning  of  the  crystalline  members  also  has  begun, 
the  entire  width  of  the  Lower  Division  in  the  vicinity  of  the  Potato  being  only 
some  six  and  a  half  miles,  as  compared  with  seven  and  a  half  miles  on  the 
Montreal,  and  with  the  same  high  dips.  But  a  yet  more  remarkable  change 
is  the  apparent  substitution  of  more  or  less  coarsely  crystalline  gray  to  black 
gabbros  for  all  the  fine-grained  diabases  of  the  lower  10,000  to  12,000  feet  of 
the  Montreal  section.  This  is  a  change  which  first  begins  on  the  Gogogashu- 
gun  River,  where  a  few  ledges  of  the  coarse  gabbro  are  seen  between  broad 
belts  of  the  finer  kinds.     On  the  Flambeau  trail,  in  the  western  part  of  T. 


THE  POTATO  EIVEE  SECTION.  231 

46,  R.  2  E.,  the  coarse  rocks  appear  more  plentifully,  and  by  the  time  the 
Potato  River  is  reached  the  finer  rocks  have  nearly  or  quite  disappeared. 
These  coarse  rocks,  as  shown  elsewhere,  are  much  like  the  coarse  gabbros 
of  Duluth  and  the  Saint  Louis  and  Cloquet  rivers,  which  they  resemble, 
moreover,  in  being  cut  by  masses  of  brick -red  granitic  porphyry,  a  large 
exposure  of  which  rock  is  to  be  met  with,  for  instance,  on  the  old  Ironton 
trail  in  the  northern  part  of  Sec.  3,  T.  45,  R.  1  W. 

For  the  rest,  the  Potato  section  is  chiefly  made  up  of  ordinary  types 
of  diabase  and  melaphyr.  At  least  two  bands  of  felsite  and  quartziferous 
porphyry  are  included.  One  of  these,  in  the  southern  part  of  T.  46,  R.  1 
W.,  is  evidently  the  same  as  that  noted  on  the  Gogogashugun,  Sec.  5,  T.  46, 
R.  2  E.,  while  the  other  much  broader  band,  which  is  exposed  on  the  river  in 
sections  14  and  15  of  T.  46,  R.  1  W.,  appears  to  belong  with  the  porphyry 
of  the  Porcupine  Mountains.  The  rock  of  the  latter  belt  is  largely  a  true 
quartziferous  porphyry,  with  a  hlac-tinted  matrix,  in  which  are  thickly  scat- 
tered minute  black  quartzes,  one-twentieth  inch  in  diameter,  and  whitish 
kaolinized  feldspars,  one-tenth  to  two-tenths  inch  across.  Faint  white 
lines  are  occasionally  seen,  and  the  whole  aspect  of  the  rock  is  very  much 
that  of  the  rock  at  the  Great  Palisades  on  the  Minnesota  coast. 

Ten  miles  farther  southwest,  along  the  strike.  Bad  River  crosses  the 
series,  and  here  the  changes  are  carried  to  a  yet  greater  extreme.  The 
black  shale  and  underlying  conglomerate  have  thinned  respectively  to  125 
and  350  feet,  while  the  entire  width  of  the  Lower  Division  to  the  imderlying 
Huronian  is  only  some  17,000  feet,  of  which  12,000  feet  are  taken  up  by 
the  coarse  gabbro.  The  remaining  fj,000  feet  are  made  up  chiefly  of  the 
typical  fine-grained  diabase  and  diabase-amygdaloid;  but  two  beds  of 
quartzose  porphyry  are  included,  the  lower  one  of  which  is  evidently  the 
same  as  the  broad  belt  of  the  Potato  River  section.  One  porphyry-con- 
glomerate has  been  noted  in  this  thickness. 

Beyond  Bad  River  the  general  trend  of  the  formation  changes  abruptly 
to  a  westerly  direction,  but  otherwise  the  conditions  remain  much  as  ob- 
served on  that  stream,  as  far  as  the  Brunschweiler  River,  save  that  the  gab- 
bro below  expands  to  a  great  width.  In  the  townships  between  Bad  and 
Brunschweiler  rivers  the   gabbro  makes  such  frequent  exposures — those 


232       COPPER-BEAEING  EOCKS  OF  LAKE  SUPERIOE. 

on  the  line  of  the  latter  stream  being  almost  continuous  from  the  head  of 
Bladder -Lake  to  the- contact  with  the  overlying  fine-grained  diabases — that 
there  can  be  no  doubt  as  to  its  occupying  the  whole  of  the  width  indicated. 
It  is  in  this  vicinity  that  the  coarse  gabbro  appears  to  occupy  a  position  of 
unconformity  to  the  underlying  Huronian  slates.^ 

A  few  miles  west  of  the  Brunschweiler  the  coarse  gabbro  exposures 
begin  to  have  scattered  among  them  others  of  the  usual  fine-grained  dia- 
bases, and  beyond  range  6  west  the  gabbro  is  no  longer  met  with.  It.  thus 
terminates  to  the  westward  much  as  it  does  to  the  eastward. 

The  entire  length  of  the  belt  occupied  by  these  coarse  gabbros  is  some 
40  miles,  its  width  ranging  from  1-J  miles  to  4 J  miles.  Three  principal 
phases  of  the  rock  occur,  viz:  orthoclase-free  gabbro,  orthoclase-gabbro 
and  hornblende-gabbro.  The  first  of  these  is  bluish-gray  to  black,  and 
ranges  from  below  medium-grained  to  very  coarse-grained,  the  crystals 
reaching  several  inches  in  length.  The  usual  constituents  are  a  very  basic 
feldspar — which,  judging  from  the  angles,  is  commonly  near  anorthite — dial- 
lagic  augite  or  true  diallage,  titaniferous  magnetite,  and  olivine.  There 
are  sundry  alteration-products  often  present,  but  the  rock  on  the  whole  is  a 
very  fresh  one.  The  second  variety  of  gabbro  is  found  especially  in  the 
more  northern  portions  of  the  gabbro  belt,  forming  apparently  continuous 
bands,  which  have  in  some  cases  been  traced  for  a  number  of  miles.  It 
is  a  red-  and  black-mottled,  or  red-,  black-  and  gray-mottled  rock,  and  com- 
monly quite  coarse-grained,  though  never  reaching  the  extreme  degree  of 
coarseness  sometimes  shown  by  the  orthoclase-gabbro  of  the  Saint  Louis. 
It  is  also  marked  by  very  abundant  and  noticeably  large  grains  of  titan- 
iferous magnetite.  Oligoclase  and  orthoclase,  both  much  altered  and  red- 
stained,  diallage  commonly  largelj^  altered  to  greenish  uralite,  and  beyond 
this  to  chlorite,  titaniferous  magnetite  and  very  abundant  apatite  are  the 
ingredients. 

Still  a  third  variety,  forming  a  belt  or  belts  some  15  miles  in  length, 
near  the  junction  with  the  Huronian,  belongs  to  what  I  have  called  horn- 
blende-gabbro in  Chapter  III.  This  rock  is  peculiar  in  containing  much 
deep-brown  intensely  dichroic  hornblende,  which,  however,  I  think  can  be 

1  See  pp.  144,  156. 


UPPER  SANDSTONES  IN  THE  BAD  RIVEE  COUNTRY.  233 

satisfactorily  proved  to  be  secondary  to  the  augitic  constituent.  Macroscop- 
ically  it  is  medium-grained,  and  from  black-  and  white-mottled  to  nearly 
black  in  color.  The  constituents  are  labradorite,  augite,  hornblende,  titan - 
iferous  magnetite,  apatite,  uralite  and  diallage ;  with  a  little  biotite,  chlorite 
and  quartz. 

In  the  townships  west  from  Bad  River,  within  the  gabbro  belt,  low 
exposures  of  a  coarse,  pinkish  granite  are  often  met  with,  and  precisely  sim- 
ilar exposures  are  found  in  the  area  occupied  by  the  upper  mica-schists 
of  the  Huronian.  This  granite  is  intrusive,  cutting  both  the  gabbro  and 
the  upper  Huronian  schists.  It  is  a  true  biotite-granite,  consisting  of  ortho- 
clase,  oligoclase,  quartz  very  rich  in  large  bubble-bearing  cavities,  and 
rather  rare  biotite. 

Thus  far,  for  the  region  west  of  the  Montreal  River,  attention  has  been 
directed  to  the  Lower  Division  of  the  Keweenaw  Series,  and  to  the  lower- 
most members  of  its  Upper  Division.  These  lower  rocks  form  the  mass  of 
a  highland  or  range  which,  on  the  Montreal,  is  only  some  two  miles  from 
the  lake  shore,  while  on  Bad  River  it  lies  twenty  miles  back  of  the  coast, 
leaving  in  front  of  it  a  broad  lowland,  underneath  which  lies  the  main  mass 
of  sandstone  of  the  Upper  Division.  On  the  Montreal  most  of  the  thickness 
of  this  upper  sandstone  is  in  sight,  and  on  the  Potato,  Bad  and  Brun- 
schweiler  rivers  some  hundreds  of  feet  of  its  lowest  portions  are  to  be 
seen.  On  the  Montreal,  Potato  and  Bad  rivers  it  stands  vertically  or 
nearly  so,  with  a  slight  inclination  to  the  north,  the  northern  dip  flattening 
somewhat  in  the  higher  layers.  On  the  Brunschweiler  a  perceptible  flat- 
tening of  the  whole  series  has  begun. 

Throughout  most  of  the  lowland  underlaid  by  these  sandstones  they 
are  covered  by  red  lacustrine  clays,  but  at  two  points — on  Bad  River  in  Sec. 
25,  T.  47,  R.  3  W.,  and  on  White  River  in  the  N.  E.  i.  Sec.  6,  T.  46,  R.  4 
W. — they  appear  in  great  force,  inclining  now  to  the  southeastward.  On  Bad 
River,  a  thickness  of  2,000  feet  is  in  sight,  trending  N.  60°  E.,  and  dipping 
S.  E.  38°;  and  on  White  River,  300  to  400  feet,  with  a  N.  40°  E.  trend,  and 
25°  S.  E.  dip.  These  southward-dipping  sandstones  indicate  the  existence 
of  a  synclinal  in  the  Upper  Division  of  the  series,  and  explain  the  wide 
surface  spread  of  the  upper  sandstones  in  this  region. 


234  COPPEE-BEAEING  BOOKS  OF  LAKE  SUPEEIOE. 


Section  V.— NOETHWESTEEK  WISCONSIN  AND  THE  ADJOINING  PAET 

OF  MINNESOTA. 

This  region  was  examined  in  considerable  detail  for  the  Wisconsin 
Greological  Survey  in  the  years  between  1873  and  1879,  by  Messrs.  Sweet, 
Strong  and  Chamberlin.  The  results  of  their  work  are  given  in  Vol.  Ill 
of  the  Geology  of  Wisconsin,  which  volume  also  contains  brief  descriptions 
by  Pumpelly  of  a  number  of  the  specimens  collected.  A  special  trip  was 
also  made  for  this  work  by  Mr.  R.  McKinlay  under  my  directions,  in  the 
valleys  of  the  Snake  and  Kettle  rivers  of  Minnesota,  with  especial  refer- 
ence to  determining  the  manner  in  which  the  Keweenawan  rocks  and  the 
Lake  Superior  basin  terminate  westwardly.  From  the  data  collected  by 
these  several  investigators  I  compile  the  following  brief  account  of  the 
region,  adding  some  conclusions  of  my  own. 

This  region  includes  two  distinct  belts  of  the  Keweenawan  rocks.  One 
of  these  is  a  continuation  of  the  belt  we  have  been  following  all  along  from 
Keweenaw  Point,  with  the  dip  as  usual  to  the  north  and  west,  while  the 
other  is  a  parallel  belt  made  up  of  strata  presenting  a  southerly  or  south- 
easterly dip,  and  forming  the  northern  side  of  a  synclinal  trough,  which 
extends  entirely  across  Wisconsin  to  the  Minnesota  line.  The  axis  of  this 
synclinal,  where  it  first  strikes  the  lake  shore  near  the  Montreal  River, 
trends  about  southwest ;  beyond  Bad  River  for  some  35  miles  it  runs  nearly 
due  west.  Turning  then  again,  it  follows  a  southwesterly  course  for  about 
80  miles,  when  it  changes  to  a  nearly  due  south  course  in  the  valley  of  the 
Saint  Croix  River,  where  the  north  and  south  belts  finally  unite,  under- 
neath the  newer  Cambrian  sandstone  of  the  Mississippi  Valley.  In  this 
region  the  two  main  divisions  of  the  series  are  plainly  recognizable.  It 
thus  follows  that  the  Upper  Division,  with  its  great  thickness  of  soft  sand- 
rock,  occupies  the  middle  of  the  synclinal,  while  the  ridges  on  each  side  are 
composed  of  the  resistant  crystalline  rocks  of  the  Lower  Division. 

The  Southern  Belt;  Numakagon  Lake  to  the  Saint  Croix  River. — In  the 
southern  belt,  including  the  northward-dipping  rocks,  with  the  one  excep- 
tion of  the  conglomerate  at  the  base  of  the  Upper  Division,  the  only  expos- 


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NJJMAKAGON  LAKE  TO  THE  ST.  CEOIX  EIVBE,  235 

ures  are  altogether  of  the  beds  of  the  Lower  Division.  The  relative  positions 
of  the  exposures,  and  the  bedding  directions  observed  at  them,  show  that 
immediately  west  of  the  district  last  described,  or  in  ranges  6  and  7  west, 
a  rapid  flattening  of  the  northern  dip  takes  place,  and  a  proportionately 
great  widening  of  the  belt  of  country  occupied  by  the  formation.  In  range 
7  west  this  width  is  as  much  as  9  miles,  the  dip  flattening  to  35°  in  the 
lowermost  belts  and  to  25°  in  the  upper  belts.  Farther  west  the  dips  flatten 
still  more,  getting  as  low  as  10°  to  15°,  and  the  surface  width  becomes  as 
much  as  12  to  15  miles. 

As  to  kinds  of  rocks,  the  exposures  are  sufficiently  frequent  to  show 
that  the  constitution  of  the  Lower  Division  of  the  series  is  the  same  in  this 
region  as  farther  east.  The  greatly  predominant  rocks  are  the  usual  plainly 
bedded  fine-grained  diabases  with  their  amygdaloids.  Melaphyrs  or  ohvine- 
diabases  are  rarer,  but  occur  somewhat  frequently.  Interleaved  porphyry- 
conglomerate  and  sandstones  have  been  observed  at  a  number  of  points, 
while  massive  quartz-porphyry  is  occasionally  to  be  seen.  No  coarse  gab- 
bro  has  been  observed  anywhere  between  Numakagon  Lake  and  the  Saint 
Croix  River.^ 

A  rather  unusual  phase  of  diabase  is  the  diabase-porphyry  of  the  ledges 
in  sections  26  and  27  of  T.  36,  R.  16  W.  These  rocks,  "in  a  greenish-gray, 
fine-grained  matrix  have  a  greater  or  less  amount  of  red  feldspar  in  porphy- 
ritic  crystals,  one-thirtieth  and  one-eighth  inch  in  diameter."  The  specific 
gravity  is  2.90.^  Under  the  microscope  the  porphyritic  crystals  prove 
to  be  a  somewhat  altered  triclinic  feldspar,  while  the  groundmass  is  chiefly 
made  up  of  smaller  plagioclases.  Intermingled  with  these  are  fibrous, 
greenish,  feebly  dichroic  or  non-dichroic  particles,  which  prove  to  be  altered 
(uralitic  and  viriditic)  diallage ;  epidote,  in  quite  abundant  grains ;  titanic 
iron  largely  altered  to  a  white  substance ;  and  pseud-amygdaloidal  chlo- 
rite. Closely  related  to  this,  and  evidently  at  about  the  same  horizon,  is 
the  dark  greenish-gray  porphyritic  rock  so  largely  exposed  at  the  Dalles 
of  the  Saint  Croix,  in  T.  34,  R.  19  W.'     In  this  rock  porphyritic  brown 

'See  Geology  of  Wisconsin,  Vol.  Ill,  pp.  .41,42;  46-48;  391-395 ;  399-428,  for  lithological  descrip- 
tions and  complete  local  details. 

'  R-  Pumpelly,  Geology  of  Wisconsin,  Vol.  Ill,  p.  41. 

=  Geology  of  Wisconsin,  Vol.  Ill ;  also  A.  Streng  and  J.  H.  Kloos  in  Leonhard  u.  Geinitz  Jahrbuch 
fiir  Mineralogie.     1877. 


236       COPPER-BEAEING  EOCKS  OP  LAKE  SUPEEIOE. 

plagioclases  and  pseud-amygdaloidal  epidote,  chlorite  and  quartz  are  im- 
bedded in  a  groundmass  consisting  principally  of  plagioclase,  with  which 
are  augite,  commonly  much  changed  to  viridite,  magnetite  or  titanic  iron 
and  epidote.  This  rock  passes  into  a  true  amj^gdaloid,  and  is  associated, 
as  is  also  the  similar  rock  of  T.  36,  R.  17  W.,  with  diabases  in  all  respects 
like  the  common  types  of  Keweenaw  Point.  It  is  of  considerable  interest 
that  rocks  closely  allied  to  these  are  found  at  a  similarly  low  horizon  on 
Silver  Mountain,  in  the  South  Range  of  Michigan,  and  again  on  the  Minne- 
sota coast  of  Lake  Superior. 

In  the  vicinity  of  Saint  Croix  Falls,  as  also  again  on  the  upper  Saint 
Croix,  and  on  its  tributaries,  the  Kettle  and  Snake,  in  Minnesota,  the  Ke- 
weenawan  rocks  are  visibly  overlain  by  the  horizontal  fossiliferous  basal 
or  Cambrian  sandstones  of  the  Mississippi  Valley,  the  sandstones  lying 
horizontally  in  the  erosion  hollows  of  the  previously  tilted  Keweenawan 
beds.  As  this  is  a  matter  of  great  theoretical  importance,  we  may  quote  at 
some  length  from  Professor  Chamberlin's  descriptions  of  the  Saint  Croix 
rocks.^    The  other' places  referred  to  are  described  in  subsequent  paragraphs. 

At  and  in  the  vicinity  of  Saint  Croix  Palls,  and  southward  from  there  to  the 
neighborhood  of  Osceola  Mills,  there  are  numerous  and  very  fine  exposures  of  the  Cop- 
per-bearing series  and  of  the  overlying  Potsdam  sandstone.  *  *  *  *  For  the  greater 
part,  they  lie  in  the  immediate  valley  of  the  Saint  Croix  river,  and  owe  their  exposure 
to  the  erosive  action  of  that  stream.  The  valley  here  is  some  400  feet  (aneroid)  below 
the  higher  plateaus  that  approach  the  stream,  within  a  mile  or  two  on  either  hand.  The 
Copper-bearing  rocks  appear  in  the  slopes  of  the  valley,  at  heights  ranging  up  to  200 
feet  or  300  feet. 

Just  below  Taylor's  Palls,  the  river  has  cut  a  deep  vertical  gorge  in  the  trappean 
rock,  forming  the  beautiful  and  picturesque  Upper  Dalles.  About  two  m^les  below  this 
a  similar  caiion  has  been  formed,  constituting  the  Lower  Dalles.  The  walls  within 
these  Dalles  are  almost  absolutely  vertical,  and  instead  of  showing  worn  faces,  like 
the  slopes  above,  present  the  regular  rough  surfaces  common  to  fissure  planes.  It 
seems  very  probable  that  the  original  worn  surface  of  the  gorge  has  been  riven  and 
thrown  down  by  the  action  of  the  frost  and  the  undermining  of  the  stream. 

In  this  vicinity,  the  relation  of  the  Potsdam  sandstone  to  the  copper-bearing 

rocks  is  most  satisfactorily  shown. 

********  * 

On  the  N.  W.  i  of  Sec.  12,  T.  33,  R.  19  W. ,  Osceola,  there  is  an  exposure  of  hor- 
izontally stratified  sandstone  in  the  side  of  a  small  ravine,  and  within  a  few  feet  is  an 

'  Geology  of  Wisconsin,  Vol.  Ill,  pp.  415-421. 


CHAMBERLIN  ON  THE  EOCKS  OF  ST.  CROIX  FALLS. 


237 


exposure  of  melaphyr'  of  the  Keweenaw  series.  The  sandstone  is  quite  hard  and 
compact,  but  the  stratification  is  undisturbed,  and  there  is  no  indication  of  met- 
amorphism.  A  short  distance  farther  north,  the  road  passes  over  an  exposure  of 
melaphyr  on  which  the  sandstone  is  seen  deposited  in  direct  contact  with  it. 

In  the  south  part  of  the  village  of  Taylor's  Falls,  near  the  summit  of  the  ridge, 
is  a  remarkable  exposure  of  the  Potsdam  in  connection  with  the  Keweenaw  series. 
Its  occurrence  is  illustrated  in  Fig.  7,  but  somewhat  idealized.  All  that  is  seen  is  an 
outcrop  of  melaphyr,  and,  on  the  exposed  face, 
a  conglomerate  formed  of  rounded  and  water- 
worn  fragments  of  melaphyr.  These  frag- 
ments are  of  all  sizes,  and  the  cementing  ma- 
terial is  a  ferruginous  sandstone  of  Potsdam 
age,  containing  occasional  Lingulepis  shells. 
In  the  vicinity  of  the  melaphyr  the  greater  part 
of  the  conglomerate  consists  of  its  fragments. 
After  inspecting  the  locality,  it  seems  evident 
that  the  melaphyr  was  an  exposed  cliff  in  the 
Potsdam  sea;  about  whose  base  large  and 
small  water-worn  fragments  of  the  melaphyr 
were  collected,  and,  the  interstices  being  filled  Fig.  7.— Showiug  uuconformaUe  contact  Ije- 
with  sand,  solidified  into  the  conglomerate  as  tweenKeweenawandiabase-porpliyry.and Pots- 
it  now  appears.  Thejunction  of  the  two  forma-  dam^sandsto.e  at  Taylor's  Falls,  Mmn.;  after 
tions  is  not  well  exposed,  as  the  sandstone     '°"^' 

graduates  into  the  conglomerate,  and  the  latter  is  banked  up  against  the  uneven  sur- 
face of  the  melaphyr.    The  unconformability,  however,  cannot  be  doubted. 

********  * 

On  the  west  side  of  the  river,  about  half  a  mile  above  Taylor's  Falls,  when  the 
water  is  low,  the  junction  of  the  Potsdam  and  Keweenaw  formations  may  be  found. 
*  #  *  #  Tjjg  fossiliferous  blue  shales  of  the  Potsdam  are  horizontally  deposited 
on  a  melaphyr,  containing  small  specks  of  pyrites.  The  melaphyr  breaks  out  in  thin 
pieces,  having  the  shales  firmly  attached.  The  junction  is  marked  by  a  dark  line 
about  one-eighth  of  an  inch  thick,  which  seems  to  cement  the  two  formations. 

The  foregoing  facts,  corroborated  as  they  are  by  much  other  data  gathered  from 
the  vicinity  and  other  parts  of  the  district,  and  elsewhere,  make  it  certain  that  the 
Copper-bearing  series  was  subjected  to  much  erosion  during  and  previous  to  the  for- 
mation of  the  Potsdam  sandstone,  by  which  valleys  were  cut  into  it  to  the  depth  of  at 
least  300  feet.  This,  doubtless,  represents  but  a  small  proportion  of  the  total  erosion 
which  the  series  had  suffered  in  the  pre-Potsdam  period. 

This  locality  presents  the  most  clear  and  unequivocal  evidence  that  the  Copper- 
bearing  series  is  much  older  than  the  Potsdam  sandstone  of  our  state,  so  much  older, 
indeed,  that  there  was  time  for  the  very  extensive  wearing  down  of  the  former  before 
the  latter  was  deposited. 

The  crystalline  rocks  of  the  Copper-bearing  series  of  this  locality,  while  varying 
somewhat,  possess  a  general  similarity,  and  are  not  diversified  by  conspicuously  dis- 
tinct beds  of  different  kinds  of  rock,  as  has  been  found  to  be  usually  the  case  in  other 
iRather  a  porphyritic  diabase,  according  to  the  nomenclature  adopted  in tbis  volume.— E.  D.  I. 


238       COPPER-BEAEING  EOCKS  OP  LAKE  SUPERIOE. 

extensive  exposures  of  the  formation.     So  true  is  this,  that  it  is  quite  difficult  to  dis- 
tinguish the  true  bedding  planes. 

#  **  ***  ***  # 

In  addition  to  these  vertical  planes  of  division,  which  are  generally  quite  smooth  and 
uniform,  but  not  persistent  to  great  depths,  there  is  another  prominent  set  which  are 
much  less  smooth,  but  much  more  persistent  and  constant  in  direction.  The  surface  of 
the  layers  separated  by  these  joints  is  nearly  uniform  to  the  general  view,  but  in  detail 
is  slightly  uneven  and  undulatory,  as  though  the  separation  took  place  not  through  the 
fracture  of  a  homogeneous  roclc,  but  by  separation  along  a  natural  division  plane. 
These  planes  are  usually  separated  by  several  feet.  They  are  confidently  believed  to 
represent  the  dip  of  the  igneous  beds.  It  is  not  presumed  that  all  of  the  layers  so 
formed  represent  separate  overflows  of  molten  material,  much  less  distinct  periods  of 
eruption  J  but  that  in  the  flowage  and  outspreading  of  the  igneous  matter,  a  somewhat 
parallel  arrangement  of  the  not  perfectly  homogeneous  substance  took  place,  giving 
rise  to  an  obscure  pseudo-stratification,  sufiicient  to  influence  the  jointing  that  subse- 
quently took  place.  At  the  same  time,  the  fact  that  the  beds  at  different  horizons  pre- 
sent different  textures,  and,  in  a  subordinate  degree,  different  mineralogical  composi- 
tion, would  seem  to  favor  the  belief  that  the  several  hundred  feet  of  the  formation 
exposed  in  the  vicinity  of  Saint  Croix  Falls,  represent  a  considerable  number  of  dis- 
tinct but  closely  successive  overflows ;  all,  perhaps,  belonging  to  one  great  period  of 
eruption.  The  latter  statement  seems  to  be  demanded  by  the  lithological  similarity  of 
the  rock,  the  slight  distinction  between  the  beds,  and  the  absence  of  detrital  deposits 
between  them.  Notwithstanding  their  obscurity,  however,  the  beds  give  to  the  out- 
crops the  distinctive  step-like  or  trappean  contour  that  has  been  previously  described 
and  figured.  This  is  best  seen  in  the  exposures  about  one  mile  east  of  Saint  Croix 
Palls  (N.  E.  i  of  Sec.  29,  and  N".  W.  i  of  Sec.  28,  T.  34,  E.  18  W.),  where  the  inclined 
ledges  follow  each  other  with  much  regularity  and  persistence,  giving  to  the  profile  of 
the  cross-section  a  serrate  outline,  notwithstanding  the  fact  that  the  glacial  agencies 
acting  from  the  northwest  tended  to  plane  down  the  edges  of  the  beds. 

It  is  upon  the  persistence  of  these  inclined  ledges,  taken  in  connection  with  par- 
allel lithological  belts,  that  our  determination  of  the  dip,  a  matter  of  some  theoretical 
interest  at  this  extremity  of  the  formation,  is  mainly  based.  The  average  of  a  large 
number  of  guarded  observations  gives  a  dip  of  about  15°  W.  by  S.  This  inclination 
to  the  south  of  west  is  quite  an  interesting  fact,  however  it  may  be  Interpreted.  To 
the  writer  it  seems  to  signify,  taken  in  connection  with  other  observations,  that  the 
trough  of  the  Lake  Superior  synclinal,  at  this  western  extremity,  curves  rapidly  south- 
ward, and  is  connected,  over  a  sort  of  saddle-back  anticlinal,  with  the  broad  strati- 
graphical  basin  that  stretches  southward  into  Minnesota;  and  that  the  igneous  beds 
overlap  this  figurative  saddle-back,  so  as,  on  their  margin,  to  really  lie  in  the  southern 
or  Mississippi  basin.  This  low  anticlinal  is  supposed  to  lie  a  little  north  of  Saint  Croix 
Palls,  and  to  be  the  low,  flattened  extremity  of  the  Laurentian  and  Huronian  heights 
that  lie  to  the  eastward — the  saddle-bow  of  our  illustration. 

The  unconformity  shown  by  Messrs.  Sweet,^  Strong,^  and  Chamberlin^ 

to  obtain  in  the  Saint  Croix  Valley  between  the  fossiliferous  Cambrian 

'  Transactions  of  the  Wisconsin  Academy  of  Sciences,  Arts,  and  Letters,  Vol.  Ill,  p.  40. 
"Geology  of  Wisconsin,  Vol.  Ill,  Part  VI. 


KEWEENAW  POINT  AND  ST,  OROIX  EOOKS  COMPAEED.       239 

sandstones  and  the  bedded  melaphyrs  and  amygdaloids  upon  which  they  lie 
being  so  plainly  indisputable,  the  latest  advocate  of  the  old  idea  of  the  con- 
temporaneousness of  these  sandstones  with  the  copper-bearing  or  Kewee- 
naw rocks  has  been  driven  to  question  the  correctness  of  the  identification 
of  the  bedded  diabases  and  amygdaloids  of  the  Saint  Croix  Vallej^  with 
those  of  Keweenaw  Point.^  It  is  therefore  proper  that  I  shoiild  insist 
here  that  this  identification  is  also  indisputable;  and  that  it  is  so  because  of 
the  absolute  identity  in  nature  and  structure  of  the  rocks  of  the  two  regions, 
and  because  the  Keweenaw  belts  have  been  followed  continuously  from  the 
eastern  end  of  Keweenaw  Point  to  the  Saint  Croix  River. 

In  support  of  the  first  of  these  assertions,  I  have  to  advance  the  follow- 
ing facts.  The  predominant  fine-grained  basic  rocks  of  the  two  regions  are 
so  completely  the  same  in  mineral  composition,  even  to  the  alteration-pro- 
ducts, that  thin  sections  of  rocks  from  the  two  districts  placed  side  by  side 
are  not  distinguishable  from  one  another.  The  only  approach  to  an  excep- 
tion to  this  statement  is  the  somewhat  gi'eater  prominence  of  prehnite  as  an 
alteration-product  on  Keweenaw  Point  than  on  the  Saint  Croix. ^  The 
rocks  of  the  two  regions  present  precisely  the  same  amygdaloidal,  pseud- 
amygdaloidal,  and  compact  phases.  The  amygdules  are  made  of  the  same 
minerals  in  both,  associated  in  the  same  ways.  Native  copper  occurs  in 
the  Saint  Croix  Valley  in  the  same  manner,  and  with  the  same  associates 
as  on  Keweenaw  Point.  Here  and  there  an  exposure  may  represent  a  dike 
so  far  as  can  be  perceived,  but  almost  everywhere  the  Saint  Croix  Valley 
rocks  present  precisely  the  same  bedded  structure  as  seen  in  those  of  Ke- 
weenaw Point.  This  is  displayed,  not  only  in  the  common  step-like  con- 
tours of  the  exposures,  but  the  individual  beds  may  be  readily  separated 
from   one   another,   each  bed  often   showing   sharply   marked  its   upper 

'  "Notes  on  the  Iron  and  Copper  Districts  of  Lake  Superior,"  by  M.  E.  Wadsworth.  Bulletin  of 
the  Museum  of  Comparative  Zoology  at  Harvard  College.  Whole  aeries,  Vol  VII.  Geological  series, 
Vol.  I,  No.  1,  p.  107. 

^Compare  E.  Pumpelly,  Geology  of  Wisconsin,  Vol.  Ill,  p.  36.  "While  the  absolute  identity  of 
the  diabases  and  melaphyr'and  of  their  varieties  and  amygdaloids,  and  of  the  interbedded  porphyry 
conglomerates  of  the  Wisconsin  area  with  those  of  Keweenaw  Point  is  evident,  I  am  struck  by  the  com- 
parative scarcity  in  the  former,  of  one  of  the  most  important  forms  of  alteration  that  abounds  in  Michi- 
gan; I  have  found  in  the  four  collections  but  one  instance  of  change  of  feldspar  to  prehnite."  With 
regard  to  this  it  should  be  said  that  the  specimens  from  Wisconsin  examined  by  Pumpelly  included 
very  few  from  the  uppermost  belts  of  the  Lower  Division,  which  carry  prehnite  much  more  conunonly. 


240       COPPEEBEAEING  EOCKS  OF  LAKE  SUPEEIOE. 

vesicular  and  lower  compact  portions.  Moreover,  where  the  dip  is  high  and 
the  exposures  %re  large,  as  on  the  Snake  and  Kettle  rivers  of  Minnesota, 
there  is  to  be  seen  a  continuous  series  of  beds,  in  all  many  hundi-ed  feet 
thick  and  in  every  respect  similar  to  the  alternations  which  obtain  on 
Keweenaw  Point.  The  same  interstratified  porphyry-conglomerates  and 
sandstones  are  met  with  in  both  regions,  and  in  both  regions  carry  at 
times  native  copper.  Interbedded  original  felsitic  porphyries  also  occur  in 
both  regions. 

In  support  of  the  second  assertion,  as  to  the  actual  continuity  of  the 
Keweenaw  Point  and  Saint  Croix  rocks,  I  have  to  say,  in  the  first  place,  that 
the  evidence  of  this  continuity  is  precisely  the  same  for  the  distance  between 
the  Montreal  and  the  Saint  Croix,  as  for  that  between  the  Montreal  and 
Keweenaw  Point,  or  even  the  distance  between  the  eastern  part  of  Kewee- 
naw Point  and  its  western  portion  at  Portage  Lake ;  that  the  continuity  has 
never  been  disputed  for  the  two  latter  distances;  and  that  it  should  there- 
fore be  accepted  at  once  for  the  first-named  distance.  The  evidence  for  all 
the  distance  between  Keweenaw  Point  and  the  Saint  Croix  is  just  as  strong 
as  that  ever  appealed  to  to  prove  the  continuity  of  geological  formations 
anywhere,  save  in  those  very  rare  and  exceptional  regions  where  the  rocks 
are  completely  bare.  This  evidence  consists  in  the  frequent  recurrence,  at 
short  intervals,  of  the  same  kinds  of  rocks,  with  the  same  structure  and 
stratigraphical  arrangement;  and  such  evidence  is  forthcoming  in  the  present 
case.  From  Keweenaw  Point  to  the  Saint  Croix,  the  formation  has  been 
traced  mile  by  mile  with  a  constant  recurrence  of  precisely  the  same  bedded 
basic  rocks,  with  the  same  amygdaloidal  and  compact  portions  to  the  beds,  of 
the  same  associated  felsitic  porphyries,  of  the  same  interstratified  porphyry- 
conglomerates,  and  of  the  same  native  copper  in  veins,  altered  amygdaloids 
and  conglomerates.  The  same  division  of  the  series  into  a  Lower  or  prevail- 
ingly eruptive  member,  and  an  Upper  or  detrital  member,  is  also  everywhere 
present.  From  Keweenaw  Point  to  the  region  of  Long  Lake,  some  even 
of  the  subordinate  members  are  recognizable  as  continuous.  For  the  par- 
ticulars of  this  evidence,  I  refer  to  the  detailed  descriptions  of  Foster  and 
Whitney's  report,  of  Vol.  Ill  of  the  Geology  of  Wisconsin,  and  of  the 
present  work;  to  the  United  States  Land  Office  township  plats;  and  to  the 


UNITED  STATES   GEOLOGICAL  SURVEY  COPPER-BEARING   ROCKS  OE  LAKE  SUPERIOR    PL. XXIV, 


MAP  SHOWING  POSITION  OF  THE  EXPOSURES  OF 

ALONG  THE  LOWER  PORTIONS  OF 


IfEWEENAWAN  ROCKS  AND  POTSDAM  SANDSTONE 

SNAItEAND  KETTLE  IWERS.lXnNUESOTA.  , 


keweejSTawan  eocks  on  snake  and  kettle  eivees.    241 

collections  of  the  Wisconsin  Geological  Survey,  and  of  the  survey  made  for 
this  report.  If  this  evidence  does  not  constitute  proof  of  continuity,  then 
no  geological  formation  in  the  United  States  has  ever  been  proved  to  be 
continuous  for  more  than  a  very  few  miles — rarely  for  more  than  a  mile — 
except  in  the  plateau  region  of  the  western  territories. 

In  the  distance  between  Numakagon  Lake  and  the  Saint  Croix  it  is 
difficult  to  estimate  the  total  thickness  of  the  Keweenawan  rocks.  The 
Upper  Division  is  not  exposed  on  this  side  of  the  synclinal,  and  the  position 
of  the  junction  with  the  older  rocks  is  rendered  uncertain  by  heavy  drift 
accumulations.  Judging,  however,  from  the  dip  and  strike  observations, 
and  from  the  outside  limits  between  which  its  surface  spread  must  lie,  it 
appears  probable  that  the  Lower  Division  of  the  Keweenaw  Series  must  have 
here  a  total  thickness  of  from  25,000  to  30,000  feet. 

The  Northern  Belt;  Snake  Biver  and  Kettle  Biver  District,  Minnesota. — 
The  western  end  of  the  trough  in  the  Keweenawan  rocks  is  concealed  by 
the  unconformably  overlying  Cambrian  sandstone  and  limestone.  The 
final  termination  southward  must  lie  on  the  Saint  Croix,  about  half  way 
between  Hudson  and  Osceola,  the  exposures  continuing  down  to  the  latter 
place.  Rounding  the  turn  and  moving  northward  now  on  the  Minnesota 
side  of  the  Saint  Croix,  we  find  the  Cambrian  sandstone  completely  con- 
cealing the  older  rocks  until  Snake  River  is  reached  in  township  39.  Here, 
on  the  lower  portions  of  Snake  and  Kettle  rivers,  and  on  the  Saint  Croix 
in  the  vicinity  of  the  mouths  of  these  streams,  typical  Keweenawan  rocks 
are  again  exposed  on  a  large  scale  with  a  trend  but  little  east  of  north  and 
a  high  easterly  dip. 

Snake  River  enters  the  Saint  Croix  on  Sec.  30,  T.  39,  R.  19  "W.  Two 
miles  below  the  mouth  of  the  stream,  in  the  N.  W.  ^  of  Sec.  7,  T.  38,  R. 
19  W.,  there  are  cliffs  of  horizontal  Potsdam  (Cambrian)  sandstone  50  feet 
high,  in  which  the  rock  is  the  usual  crumbling,  quartzose,  light-colored 
sandstone  commonly  seen  in  the  Mississippi  Valley.  The  same  rock  is  ex- 
posed on  the  Snake  River  in  the  southern  part  of  Sec.  36,  T.  39,  R.  20  W. 
Above  this.  Snake  River  is  without  rock  exposures  until  Sec.  "24,  T.  39,  R.21 
W.  is  reached.     From  here  there  are  exposed  continuously  in  the  bed  and  on 

16  L  s 


242       COPPER-BEAEING  EOCKS  OF  LAKE  SUPERIOR. 

the  banks  of  the  stream  to  the  dam  at  Chegwatona  Lake,  near  the  east  line 
of  section  27,  beds  of  diabase  and  diabase-amygdaloid,  with  interbedded 
porphyry-conglomerates,  trending  N.  10°  to  20°  E.,  and  dipping  60°  to  75° 
south  of  east.  The  rocks,  as  shown  by  the  thin  sections,  are  in  every  respect 
identical  with  the  fine-grained  diabases  and  diabase-amygdaloids  of  Ke- 
weenaw Point.  The  following  is  an  account,  kindly  furnished  by  Professor 
T.  C.  Chamberlin,  of  the  succession  displaj^ed  at  this  place.-^  His  measure- 
ments were  only  rapid  pacings,  and  his  examinations  did  not  begin  quite  so 
far  down  stream  as  the  last  ledges  located  by  Mr.  McKinlay. 

The  locality  was,  however,  visited  after  having  studied  in  detail  the  adjacent 
Wisconsin  formations,  and  the  observations  were  made  especially  with  reference  to 
the  relations  of  these  rocks  to  the  Wisconsin  and  Michigan  series,  and,  so  far  as 
general  structural  questions  are  concerned,  were  entirely  conclusive  in  the  judgment 
of  the  observer. 

1.  The  uppermost  exposure  examined,  i.  e.,  the  one  lowest  down  the  river,  the  dip 

being  eastward  down  stream,  is  a  diabasic  amygdaloid,  much  altered,  con- 
,  taining  malachite.     The  exposure  is  low  and  small. 

2.  Ten  paces  up  the  stream,  i.  e  ,  westward,  there  is  a  similar  but  less  amygdaloidal 

rock  in  low  exposures. 

3.  Fifteen  paces  further  there  occurs  a  similar  rock  which  continues  exposed  for  26 

paces. 

4.  This  is  succeeded  by  a  conglomerate  of  the  Keweenawan  type,  having  a  surface 

exposure  of  between  6  and  7  paces. 

5.  This  is  followed  (underlain)  by  about  8  paces  of  mixed  conglomerate  and  amyg- 

daloid, that,  for  want  of  time,  was  not  studied  with  sufficient  thoroughness  to 
determine  its  true  nature.  It  appeared,  however,  to  consist  of  the  shattered 
fragments  of  the  upper  vesicular  portion  of  a  lava  flow  mingled  by  wave 
action  with  water- worn  pebbles  of  more  distant  derivation. 

6.  This  is  followed  by  about  45  paces  of  diabase,  more  or  less  amygdaloidal,  and  be- 

longing to  the  more  common  type  of  Keweenawan  diabases. 

7.  This  is  underlain  by  conglomerate  of  the  common  Keweenawan  sort,  having  a 

surface  width  of  about  12  paces. 

8.  This  is  succeeded  by  diabase  and  amygdaloid,  more  or  less  concealed  and  varying 

in  character,  for  a  space  of  79  paces. 

9.  This  is  followed  (underlain)  by  a  reddish  very  amygdaloidal  rock,  which,  in  6  to  7 

paces,  graduates  into 

10.  A  bed  of  the  common  Keweenawan  diabase,  which  has  a  surface  extent  of  12 

paces. 

11.  This  is  succeeded  by  a  reddish-brown  amygdaloid  for  10  to  12  paces. 

12.  Then  follows  a  dark-gray  diabase,  including  red  diabasic  rock,  for  about  12  paces. 

13.  This  is  followed  for  about  10  paces  by  amygdaloidal  rock. 

14.  This  again  by  diabase  for  23  paces. 

'  From  unpuljlislied  notes,  made  in  1878. 


KEWEENAW  AN  EOCKS  OF  SNAKE  AND  KETTLE  EIVEES.     243 

15.  Then  follows  a  mottled  igueous  rock  for  44  paces,  graduating  into  the  common  dia- 

basic  trap. 

16.  This  gives  place  to  a  coarse  red  conglomerate  of  pronounced  Keweeuawan  type 

containing  porphyry  pebbles.     It  has  a  surface  width  of  35  paces. 

17.  This  is  underlain  by  amygdaloidal  diabase  for  12  paces. 

18.  There  follows  this  again  a  coarse  red  Keweenawan  conglomerate  with  associated 

shale.  This  presented  favorable  conditions  for  deiinitely  ascertaining  the 
strike  and  dip,  which  were  found  to  be,  strike,  N.  10°  to  15°  E. ;  dip,  60°  east- 
ward (E.  10°  to  15°  S).    This  stratum  has  a  surface  width  of  10  paces. 

19.  Under  this  lies  a  fine-grained  diabase,  with  a  surface  exposure  of  18  paces. 

20.  This  gives  place  to  a  band  of  amygdaloid,  3  paces  across,  followed  by  trap  of  vary- 

ing character  for  a  width  of  75  paces. 

A  gap,  estimated  to  exceed  one-half  mile,  ensues,  in  which  there  appeared  to  be 
no  exposures  in  the  north  bank  of  the  river.  (Jn  the  south  side,  rock  with  harmonious 
dip  could  be  seen  at  some  points,  but  it  was  not  visited. 

At  the  dam  at  the  outlet  of  Ghegwatona  Lake  there  are  much  altered  amygda- 
loids  and  diabases  in  alternating  series  that  strike  from  N.  to  N.  10°  E.,  and  dip  from 
60°  to  70O  eastward.  The  amygdaloidal  bands  are  conspicuous,  but  not  sufficiently 
sharply  defined  to  give  very  precise  strike  and  dip,  but  they  harmonize  well  with  the 
undoubted  observations  given  above  and  the  more  general  ones  made  along  the  whole 
section,  so  that  the  attitude  of  the  series  is  perfectly  certain.  The  rocks,  whether  com- 
pact diabases,  amygdaloids,  or  conglomerates,  are  typically  Keweenawan  in  aspect,  and 
leave  no  room  for  doubt  that  they  belong  to  the  copper-bearing  series  and  form  the 
western  and  perhaps  terminal  margin  of  the  Lake  Superior  synclinal  trough. 

Native  copper  occurs  in  these  rocks  on  Snake  River  both  in  the  con- 
glomerates and  some  of  the  bands  of  altered  amygdaloid,  and  in  such 
quantity  near  the  surface  as  to  promise  success  to  mining  enterprise. 

Retui-ning  now  to  the  Saint  Croix  River,  we  find,  on  ascending  from  the 
mouth  of  the  Snake,  an  exposure  of  the  horizontal  light-colored  Potsdam 
sandstone  in  Sec.  30,  T.  39,  R.  19  W.,  and  in  the  N.  W.  J  of  Sec.  29, 
ledges  of  the  usual  fine-grained  diabase  striking  north  and  south.  On  the 
N.  E  ^  of  Sec.  20,  are  again  flat-lying  ledges  of  a  fine-grained  diabase, 
apparently  striking  north  and  south  with  a  very  low  eastern  dip.  At  this 
place  native  copper  has  been  obtained  in  a  small  trial  shaft  near  the  river 
edge.  The  lower  part  of  the  shaft  penetrated  to  an  amygdaloidal  laj^er.^ 
The  usual  light-colored  horizontal  Potsdam  sandstone  is  exposed  near  by. 

Kettle  River  enters  the  Saint  Croix  near  the  S.  E.  corner  of  Sec.  8,  T. 
39,  R.  19  W.  From  this  point  the  upward  course  of  the  stream  is  nearly 
due  north  for  some  three  miles,  this  direction  being  apparently  induced  by 
the  north  and  south  strike  of  the  rocks.     In  this  distance  are  some  five  or 


1  Geology  of  Wisoonsiii,  Vol.  Ill,  p.  427. 


244       COPPEE-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 

six  long,  low  diabase  ledges  at  the  edge  of  the  stream,  apparently  with  a 
north  and  south  trend.  In  the  N.  W.  i  of  the  N.  W.  ^  of  Sec.  32,  T.  40, 
R.  19  W.  the  east  bank  of  the  stream  shows  a  diabase  ledge  15  feet  high 
and  35  rods  long.  Just  opposite,  on  the  other  side  of  the  stream,  is  a  flat- 
lying  reddish  conglomerate.  Porphyry  conglomerate  occurs  again  on  the 
east  bank  of  the  river  near  the  center  of  the  S.  E.  ^  of  Sec.  29,  T.  40,  R 
19  W.,  and  again  in  a  large  exposure  a  mile  farther  up  stream  in  the  N.  E. 
^  of  the  S.  E.  ^  of  Sec.  19,  where  it  plainly  lies  at  a  very  flat  angle.  These 
three  exposures  appear  all  to  be  part  of  the  same  conglomerate  bed.  Six 
hundred  paces  up  stream  from  the  last  exposure,  typical  diabase  and  dia- 
base-amygdaloid are  in  sight. 

Above  this  point  the  river  is  without  exposure  for  about  five  miles,  the 
upward  course  of  the  stream  in  this  distance  making  over  three  miles  of 
westing.  On  the  north  line  of  Sec.  3,  T.  40,  R  20  W.,  however,  large  expos- 
vires  of  diabase  begin  again.  Through  Sec.  35,  T.  41,  R.  20  W.  the  river 
pursues  a  nearly  southerly  course,  and  on  the  east  side,  continuing  for  over 
half  a  mile,  is  a  west-facing  cliif  10  to  30  feet  high  of  the  typical  fine-grained 
diabase.  The  east  slope  of  the  ledge  is  gradual,  and  the  strike  and  dip  are 
plainly  to  be  made  out,  as  respectively  N.  6°  E.,  and  50°  E.  On  the  north 
line  of  the  same  section,  and  again  in  section  22,  similarly-placed  ledges 
are  largely  exposed,  the  river  making  in  this  distance  about  a  mile  of  west- 
ing, so  that  between  the  south  line  of  T.  41,  R  20  W.  and  the  expos- 
ures in  section  22  the  river  crosses  a  mile  in  width  of  Keweenawan  beds, 
with  an  average  eastern  dip  of  50°.  The  last  place  is  of  great  interest,  for 
only  300  paces  north  of  the  stream,  and  directly  in  the  course  of  the  north- 
ward-trending diabases,  is  a  cliff  of  horizontal  light-colored  Cambrian  sand- 
stone 40  feet  high  and  several  hundred  paces  in  length.  The  occurrences 
at  this  point  are  represented  in  Figs.  8  and  9. 

Four  miles  west  of  here,  on  the  line  of  the  Saint  Paul  and  Duluth  Rail- 
road, near  Hinckley,  the  same  light-colored  sandstone  is  quarried. 

The  exposures  in  section  22  are  the  last  of  Keweenawan  rocks  seen  in 
ascending  Kettle  River.  Six  miles  farther  north,  however,  near  the  south 
line  of  Sec.  22,  T.  42,  R  20  W.,  the  horizontal  light-colored  Cambrian  sand- 
stone begins  to  make  large  exposures,  which  continue  without  break,  either 


UNCONFORMITY  ON  KETTLE  EIVER. 


245 


in  the  bed  of  the  stream,  or  in  cliffs  20  to  75  feet  high,  on  either  side,  for 
5  miles,  to  the  north  line  of  Sec.  35,  T.  43,  R  20  W.  Above  this  for  3J 
miles  the  sandstone  does  not  appear  in  the  river,  but  is  seen  here  and  there 
on  hillsides  near  the  stream.    Throusfh 


sections  11  and  3,  T.  43,  R.  20  W.  the 
sandstone  reappears  in  the  stream,  the 
last  exposure  found  lying  200  paces 
south  of  the  north  line  of  section  3. 

From  this  point  up  stream  for 
some  nine  miles  no  exposures  were 
found.  Then,  through  sections  9  and 
4,  of  T.  45,  R.  20  W.,  and  32,  29,  and 
28,  of  T.  46,  R.  20  W.,  frequent  out- 
crops of  mica-schist  are  met  with  in 
the  hillsides  near  the  river. 

These  briefly-stated  facts  with 
regard  to  the  region  of  the  Snake  and 
Kettle  rivers,  fui'ther  illustrated  by 
the  map  of  Plate  XXIV,  will  serve  to 
render  certain  three  very  important 
conclusions,  viz:  (1)  the  diabases  and 

diabase-amygdaloids  and  interbedded      Fig.  8.— Map  of  Exposures  on  Kettle  River,  Sec. 

„      ,  .       _       22,  T.  41,  E.  20  W.,  Minnesota.     Scale  4  inches  to 

porphyry-conglomerates  ot    this  dis-  the  mile. 

trict  are  in  all  respects  like  those  of  Keweenaw  Point;  (2)  the  light-colored 
horizontal  Cambrian  sandstones  overhe  these  bedsunconformably;  (3)  these 
Keweenawan  beds,  with  a  trend  but  little  east  of  north,  present  an  easterly 
dip  which  at  from  5  to  8  miles  west  of  the  Saint  Croix  reaches  50°  to  7o° 
and  which  flattens  rapidly  eastward,  becoming  very  low  on  the  Saint  Croix 


^Sevpse-nxx:mm-  d^'^frect^o'^     


Fig.  9. — Section  on  line  A  B  of  Fig 
to  the  inch. 


8.     Scale,  horizontal,  8  inches  to  the  mile;  vertical,  300  feet 


itself     The  first  two  of  these  conclusions  are  but  confirmations  of  those 
reached  farther  down  the  Saint  Croix,  but  the  last  is  of  the  greatest  interest 


246       COPPEE-BBARING  EOCKS  OF  LAKE  SUPERIOR. 

and  importance  in  its  bearing  upon  the  structure  of  the  western  termination 
of  the  Keweenawan  trough,  as  is  shown  in  a  subsequent  chapter. 

The  exposures  of  Huronian  schists  on  the  upper  Kettle  Eiver,  taken 
together  with  those  at  Moose  Lake,  on  the  Saint  Paul  and  Duluth  Eailroad, 
and  those  at  Thompson,  on  the  Saint  Louis  River,  are  also  of  great  interest, 
since  they  form  a  line  beyond  which  it  is  certain  that  the  Keweenawan  rocks 
cannot  pass  to  the  westward. 

Upper  Saint  Croix.— For  30  miles  above  the  mouth  of  the  Kettle  River 
the  Saint  Croix  shows  no  rock  of  any  kind.  Then  come  large  exposures  of 
southeastward-dipping  sandstone,  extending  for  many  miles  along  the  stream. 
I  quote  in  this  connection  from  Professor  Chamberlin's  description:^ 

Prom  Sec.  4  (T.  43,  E.  13  W.)  to  the  southern  line  of  the  county^  (Sec.  33,  T.  43,  E. 
14  W.),  the  bed  of  the  stream^  is  almost  continuously  composed  of  sandstone  and  con- 
glomerate. The  greater  portion  of  this  is  the  common  red  sandstone  of  the  series. 
The  lower  mile  and  a  half  is  conglomerate,  and  probably  corresponds  in  stratigraphical 
equivalence  to  the  conglomerates  of  Sees.  27  and  14,  T.  44,  E.  13  W.,  above-described, 
as  it  lies  in  the  line  of  strike,  and  bears  a  similar  relation  to  the  crystalline  strata  on 
the  northwest.  By  consulting  the  map,  it  will  be  seen  that  from  Chase's  dam  to  the 
county  line,  a  distance  of  about  11  miles,  the  river  runs  an  almost  direct  course,  and 
■with  slight  interruptions  is  bedded  on  sandstones  and  conglomerates.  A  casual  glance 
will  show  that  tlie  stream  runs  closely  with  the  strike  of  the  strata.  A  more  careful 
study  makes  it  appear  that  the  river  crosses  the  strata  at  a  very  small  angle,  passing 
from  higher  to  lower  beds.  Near  the  county  line,  however,  the  river  turns  southward 
and  pursues,  for  about  three  miles,  a  southerly  course.  This  brings  it  over  higher  (geo- 
logically) strata.  For  a  little  more  than  a  mile,  however,  it  is  bedded  in  drift,  but 
near  the  north  line  of  Sec.  9  (T.  43,  E.  14  W.),  the  sandstone  of  the  series  reappears 
in  the  bed  of  the  river  and  extends  across  it,  causing  rapids  and  forming  occasional 
low  exposures  in  the  banks.  The  ledges  show  fine  ripple-marks  and  occasionally  rain- 
drop impressions.  They  are  more  indurated  and  seem  to  contain  more  quartz  and  less 
argillaceous  material  than  those  previously  described.  This  sandstone  again  becomes 
concealed  at  the  south  line  of  the  section,  but  reappears  in  the  bed  of  the  river  in  the 
Indian  village  in  the  N.  W.  J  of  the  N.  W.  i  of  Sec.  21,  (T.  42,  E.  14  W.),  about  a  mile 
below.  These  are  probably  the  highest  beds  of  the  Keweenaw  group  exposed  in  the 
district. 

ISText  immediately  underlying  this  sandstone  and  conglomerate  series,  so  far  as  the 
outcrops  show,  there  appears  to  be  a  diabase,  little  exposed,  underlaid  by  a  stratum 
of  easily-recoguized  melaphyr,  forming  at  the  surface  outcrops  along  a  belt  lying 
parallel  to  the  sandstones.    This  is  the  typical  Keweenaw  melaphyr  described  by 

'Geology  of  Wisconsin,  Vol.  Ill,  p.  424-4!i7. 
i!  Douglas  County,  Wisconsin. 
3  The  Saint  Croix. 


UNITED  STATES   GEOLOGICAL  SURVEY 


l^.SWyi'. 


COPPER-BEARING  ROCKS  OF  LAKE  SUPERIOR   PL,  XXV 

R.xm-vr. 


JMAP  SHOW^I^G  POSITIONS  OF  EXPOSURES  OF  KEAM5ENAWAN  ROCKS 

IN  THE  UPPER  ST  CROrS  VALLEY ^VlSCO:S Sm , 

BASED  OH  TJEPORT  OF  MOSES  STROKG  -GEOLOGY  OF  YflSCONSTl\r,  VOL  TE. 
Scale  2.3  miles  to  1  in  ell. 


KOCKS  OF  THE  UPPER  ST.  CKOIX.  247 

Professor  Pumpelly  on  page  32,  aud  identical  with  stratum  108  of  the  Eagle  river 
section.'  It  is  found  in  the  N.  E.  a  of  Sec.  14  and  the  N.  W.  i  of  Sec.  27,  T.  44,  E.  13 
W.,  the  N.  W.  i  of  Sec.  28,  T.  43,  E.  14  W.,  and  at  the  falls  on  Chase's  brook,  in  the 
IT.  E.  i  of  the  N.  W.  i  of  Sec.  16,  T.  42,  E.  15  W. 

The  first  three  of  these  may  be  joined  by  a  nearly  straight  line  about  12  miles  in 
len£<th,  whose  course  will  be  about  N.  48°  E.,  or  nearly  the  average  observed  strike. 
If  this  line  be  projected  it  will  pass  to  the  southeast  of  the  melaphyr  exposure  on 
Chase's  brook,  and,  if  extended  in  the  opposite  direction  it  will  pass  about  an  equal 
distance  from  an  outcrop  of  precisely  similar  rock  found  on  Moose  creek,  in  the  N.  E. 
i  of  the  N.  W.  i  of  Sec.  2,  T.  44,  E.  13  W.  It  would  appear  highly  probable,  then, 
that  the  first  three  exposures  belong  to  the  same  stratum,  and  that  the  remaining  two 
represent  a  lower  bed.  This  is  confirmed  by  the  existence  of  a  similar  melaphyr  near 
the  center  of  the  west  line  of  the  N.  W.  i  of  Sec.  21,  T.  43,  E.  14  W.,  and  also  one  in 
the  S.  W.  i  of  Sec.  10,  either  of  which  might  readily  be  referred  to  the  lower  bed, 
though  it  is  not  so  apparent  that  both  could.  Entering  more  into  detail,  we  observe 
that  the  outcrop  of  the  melaphyr  in  the  N.  W.  i  of  the  F.  B.  ^  of  Sec.  27,  T.  44,  E. 
13  W.,  rises  only  about  10  feet  high  and  forms  the  bank  of  a  creek.  The  rock 
agrees  completely  with  the  description  of  the  typical  Keweenaw  rock  previously 
referred  to.  The  formation  crops  out  quite  continuously  along  the  stream  as  far  as 
the  forks  in  the  K.  E.  I  of  Sec.  28.  Here  it  is  found  to  be  dark,  coarse-grained,  rather 
soft,  containing  much  chlorite,  and  crumbling  readily  on  weathering,  and  no  longer 
possesses  the  distinctive  melaphyr  characters.  On  the  line  between  Sees.  22  and  27 
we,  however,  find  the  typical  melaphyr  again. 

Following  the  line  of  strike  into  the  S.  E.  i  of  the  N.  E.  i  of  Sec.  14  of  this 
township  (T.  44,  E.  13  W.),  we  find  on  Moose  creek  the  same  melaphyr.  Its  dip 
here  appears  to  be  about  18°  to  S.  30°  E.  A  few  yards  below  the  ledges  are  traversed 
by  veins  of  epidote,  with  some  indications  of  copper. 

The  rock  is  here  very  amygdaloidal,  carrying  chlorite  as  a  cell-filling,  dip  20°.  A  few 
rods  below  we  find  a  heavy,  firm,  :flne-grained,  dark-greenish,  diabase-like  rock.  The 
surface  of  this  presents  very  finely  preserved  glacial  grooves,  having  a  direction  S.  13° 
W.  Some  are  wide  and  shallow,  while  others  are  narrow,  sharply-defined  hair  lines.  In 
the  ]Sr.  W.  I  of  the  S.  E.  |  of  this  section  there  are  also  some  small  ledges  of  fine-grained 
diabasoid  rock,  and  in  the  S.  W.  ^  of  the  S.  E.  i  of  the  section  we  encounter  the  con- 
glomerate before  described.  Following  up  Moose  creek  to  Sec.  2,  we  find  in  the  IST.  W. 
i  of  the  S.  E.  i  first  a  very  hard,  fine-grained,  nearly  black  diabase,  above  which, 
about  100  yards,  there  appears  a  coarser  crystalline  diabase  and  diabase  pseudo- 
amygdaloid,  containing  patches  of  epidote,  quartz  and  considerable  calcite,  though  the 
rock  is  not  generally  amygdaloidal.  There  are  to  be  found  occasionally  specks  of  mala- 
chite. The  dip  measurements  were  17°  and  20°  S.  E.  The  ledges  are  much  fissured 
and  broken  in  all  directions.  Near  the  center  of  the  section,  low  ledges,  along  the 
west  side  of  the  stream,  exhibit  a  coarse-grained  rock,  somewhat  resembling  the  mela- 
phyr found  farther  upthe  stream,  presently  to  be  described.  Above  this  in  the  N.  W. 
i  of  the  section,  there  first  appears  a  diabase  of  medium  grain  and  greenish-gray  color, 
and  about  100  yards  farther  up,  on  the  left  bank  of  the  stream,  a  low  outcrop  of  soft, 
very  dark,  diabase  pseudo-amygdaloid,  containing  chlorite,  quartz,  orthoclase  and 

'See  Geological  Survey  of  Michigan,  1869-1873,  Vol.  I,  Part  II,  p.  136. 


248       COPPER-BEARING  ROCKS  OF  LAKE  SUPERIOR. 

prehnite.  About  an  eighth  of  a  mile  above  this,  and  only  a  short  distance  below  the 
town  line,  the  west  bank  of  Moose  creek  exposes  the  typical  melaphyr  previously  men- 
tioned, characterized  by  a  dark-green  color,  fine-grain,  peculiar  irregular  fracture  and 
large  reflecting  surfaces  of  satin-like  luster. 

Passing  due  southwestward  about  seven  miles  into  the  S.  W.  ^  of  Sec.  6  we  en- 
counter a  dark,  fine-grained  diabase,  occasionally  amygdaloidal,  with  calcite.  A  vein 
about  two  inches  wide  was  observed,  which  carried  considerable  native  copper  in  films 
and  small  particles,  associated  with  calcite  and  epidote.  In  the  rock  there  are  also  parti- 
cles of  epidote  carrying  copper.  These  are  in  the  bed  of  the  brook,  and  overflowed  in 
high  water.  A  short  distance  above,  the  rock  becomes  softer  and  contains  large  amyg- 
dules  of  chlorite  with  frequently  a  core  of  calcite.  These  ledges  extend  along  the 
stream  for  about  half  a  mile.  At  the  dam,  near  the  town  line,  there  is  a  ledge  of  highly- 
altered  diabase-amygdaloid  containing  calcite,  chlorite  and  epidote. 

To  the  west  of  the  center  of  Sec.  8,  T.  43,  R.  13  W.,  there  is  a  fine  exposure  of 
conglomerate,  having  a  dip  of  about  14°  iu  a  direction  S.  50°  E.  It  is  traversed  by 
two  I'egular  systems  of  joints  on  courses  N.  34°  E.  and  N.  56°  W.,  by  reason  of  which 
it  is  cut  into  regular  cubical  blocks.  Judging  from  the  drift,  the  western  portion  of 
the  S.  E.  ^  of  the  adjoining  Sec.  5  is  underlaid  by  conglomerate. 

Passing  over  an  interval  of  about  five  miles,  in  which  no  outcrops  are  known  to 
exist,  we  find  in  the  S.  E.  J  of  Sec.  6,  T.  43,  R.  14  W.  a  wide,  low,  northeasterly- 
trending  ridge,  presenting  bared  rock  at  one  point,  which  appears  to  be  a  diabase 
of  very  fine,  close  grain,  and  dark  color,  coated  with  a  thin,  light-colored  crust  due 
to  weathering.  In  the  jST.  E.  ^  of  Sec.  9  of  this  township  there  is  a  ridge  composed 
of  rather  soft,  fine-grained,  dark,  reddish-brown  diabase  amygdaloid  (specimen  425), 
weathering  to  a  dirty  lilac  hue.  It  appears  to  be  much  altered.  This  ridge,  in  common 
with  those  of  the  vicinity,  presents  an  abrupt  declivity  on  the  northwest  and  a  gentle 
slope  in  the  opposite  direction,  the  same  phenomenon  observed  so  frequently  on  the 
opposite  side  of  the  Saint  Croix  Valley,  but  reversed  in  direction.  It  is  scarcely  nec- 
essary to  repeat  that  it  is  due  to  the  inclination  of  the  strata  whose  projecting  edges 
form  the  ridges. 

In  the  S.  W.  ^  of  Sec.  10  (T.  43,  R.  14  W.)  there  is  a  similar  ridge  composed  of 
melaphyr,  and  already  referred  to.  A  short  distance  south  of  this,  in  the  adjoining  sec- 
tion (N.  W.  I  of  Sec.  15),  there  is  a  similar  ridge,  but  of  diabase,  beyond  which  is  still 
another,  the  rock  of  which  is  a  dark  brown  and  black,  hard,  fine-grained  crystalline 
diabase,  containing  occasional  amygdules  of  chlorite.  It  resists  weathering  well,  and 
only  shows  a  thin,  light,  dirty  grayish  coating  of  weathered  substance.  The  trend  of 
these  ridges  is  northeasterly  with  the  strike  of  the  strata. 

In  the  S.  W.  i  of  Sec.  15  (T.  43,  R.  14  W.),  in  a  large  hill,  there  is  a  small  denuded 
area  of  rock,  of  hard,  close,  minutely  crystalline  texture,  reddish-brown  color,  and 
rough,  uneven  fracture.  It  contains  scattered  aggregations  of  chlorite.  It  appears 
to  be  an  altered  melaphyr. 

In  the  S.  R.  ^  of  Sec.  17  (T.  43,  E.  14  W.)  there  is  a  long  ridge  of  melaphyr  that 
appears  to  be  a  continuation  of  that  above  noted  in  Sec.  10,  and  is  probably  to  be 
correlated  with  that  in  Sec.  2,  T.  44,  E.  13  W.,  and  that  at  the  falls  on  Chase's  brook. 
Sec.  16  (T.  42,  E.  15  W.).  There  is  a  like  rock  found  near  the  center  of  the  west  line  of 
the  N.  W.  i  of  Sec.  21. 


BOOKS  OF  THE  UPPEE  ST.  OEOIX,  249 

In  the  S.  W.  J  of  Sec.  22  (T.  43,  E.  14  W.)  there  is  an  outcrop  of  a  fine-grained, 
hard,  reddish-brown,  crystalline  rock,  probably  a  diabase.  It  forms  the  nucleus  of  a 
hill. 

In  the  S.  E.  |  of  the  N.  W.  I  of  Sec.  28  there  is  a  small  uncovered  area  of  typical 
melaphyr,  which  probably  belonged  to  the  same  stratum  as  those  situated  in  Sees. 
14  and  27,  T.  44,  E.  13  W.,  as  already  stated.  Prom  this  point,  for  a  distance  of  about 
30  miles  down  the  Saint  Croix,  no  exhibitions  of  rock  in  place  of  any  kind  are  known 
to  exist. 

The  following  is  a  description  of  the  same  district  by  Mr.  E.  T.  Sweet  :^ 

In  the  hanks  and  channel  of  Moose  river,  on  Sec.  2,  T.  44,  E.  13  W.,  there  are 
low  ledges  of  melaphyrs  and  diabases,  dipping  18°  S.  35°  E.  These  are  conformably 
overlaid  by  fine  conglomerates  and  coarse  sandstones.  The  pebbles  of  the  con- 
glomerate have  nearly  all  been  directly  derived  from  the  underlying  crystalline  rocks, 
and  are  held  together  by  a  coarse,  red,  sandy  matrix.  None  of  the  very  coarse  or 
bowlder  conglomerates  noticed  on  the  northward-dipping  belt,  in  Ashland  county,  and 
on  the  Saint  Croix  river,  were  observed  here.  In  following  Moose  river  southward, 
towards  its  mouth,  several  small  exposures  of  the  fine  conglomerate  were  seen,  but  it 
apparently  has  no  great  thickness,  for  it  soon  grades  into  coarse,  reddish  sandstone, 
and  that  finally,  after  reaching  the  Saint  Croix,  into  quite  fine-grained,  red  sandstone, 
often  somewhat  argillaceous. 

The  most  northern  exposure  of  this  sandstone  on  the  Saint  Croix,  is  at  the  head 
of  a  small  lake  about  a  mile  above  the  mouth  of  Moose  river.  The  outcrop  is  in  the 
east  bank.  The  layers  are  hard  and  thin,  and  contain  many  red  argillaceous  spots. 
Indurated  smooth  slabs  come  out  readily.  A  few  of  the  layers  are  finely  ripple- 
marked.  The  strike  is  N.  53°  E.  and  the  dip  14°  S.  This  place  is  a  short  distance 
below  Chase's  dam,  on  Sec.  36,  T.  44,  E.  13  W.  For  five  miles  along  the  Saint  Croix 
below  Moose  river,  a  few  small  exposures  only  are  seen.  On  Sec.  8,  T.  43,  E.  13  W. 
the  sandstone  is  exposed  in  the  banks  five  or  six  feet  high.  At  the  first  considerable 
exposure,  the  rock  is  fine  grained,  very  thin  bedded,  and  argillaceous.  Circular  red- 
dish and  bluish  spots  of  indurated  clay  are  of  frequent  occurrence  in  the  layers.  The 
strike  is  N.  55°  E.  and  the  dip  13°  S.  There  are  two  well-marked  systems  of  joints; 
one  trending  N.  28°  W.,  and  the  other  K".  55°  E.  A  short  distance  below  here,  there 
is  a  somewhat  larger  exposure,  showing  a  strike  of  N.  53°  E.,  and  dip  of  14°  S.  Below 
this,  for  a  distance  of  ten  or  twelve  miles,  tilted  sandstones  often  form 'the  bed  of  the 
stream,  although  they  are  seldom  seen  in  the  banks.  In  the  banks  of  Eocky  Eun  on 
Sec.  9,  T.  42,  E.  14  W.,  a  half  mile  from  the  Saint  Croix,  the  sandstones  are  again 
largely  exposed.  In  the  banks  and  channel  of  the  Saint  Croix,  at  Pine  Eapids,  a 
quarter  of  a  mile  below  Sawyer's  old  dam,  on  the  S.  W.  J  of  Sec.  16,  T.  42,  E.  16  W., 
is  the  most  southerly  outcrop  of  the  southward-dipping  sandstone  observed.  The 
rock  is  very  similar  to  that  above  described.    The  strike  is  N.  52°  E.  and  the  dip  16°  S. 

These  descriptions,  with  the  map  I  have  constructed  from  them  (Plate 
XXV),  will  serve  to  show  plainly  enough  that  we  have  in  this  region  some 

'Geology  of  Wisconsin,  Vol.  Ill,  p.  349. 


250       COPPEE-BEAEING  BOOKS  OF  LAKE  SUPEEIOE. 

thousands  of  feet^  of  sandstone  underlain  by  a  much  greater  thickness  of 
fine-grained,  basic  eruptive  rocks  (diabases  and  melajahyrs),  with  included 
felsitic  porphyries  and  porphyry-conglomerates;  in  other  words,  that  we 
have  the  same  Upper  and  Lower  Divisions  of  the  series  recognized  farther 
east.  The  upper  sandstones  are  the  same  as  those  seen  on  White  and  Bad 
rivers,  in  Ashland  County,  Wisconsin,  dipping  southward,  as  was  first 
shown  b}''  Mr.  E.  T.  Sweet,  who  was,  however,  in  error  in  supposing  that 
the  sandstones  exposed  on  the  Kettle  River  are  also  a  continuation  of 
these  ^  The  latter,  as  already  shown,  belong  with  the  light-colored  Potsdam 
sandstone  of  the  Mississippi  Valley.  The  Keweenawan  upper  sandstones 
do  not  extend  so  far  to  the  west. 

Douglas  County  Copper  Range. — For  12  or  14  miles  northward  from 
the  exposures  in  the  valley  of  the  Upper  Saint  Croix  the  country  is  heavily 
drift-covered,  and  no  exposures  have  ever  been  observed.  On  the  northern 
side  of  this  area,  however,  is  again  a  belt,  2  or  3  miles  in  width,  in  which 
there  are  frequent  exposures  of  bedded  Keweenawan  diabases,  dipping 
southward.  This  belt  forms  a  bold  range,  or  series  of  ranges,  north  of 
which,  and  extending  to  the  shores  of  Lake  Superior,  is  a  lowland  under- 
lain by  horizontal  sandstone.  This  district  has  been  examined  in  some  de- 
tail by  Mr.  E.  T.  Sweet,  from  whose  description^  I  cull  the  following  facts, 
quoting  his  own  words  whenever  practicable : 

The  line  of  junction  on  the  north  ^  is  somewhat  curved,  but  in  the  main  pursues  an 
B.  N".  E.  course,  nearly  parallel  with  the  strike  of  the  crystalline  strata.  *  *  *  * 
Most  of  the  northward-iiowing  streams  in  Douglas  county  leave  the  crystalline  rocks 
and  enter  upon  the  sandstone  district  through  deep  gorges  and  in  wild  and  precipitous 
falls.  In  the  walls  of  these  gorges  both  formations  are  usually  beautifully  exposed,  but 
the  sandstone,  for  a  distance  of  from  twenty  to  three  or  four  hundred  feet  from  where 
we  would  expect  to  iind  the  point  of  contact,  has  evidently  been  affected  by  some  great 
lateral  pressure,  for  we  find  the  layers  broken  into  short  lengths,  and  tilted  at  various 
angles,  generally  to  the  northwest  or  from  the  line  of  strike  of  the  crystalline  rocks. 
In  following  down  the  stream  the  sandstone  layers  in  the  walls  of  the  gorge  gradually 
show  the  effects  of  less  disturbing  influences,  and  finally  assume  horizontality  and 

'  Mr.  Sweet's  estimate,  Geology  of  Wisconsin,  Vol.  Ill,  p.  350,  is  excessive. 

^  "Notes  on  the  Geology  of  Northern  Wisconsin."  Transactions  of  the  Wisconsin  Academy  of 
Science,  Arts,  and  Letters,  Vol.  Ill,  p.  48,  1876. 

3  Geology  of  Wisconsin,  Vol.  Ill,  p.  336,  et  seq. 
■•With  the  horizontal  sandstone  just  mentioned. 


COPPEE  EANGE  OF  DOUGLAS  COUJSTTY,  WISCONSIN.  251 

regular  bedding.  On  Middle  river  the  original  lines  of  depositon  have  been  entirely 
obliterated,  and  the  very  argillaceous  sandstone  transformed  into  a  transverse  cleav- 
ing slate,  somewhat  micaceous. 

That  the  Keweenawan  eruptive  rocks  are  bedded,  in  a  certain  sense,  there  can  be  no 
doubt.  The  dip  and  strike  *  *  *  *  can  be  determined  at  almost  any  locality,  where 
the  rocks  are  exposed,  with  but  little  difficulty.  *  *  *  *  Owing  to  a  great  inequal- 
ity in  the  hardness  of  the  different  beds,  the  softest  have  been  eroded,  and  the  hardest 
and  firmest  have  remained ;  and  now  outcrop  in  the  form  of  bare  ridges  of  rock.  These 
ridge-like  exposures  are  very  prominent  and  characteristic  between  Black  river  falls 
and  the  Aminicon  river.  They  appear  to  arise  directly  from  the  soil,  like  a  great 
stone  wall,  and,  at  least  in  one  or  two  instances,  pass  across  the  country  for  a  mile  or 
more  in  an  almost  straight  line,  with  a  height  of  30  or  40  feet,  and  a  thickness  of  50  to 
100  feet.  Other  ridges  vary  in  length  from  a  few  feet  to  a  quarter  of  a  mile  or  more. 
On  the  north,  the  face  of  each  ridge  is  usually  precipitous  and  somewhat  jagged, 
owing  to  the  exposure  of  the  edges  of  the  layers.  The  south  side  descends  to  the  soil 
with  the  inclination  of  the  bedding.  Ordinarily  the  dip  is  readily  obtained,  and  the 
trend  of  the  ridges  is  usually  the  strike  of  the  formation.    East  of  the  Aminicon,  the 

wall-like  exposures  are  less  prominent. 

#**  #***## 

By  far  the  most  common  rock  is  a  greenish  to  dark-gray  diabase.  Near  the  sand- 
stones on  the  north,  this  rock  often  has  an  amygdaloidal  structure.  There  are  also 
beds  or  layers  of  amygdaloidal  diabase  1,000  or  2,000  feet  south  of  the  sandstones. 
The  amygdules  are  usually  either  epidote,  prehnite  or  chlorite.  A  less  common  rock, 
but  one  forming  massive  beds,  is  a  finegrained,  nearly  black  kind,  having  a  marked 
conchoidal  fracture,  and  differing  much  from  the  ordinary  diabase  (Pnmpelly's  "ash- 
bed  type").  Coarse-grained,  red-and-black-mottled  basic  rocks  (gabbro)  also  occur. 
Felsitic  porphyry  has  been  seen  only  on  Black  Eiver.' 

Native  copper  occurs  throughout  this  district  in  three  ways,  accord- 
ing to  Mr.  Sweet:— ^ 

1.  Indiscriminately  scattered  through  belts  of  epidotic  and  calcareous  rock  of 
various  thicknesses,  and  lying  usually  with  the  bedding  of  the  formation,  as  at  the  Per- 
cival  mine.  2.  Irregularly  disseminated  fine  particles  of  native  copper  in  the  layers 
or  beds  of  diabase,  as  at  the  Fond  du  Lac  mine.  3.  In  true  fissure  veins,  as  at  the 
Wisconsin  mine. 

Mining  has  been  attempted  at  a  number  of  points,  but  unsuccessfully. 

The  following  are  further  quotations  from  Mr.  Sweet's  descriptions: 

The  most  western  exposure  of  the  eruptive  roclcs  of  the  Western  Lake  Superior 
district  that  I  have  seen,  is  a  short  distance  west  of  the  lower  falls  of  Black  river.^ 

1  Having  examined  in  detail  a  complete  suite  of  roclcs  from  this  region,  I  find  them  identical  in 
all  respects  with  those  of  the  Kewsenaw  Series  farther  east.  See  also  E.  Pumpelly,  Geology  of  Wis- 
consin, pp.  42  and  48.    R.  D.  I. 

'  Geology  of  Wisconsin,  p.  357. 

=  Do.,p.  340. 


252       COPPER-BEAEING  ROCKS  OF  LAKE  SUPEEIOE. 

The  lower  falls  of  Black  river  are  near  the  S.  E.  corner  of  Sec.  21,  T.  47,  E.  14 
W.  In  this  vicinity,  the  exposures  are  the  largest  and  most  interesting  of  any  observed 
in  the  district.  Here  the  river  has  cut  a  gorge  through  the  crystalline  rocks  of  the 
Keweeuawan  and  the  breccia  conglomerate  and  sandstone  of  the  Lower  Silurian,  to  a 
depth  varying  from  100  to  180  feet,  and  having  a  length  of  nearly  one-half  mile.  Along 
the  walls  of  the  gorge,  the  measures  are  most  beautifully  exposed,  but  great  difficulty 
has  been  experienced  in  satisfactorily  making  out  their  relations.  *  *  *  *  At 
the  head  of  the  rapids,  which  extend  about  100  feet  above  the  falls,  a  dark-colored, 
fine-grained,  hard  diabase  of  the  ashbed  type  occurs.  Although  quite  indistinct,  it 
appears  to  have  a  bedded  structure.  At  the  immediate  head  of  the  falls,  this  is  suc- 
ceeded by  a  red  felsitic  porphyry,  and  this,  again,  by  the  common  diabase.  The  third 
bed,  over  which  is  the  main  fall,  is  a  dark-gray,  fine-grained  diabase,  having  also  an 
indistinct  bedding.  About  75  feet  above  the  foot  of  the  great  fall,  in  the  left  or  west 
wall  of  the  gorge,  there  is  a  vertical  fissure  8  inches  wide,  formed  by  two  smaller 
fissures  dipping  towards  each  other,  and  making  an  angle  of  about  40°.  Owing  to 
talus,  the  fissure  can  be  seen  only  a  distance  of  10  or  15  feet.  It  is  filled  with  a  soft, 
clay-like  sandstone.  The  walls  on  either  side  of  the  fissure  are  very  dark-colored,  soft 
and  unctuous.    The  rock  is  a  chloritic  alteration  of  the  diabase. 

Just  above  the  head  of  a  small  fall,  near  the  foot  of  the  great  fall,  a  fine-grained, 
reddish-brown  diabase  comes  in,  which  is  frequently  amygdaloidal.  The  dip  here  is  46° 
in  a  direction  about  S.  20°  E.  This  extends  along  the  wall  of  the  gorge  nearly  300  feet, 
and  gradually  grades  on  the  west  side  into  a  diabase-breccia.  The  transition,  in  fact, 
is  so  imperceptible  that  it  is  impossible  to  locate  exactly  the  point  at  which  the  dia- 
base ceases  and  the  breccia  commences.  This  is  partially  owing  to  the  fact  that  both 
diabase  and  breccia  walls  are  considerably  decomposed,  and  partially  to  the  further 
fact  that  the  breccia  contains  immense  inclosed  masses  of  the  diabase.  In  the  east 
wall  tbere  is  no  breccia.  'Near  the  southern  limit  of  the  breccia  the  rock  consists 
almost  exclusively  of  angular  grains  and  masses  of  diabase,  while  in  the  north,  there 
is  a  notable  proportion  of  reddish  sand,  which  seems  to  be  the  matrix  or  cementing 
material.  Imbedded  in  the  face  of  the  breccia  are  a  number  of  highly  indurated 
layers  of  reddish  sandstone,  from  4  to  60  feet  in  length,  and  inclined  at  different 
angles,  some  of  them  being  vertical.  About  50  feet  above  the  stream,  and  near  the 
foot  of  a  cliff  a  hundred  feet  high,  are  two  of  these  layers  resting  together.  They  are 
2  feet  thick,  40  feet  long,  straight,  and  dip  26°  to  the  IST.  70°  W.  These  layers  are 
really  quartzite.  They  have  a  dark-brown  color,  coarse  granular  structure,  and  con- 
tain a  few  disseminated  grains  of  delessite. 

One  hundred  and  fifty  feet  from  the  breccia,  in  the  left  bank  of  the  stream,  there 
is  a  bed  of  conglomerate,  arising  directly  from  the  stream,  30  feet  in  thickness. 
Interstratified  with  the  conglomerate  are  a  few  layers  of  sandy  shale.  The  dip  of 
this  bed  of  conglomerate  is  25°  in  a  direction  S.  20°  W.  The  dip,  however,  is  not 
uniform.  For  a  few  rods  down  the  stream  it  is  20°,  and  25  feet  farther,  only  15°  in  the 
same  direction.  The  pebbles  are  from  one-half  an  inch  to  three  inches  in  diameter, 
and  are  principally  white  amorphous  quartz.  About  a  third  of  them  are  diabase, 
much  more  angular  than  the  quartz  pebbles,  some  are  sandstone,  and  a  few  are  them- 
selves conglomerate.  The  matrix  is  red  sand.  On  the  east  or  right  side  of  the  stream 
there  is  also  a  bed  of  conglomerate,  which  is  underlaid  by  thin-bedded  sandstone.    This 


UNCONFORMITY  ON  BLACK  EIVER,  WISCONSIN. 


253 


bed  dips  42°  to  the  N.  10°  E.  *  *  *  *  In  this  coDglomerate  bed  there  are  white 
quartz,  gray  quartzite,  diabase  and  sandstone  pebbles,  with  reddish  sand  as  the 
cementing  material.  It  forms  so  incoherent  a  mass  that  most  of  the  pebbles  may  be 
readily  picked  out  with  the  hand.    In  other  respects  it  is  very  similar  to  the  bed  on 


Fig.  10.  Section  on  the  gorge  of  Black  Eiver,  Douglas  County,  Wis.  (after  Sweet).  1.  Diabase, 
ashbed  type.  2.  Felsitic  porphyry.  3.  Gray  diabase.  4.  Eeddish-brown  diabase.  5.  Diabase-brec- 
cia. 6.  Shaly  sandstone.  7.  Porphyry  and  qnartz-conglomerate.  8.  Unexposed.  9.  Broken  and 
inclined  red  sandstone.  10.  Horizontal  red  sandstone.  Horizontal  scale  300  feet  to  the  inch.  Ver- 
tical scale  150  feet  to  the  inch. 

the  west  side  of  the  stream.  The  space  above  the  conglomerate  for  about  50  feet  is 
covered  with  talus  which  has  fallen  down  from  the  nearly  vertical  cliff  of  diabase. 
Many  hundred  tons  weight  have  recently  fallen  from  this  cliff".  All  of  the  fragments 
are  very  small,  angular,  find  much  weathered.  Numerous  stains  of  copper  carbonate 
were  noticed  in  the  fragments.    The  dip  here  appears  to  be  36°  to  the  SE. 

On  the  opposite  side  of  the  stream,  above  the  conglomerate,  there  are  two  small  ex- 
posures of  shaly  sandstone.  The  upper  exposure  dips  29°  to  the  S.  20°  W.,  directly 
towards  a  diabase  cliff'  only  50  feet  distant  and  40  feet  high.  Clinging  to  the  face  of 
the  cliff  are,  at  two  or  three  places,  patches  of  diabase-breccia,  a  foot  or  more  thick. 
Seventy-five  or  one  hundred  feet  northwest  from  the  cliff  of  diabase,  in  the  wall  of  the 
gorge,  dark-reddish  and  somewhat  indurated  sandstones  are  found.  The  layers  at 
first  are  broken  into  blocks  from  4  to  10  feet  in  length,  and  are  inclined  at  various 
angles,  usually  to  the  NW.  Gradually  they  assume  more  and  more  of  a  distinctly 
bedded  structure,  and  finally,  in  the  distance  of  a  few  hundred  feet,  grade  into  reg- 
ularly bedded  reddish  sandstone,  with  a  dip  of  only  4°  or  5°  to  the  N  W.  The  nearly 
horizontal  sandstone  is  found  in  the  banks  of  the  stream  at  intervals  for  several  miles. 
It  resembles  in  all  respects  that  occurring  along  the  lake  shore  in  Bayfield  County 
and  at  the  base  of  the  Apostle  Islands.  In  the  east  wall  of  the  gorge,  about  100  feet 
west  of  the  conglomerate,  there  is  a  perpendicular  ledge  of  the  sandstone  over  100 
feet  high.  The  layers  near  the  top  are  thin  and  shaly,  and  dip  30°  in  a  direction  N. 
10°  W.  Near  the  bottom,  the  layers  have  the  same  inclination,  but  are  coarse-grained 
and  from  6  to  8  feet  in  thickness.  Scattered  through  the  layers  are  diabase  and 
quartz  pebbles  an  inch  or  less  in  diameter.  Several  hundred  feet  still  farther  along 
the  gorge,  the  layers  upon  this  side  become  nearly  horizontal,  and  are  often  interstrat- 
ified  with  yellowish  and  white  sandstone. 


254       COPPEE-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 

'By  the  tortuous  course  of  the  stream,  the  Upper  Fall  of  Black  river  is  found 
IJ  miles  above  the  Lower  Fall.  Diabase  crops  out  iu  low  ledges,  frequently  ou  either 
side  of  the  very  rapid  current.  The  fall  is  near  the  east  quarter-post  of  Sec.  28,  T.  47, 
E.  14  W.  The  descent  is  31  feet  vertical,  over  a  layer  or  bed  of  diabase  dipping  about 
40°  to  the  S.  30°  E.  One  hundred  feet  below  the  fall,  in  the  bank  of  the  river,  amyg- 
daloidal  diabase  occurs,  having  the  characteristic  greenish  color  from  the  presence 
of  epidote.  About  200  feet  above  the  fall  amygdaloid  again  comes  in.  This  is  the 
most  southern  exposure  found  on  Black  river. 

#  #,#  *  *  #  *  *  * 

^  Starting  oii'  in  a  northeasterly  direction  from  the  Lower  Palls  of  Black  river, 
is  a  somewhat  broken  line  of  outcrops,  in  the  form  of  I'ounded  rock  ridges.  All  trend 
in  a  more  or  less  N.E.  and  S.W.  direction ;  and  vary  in  length  from  a  few  feet  to  a  hun- 
dred yards. 

•^  •Tp  '^  'fe  rfr  'ff  ^  ^  P^ 

^At  Copper  creek,  the  Keweenawan  beds  are  found  for  a  distance  of  nearly  one 
mile  along  the  channel  of  the  stream,  and  in  places  arise  to  the  height  of  about  100 
feet  above  it.  In  the  most  southern  outcrops  the  predominating  rock  is  a  dark-colored 
diabase.  In  the  most  northern,  the  rock  is  usually  the  more  common  reddish  altered 
diabase.  #  #  *  *  From  the  union  of  the  forks,  the  creek  has  a  tortuous  course  in 
a  general  northwesterly  direction.  It  passes  through  a  gorge  or  valley,  a  little  wider 
than  the  canon  of  Black  river.  The  rock  on  each  side,  for  a  quarter  of  a  mile,  is  close- 
grained,  dark  reddish-gray  diabase,  which  frequently  rises  in  great,  smooth,  rounded 
or  sloping  exposures,  to  the  height  of  from  100  to  135  feet  above  the  stream.  Along 
the  stream  the  bedding  is  quite  indistinct.  The  layers,  if  such  they  may  be  called,  are 
often  50  to  60  feet  in  thickness.  The  most  northern  exposure  in  the  banks  of  the 
stream  is  on  the  west  or  left  side,  and  slopes  from  the  creek  at  an  angle  of  30°  or  40° 
to  the  height  of  90  feet.  The  rock  is  a  dark  gray  amygdaloidal  diabase,  which  readily 
weathers  to  a  greasy-feeling,  soft  chloritic  rock.  It  breaks  into  small,  sharp  angular 
pieces,  which  is  due  to  the  almost  innumerable  small  seams  or  joints  traversing  the 
rock  in  every  direction.  Ou  the  north  of  this  exposure  there  is  a  gully,  about  15  feet 
across,  showing  nothing  except  sand,  clay,  and  loose  rocks  from  the  adjacent  cliffs. 
In  the  north  side  of  the  gully,  there  are  short  layers  of  sandstone  having  a  high  dip 
to  the  northwest.  Immediately  adjoining  them  is  a  cliff  of  dark  reddish,  coarse- 
grained, somewhat  indurated  sandstone.  The  layers  are  broken  into  short  lengths, 
and  dip  60°  to  the  northwest.  The  bedding  for  a  hundred  feet  is  not  well  marked,  and 
for  several  hundred  feet  still  farther  along  the  stream,  it  shows  distinct  evidences  of 
having  at  some  time  been  subjected  to  great  lateral  pressure,  for  the  layers  are  broken 
into  lengths  of  from  2  to  10  feet,  the  thickest  often  presenting  the  appearance  of  a 
transverse  conchoidal  fracture.  Between  the  beds  are  frequently  thin  layers  of  fine- 
grained white  sandstone,  some  portions  of  which  have  been  manufactured  into  grind- 
stones, said  to  be  of  very  fine  quality.  Many  of  the  reddish  layers  are  themselves 
mottled,  by  containing  spheres  of  white  sandstone  a  quarter  of  an  inch  or  more  in 
diameter.  Iu  the  course  of  a  few  hundred  feet  the  broken  layers  gradually  assume 
regular  and  undisturbed  bedding,  having  a  dip  of  only  4°  to  the  N.W.    The  layers 

■  Geology  of  Wisconsin,  Vol.  Ill,  p.  343.  ^Do.,  p.  344. 


COPPEE  EANGE  OP  DOUGLAS  COUNTY,  WISCOI^SIN. 


255 


are  from  one  half  iucli  to  two  feet  tliick,  often  finely  cross-laminated,  and  frequently 
show  most  beautiful  ripple  marks.  They  are  also  finer  grained,  and  of  a  much  lighter 
red  color  than  the  broken  layers. 


Fig.  11. 


Section  in  tlie  gorge  of  Copper  Creek,  Doiigla 
Talus.    3.  Broken  and  inclined  sandstone. 


NW: 


i  Co.,  Wis.     (Afcer  Sweet.)     1.  Diabase.    2. 
4.  Horizontal  red  sandstone. 


The  following  section  illustrates  the  relations  of  the  diabases  and  sandstones  in 
the  left  bank  of  the  creek.  On  the  opposite  side  they  were  not  exposed  together.  It 
will  be  observed  that  conglomerate  and  breccia  do  not  occur.  No  sandstones  were 
observed  in  the  vicinity  of  Copper  creek,  except  in  the  banks  of  the  stream. 


'  About  one  mile  northeast  from  the  mining  location  at  Copper  creek,  there  com- 
mences a  very  remarkable  exposure.  It  is  in  the  form  of  a  rock  ridge  or  great  wall  of 
stone,  almost  perfectly  straight,  nearly  a  mile  long,  40  feet  high,  jagged  and  nearly 
vertical  on  the  north  side;  the  top,  for  a  considerable  distance,  as  smooth  and  level  as 
a  sidewalk,  from  10  to  30  feet  wide ;  the  south  side,  even  and  sloping,  and  20  or  30  feet 
above  the  soil.  *  *  *  *  2  The  trend  of  the  ridge  is  N.  55°  E.,  which  is  exactly  the 
direction  of  the  strike,  and  the  dip  is  36°  S. 

Passing  off  from  the  eastern  end  of  this  exposure,  there  are  two  lines  of  similar, 
but  much  smaller,  outcrops.  One  goes  nearly  east  to  the  Aminicon  Eiver,  in  Sec.  17, 
where  it  divides  into  two  or  three  general  lines,  the  individual  outcrops  lapping  past 
each  other,  and  each  preserving  its  trend  of  IST.  55°  to  60°  B.  The  second  passes  north- 
easterly across  Sec.  12,  and  Sees.  7  and  8,  T.  47,  E.  13  W.  Here  the  exposures  are 
not  numerous,  but  are  in  a  nearly  straight  line.  Most  of  them  are  in  theTorm  of  short 
ridges.  *  *  *  *  Upon  Sec.  1,  directly  southeast  from  the  Wisconsin  mine, 
exposures  are  very  numerous.  They  are  usually  short  and  ridge-like,  varying  in  length 
from  a  few  feet  to  an  eighth  of  a  mile;  in  width,  at  the  surface  of  the  drift,  from  ten  or 
20  to  200  feet;  and  in  height  from  ten  to  seventy-five  feet.  They  trend  from  20°  to  30° 
east  of  north ;  somewhat  nearer  north,  it  will  be  observed,  than  those  a  few  miles  to  the 
west.  They  are  separated  from  each  other  by  drift-filled  valleys  from  100  to  700  feet 
wide.  A  short  distance  to  the  east  of  most  of  the  small  ridges  is  one  very  prominent 
ridge,  or  rather  mound-like  exposure,  several  hundred  feet  long  and  75  feet  high.  The 
northern  face  is  very  precipitous,  and  the  bedding  is  very  distinct.  The  dip  is  30°  to 
the  S.  60°  E. 


1  Geology  of  Wisconsin,  Vol.  Ill,  p.  345. 


^Do.,  p.  346. 


256       COPPEE-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 

'Kear  the  southeast  corner  of  Sec.  32,  T.  48,  E.  12  W.,  is  the  most  southern  ex- 
posure in  the  banks  of  the  Aminicon  river.  There  are  several  between  here  and  the 
crossing  of  the  Superior  and  Bayfield  post-road,  a  half  mile  below.  The  rock  is  a 
coarse-grained  gabbro,  composed  of  red-stained  plagioclase,  a  gray  plagioclase,  and  a 
soft  greenish  altered  (chloritic)  diabase,  with  bright,  shining  grains  of  magnetite. 
Below  the  bridge,  a  dark-colored,  coarse-grained  variety  occurs,  in  which  the  several 
minerals  show  much  less  alteration. 

'Kearthe  east  and  west  line,  running  between  Sees.  20  and  29,  is  the  junction  of 
the  reddish  sandstones  with  the  crystalline  rocks.  For  25  feet  only  have  the  sand- 
stones been  disturbed.  In  this  distance  they  are  broken  into  short  lengths,  and  dip 
northwest  from  the  crystalline  strata  at  angles  varying  from  60°  to  20°,  after  which 
they  become  horizontal  and  show  two  well-marked  systems  of  vertical  joints.  The 
direction  of  one  system  is  N.  60°  E.,  and  of  the  other  N.  and  S.  At  the  immediate 
line  of  junction,  or  where  the  sandstone  is  removed  from  the  crystalline  rocks  but  a 
very  few  inches,  the  layers  of  sandstone  have  a  facing  of  fine  conglomerate,  from  a 
few  inches  to  a  foot  or  more  in  thickness.  The  pebbles  consist  of  quartz  and  melaphyr 
or  diabase,  and  are  cemented  by  sand.  The  sandstone  layers  at  some  distance  from 
the  junction  also  contain  small  pebbles  of  the  adjacent  crystalline  rocks.  The  sand- 
stone layers  are  much  softer  than  usual,  uniformly  reddish  in  color,  and  in  thickness 
vary  from  1  to  24  inches. 

Between  the  Aminicon  and  Middle  rivers  there  are  very  few  exposures  of  the 
copper- bearing  rocks,  and  none  in  Sees.  33  and  34,  where  we  would  expect  to  find 
them.  The  country  is  somewhat  lower  than  the  remainder  of  the  range,  and  covered 
with  drift.  Near  the  center  of  Sec.  4,  T.  47,  E.  12  W.,  there  are  two  small  exposures 
of  fine-grained,  blackish  diabase,  separated  by  a  few  rods  of  drift.  Similar  rocks  are 
again  visible  in  Sec.  3,  and  along  the  banks  of  Middle  river,  for  about  a  mile  in  sec- 
tions 2  and  3.5.    They  are  all  small  exposures.    In  Sec.  35  the  rock  is  often  porphyritic. 

A  iierfect  Jietwork  of  minute  laumontite  veins  occurs  at  one  or  two  localities. 

********* 

2  Near  the  south  line  of  Sec.  24,  T.  48,  E.  12  W.,  Middle  river  cuts  through  the 
northern  face  of  the  range,  leaving  a  large  surface  of  the  rock  exposed.  On  the  south, 
the  stream  has  exposed  between  300  and  400  feet  of  a  dark-brown  diabase,  being  in 
places  somewhat  amygdaloidal.  This  outcrop  is  nowhere  more  than  25  feet  in  height. 
A  hundred  yards  northwest  of  it,  in  the  left  bank  of  the  stream,  is  an  exposure  of 
soft,  dark- reddish,  much  altered  and  decomposed  diabase-amygdaloid,  carrying  numer- 
ous small  veins  or  "  strings  "  of  laumontite  and  calcite.  This  rock  weathers  easily,  and 
is  rapidly  crumbling  away.  It  is  very  similar  to  that  described  as  occurring  at  the 
forks  of  Copper  creek.  Some  of  the  small  veins  of  calcite  were  noticed  stained  with 
copper  carbonate.  This  rock  is  found  along  the  stream  for  200  feet,  is  about  50  feet  in 
height,  and  is  very  indistinctly  bedded.  On  the  north  it  is  separated  from  a  south- 
ward-dipping and  shaly  rock  by  a  gully  30  feet  across. 

********* 

^Near  the  crystalline  rock,  the  bedding  has  been  entirely  obliterated,  and  the  out- 
crop presents  the  appearance  of  a  bed  of  shale  dipping  79°  to  the  S.  20°  E.    The 
layers  are  from  one-sixteenth  of  an  inch  to  an  inch  in  thickness.    All  of  them  contain 
flakes  of  mica,  and  the  most  of  the  layers  are  reddish-colored.    But  thin,  hard,  light- 
'  Geology  of  Wisconsin,  Vol.  Ill,  p.  346.  ^Do.,  p.  347. 


COPPEE  EANGE  OF  DOUGLAS  COUNTY,  WISCONSIN.  257 

colored,  slaty  layers  occnr  between  the  reddish,  shaly  layers.  In  the  course  of  75  feet, 
the  exposure  begins  to  show  evidences  of  horizontalbedding  with  a  transverse  slaty 
cleavage,  as  shown  in  the  cut.  Beyond  this,  the  layers  are  broken  and  dip  somewhat, 
and  only  one  or  two  of  them  show  the  slaty  cleavage.  After  another  bed  of  slate 
and  shale,  as  shown,  and  attempts  at  slaty  cleavage,  the  rock  becomes  evenly  bedded 
and  horizontal.  It  is  fine-grained,  and  more  aluminous  than  is  usual  with  the  Lake 
Superior  sandstones.  There  are  no  conglomerates,  breccias,  or  thick  layers  of  coarse 
sandstone  seen  in  the  vicinity. 

Eastward  from  Middle  river,  neither  the  copper-bearing  rocks  nor  sandstones  are 
exposed  for  a  distance  of  about  7  miles.  Between  Middle  river  and  the  high  ground, 
locally  known  as  the  BruM  range,  the  country  is  quite  level,  somewhat  swampy,  and  has 
an  elevation  of  about  400  feet.  The  Brul6  range,  commencing  in  T.  47,  E.  11  W.,  attains 
an  altitude  on  the  broad  summit  of  about  540  feet.  Towards  the  western  end  are  two 
or  three  small  exposures  of  fine-grained,  dark-gray  diabase.  On  Sec.  29,  T.  48,  E.  10 
W.,  is  an  exposure  of  diabase,  fine  conglomerate  and  sandstone,  very  similar  to  that 
described  as  occurring  on  the  Aminicon  river.  In  the  NE.  i  of  Sec.  28,  at  the  falls 
of  a  small  stream,  is  an  exposure  of  dark-gray  amygdaloidal  diabase,  a  few  feet  in 
height,  and  10  or  12  yards  long.  A  test  pit  was  sunk  here  on  what  appears  to  be  a 
small  quartz  and  epidote  vein  bearing  altered  amygdaloid,  by  the  North  American 
Fur  Company,  in  1847.  A  small  quantity  of  iron  pyrites  and  copper  carbonate  were 
observed  with  the  debris.    The  dip  here  is  30°  S.  40°  E. 

No  exposures  are  found  from  here  to  the  Percival  mining  location,  a  mile  to  the 
east.  At  this  location  the  natural  exposures  are  small  and  few  in  number,  but  the 
underlying  rock  is  lightly  covered  with  drift,  which  may  be  easily  removed.  The  rock 
is  usually  a  dark  greenish-gray  amygdaloidal  diabase,  carrying  in  places  shot  and 
nugget  copper.  The  bedding  is  not  very  distinct.  At  the  wagon-bridge  across  the 
BruM,  on  Sec.  23,  and  occasionally  along  the  banks  of  the  stream  for  a  half  mile 
below,  there  outcrop  low  ledges  of  a  dark-colored  amygdaloidal  diabase.  A  quarter 
of  a  mile  below  these  exposures  there  are  low  ledges  of  nearly  horizontal,  reddish, 
heavily-bedded  sandstone  in  the  banks  of  the  stream.  Similar  sandstones  are  found 
along  the  banks  and  in  the  channel,  forming  numerous  rapids  nearly  to  the  mouth  of 
the  river.  No  exposures  were  found  on  the  steep  sides  of  the  valley,  although,  in  the 
vicinity  of  the  sandstones  and  crystalline  rocks,  it  is  over  200  feet  deep,  and  not  half 
a  mile  across  from  summit  to  summit.  On  the  northeast  quarter  of  Sec.  23,  a  short 
distance  north  of  the  Bayfield  road,  there  is  a  very  prominent  and  high  exposure,  form- 
ing the  western  end  of  that  portion  of  the  range  east  of  the  Brule  river.  The  summit 
of  the  bare,  bald  cliff  is  301  feet  above  the  bridge  across  the  stream  at  its  foot,  or  553 
feet  above  Lake  Superior. 

********  # 

'From  the  altitudes,  trend,  and  general  appearance  of  the  ridges  or  abrupt  ele- 
vations to  the  south,  a  few  miles  from  the  lake  shore,  in  Bayfield  county,  we  should 
expect  to  find  crystalline  rock  exposures,  occasionally,  as  in  Douglas  county.  The 
streams  are  numerous,  and,  like  those  to  the  west,  cut  deep  and  narrow  valleys 
through  the  drift,  near  the  abrupt  ascent  of  the  country  to  the  south,  and  nearly  all 
expose  small  ledges  of  horizontal,  reddish  Lake  Superior  sandstone,  but  none  of  them 
appear  to  have  uncovered  the  Keweenawan  rocks.    In  the  southwest  part  of  T.  50,  E. 

'  Geology  of  Wisconsin,  Vol.  Ill,  p.  348. 
17  L  S 


258       COPPEE-BEAEIXG  ROCKS  OF  LAKE  SUPEEIOK. 

7  W.,  between  Erog  and  Cranberry  rivers,  there  is  a  ridge  having  an  altitude  of  over 
400  feet,  and  covered  with  many  large  angular  bowlders  of  diabase  and  other  Ke- 
weenawan  eruptive  rocks.  It  presents  a  striking  resemblance  to  the  range  a  short 
distance  west- of  the  BruM  river.  On  the  NE.  ^  of  Sec.  20,  T.  50,  E.  6  W.,  a  few 
rods  north  of  the  shore  of  Siskowit  lake,  there  is  a  flat  circular  bowlder  of  brecciated 
conglomerate,  18  feet  in  diameter.  A  few  of  the  angular  and  some  of  the  round 
pebbles  are  diabase,  but  the  most  of  them  are  felsitic  porphyry.  The  matrix  is  red- 
dish sand.  The  bowlder  is  evidently  stratified,  and,  as  it  lies,  has  an  inclination  of 
20°  S.  Owing  to  the  frail  nature  of  the  rock,  it  probably  never  traveled  any  consid- 
erable distance.  This  is  one  of  the  numerous  so-called  outcrops  of  crystalline  rocks 
to  which  I  was  directed  by  some  of  the  citizens  of  Bayfield.  Others  were  found  to  be 
large  bowlders,  trains  of  bowlders,  or  ledges  of  sandstone. 

The  most  important  points  established  by  Mr.  Sweet's  investigations 
in  this  district  are  (1)  the  identity  between  the  cupriferous  rocks  here  ex- 
posed and  those  of  Keweenaw  Point ;  (2)  the  existence  of  a  rather  low 
southern  dip,  and  of  a  northeasterly  strike,  ranging  from  east-northeast  on 
the  west,  to  noilh-northeast  on  the  east ;  (.'i)  the  absence  of  diabase  ledges 
on  the  highlands  of  Bayfield  County;  (4)  the  unconforamble  contact  be- 
tween the  red  sandstone  which  borders  Lake  Superior  west  of  the  Monti-eal, 
and  the  Keweenawan  beds. 

The  contacts  between  these  formations  described  by  Mr.  Sweet  as 
showing  on  Black  River,  Copper  Creek,  Aminicon  River,  and  Middle  River, 
are  also  of  great  interest  on  account  of  the  peculiar  disturbances  they  pre- 
sent. These  distui'bances  find  their  explanation,  in  part,  as  it  appears  to 
me,  in  the  irregularities  of  an  unconformable  contact,  and  in  the  pressure 
exerted  by  the  deep-seated  Keweenawan  beds  against  the  more  shallow 
sandstone,  but  also  in  large  measure  in  a  faulting  that  has  taken  place 
along  the  contact  line.  There  is  much  in  common  between  this  fault  and 
that  on  the  south  side  of  Keweenaw  Point.  There  are  only  two  ways  in 
which  the  conclusion  might  be  avoided  that  we  have  here  to  deal  with  a 
true  non- conformable  contact.  These  are  to  suppose,  either,  as  Whittlesey^ 
and  Norwood^  did,  that  the  crystalline  rocks  here  ai'e  true  intrusive  or  dis- 
turbing masses,  or  dikes,  and  hence  newer  than  the  sandstone  with  which 
they  are  now  in  contact,  or  that  the  sandstones  belong  to  the  Upper  Division 
of  the  Keweenaw  Series,  let  down  here  by  a  great  fault  to  a  lower  level 
than  that  occupied  by  the  diabases  which  belong  normally  under  them. 

'  "Physical  Geology  of  Lake  Superior."    Proc.  Am.  Assoc.  Adv.  Sci.,  Detroit  Meeting,  1875,  p.  67. 
^  Owen's  Geological  Survey  of  Wisconsin,  Iowa,  and  Minnesota,  p.  305. 


POSITION  OP  THE  WESTEEN  SANDSTONE.  259 

The  former  of  those  suppositions  is,  however,  at  once  forbidden  by  the 
bedded  structure  of  the  crystalline  rocks,  by  their  manifest  identity  with 
the  bedded  rocks  of  Keweenaw  Point,  which  are  never  dikes,  and  also  by 
the  abu.ndant  occurrence  in  the  sandstone,  near  its  contact  with  the  crystal- 
line  rocks,  of  pebbles  worn  from  these  rocks.  The  second  supposition  is 
much  more  plausible.  It  has  against  it,  however,  the  nature  of  the  sand- 
stone— which  resembles  more  nearly  the  Eastern  Sandstone  of  Keweenaw 
Point  than  the  sandstones  of  the  Keweenaw  Series — and  also  the  occur- 
rence in  it  of  the  pebbles  just  mentioned.  Were  these  pebbles  merely  such 
as  are  apt  to  occur  in  the  conglomerate  bands  of  the  upper  Keweenawan 
sandstones,  they  would  not  be  restricted,  as  they  are,  to  the  immediate  con- 
tact of  the  two  formations,  but  would  be  regularly  distributed  through  the 
sandstone  without  any  reference  to  the  faulting  line. 

But  a  still  greater  difficulty  to  overcome  on  this  theory  is  one  con- 
nected with  the  general  structural  features  of  this  part  of  the  Lake  Supe- 
rior trough.  If  this  sandstone  belongs  with  the  Keweenaw  Series  at  all,  i.  e., 
belongs  with  the  diabases  in  contact  with  it,  then  it  must  belong  with  the 
Upper  Division  of  the  series.  This  must  be  so,  because  it  is  continuous 
with  the  horizontal  sandstones  of  the  Apostle  Islands  and  Bayfield  County 
coast,  which,  if  they  were  to  be  placed  in  the  Keweenaw  Series  at  all,  must 
belong  in  its  Upper  Division,  because  they  lie  directly  across  the  trough  of  the 
north  Wisconsin  synclinal,  where  only  upper  Keweenawan  sandstones  could 
lie.  Now,  westward  from  the  contacts  on  the  Black  and  other  rivers  of  Doug- 
las County,the  same  sandstonecontinues until, ontheSaintLouisRiver,above 
Fond  du  Lac,  it  is  found  resting  on  the  Huronian  slates  in  such  a  manner 
as  to  render  certain  its  original  deposition  in  that  position.  Westward 
from  the  eastern  point  of  the  contact  in  Douglas  County,  between  this  sand- 
stone and  the  south-dipping  diabases,  the  contact  line  cuts  across  the  Ke- 
weenawan belts  until  it  finally  reaches  the  Huronian  slates.  It  is  impossi- 
ble to  conceive  of  a  fault  by  which  the  upper  sandstone  could  have  been 
let  down  into  such  a  position.  There  thus  seems  to  be  no  escape  from  the 
conclusion  that  in  the  Western  Sandstone  we  are  deahng  with  the  same 
overlying  sandstone  that  presents  itself  on  the  south  side  of  Keweenaw 
Point,  and  that  the  Douglas  County  contact  line  is  one  of  unconformability 
complicated  by  faulting. 


CHAPTER   VII. 

THE   KEWEENAWAN    ROCKS    OF    THE    NORTH  AND   EAST 
SHORES  OF  LAKE  SUPERIOR. 

Introductory. — Large  rock  exposures  of  tlie  North  and  East  Shores. — Contrast  between  the  North 
and  South  Shores. — Distribution  of  the  Keweenawan  rocks  of  the  North  and  East  Shores. 

Section  I.  The  Minnesota  coast. — Examinations  made  on  this  coast. — Lakewarddip  of  the  rocks. — 
General  review  of  relation  of  coast  line  to  strike  and  dip  of  the  rocks. — Crescentic  courses  of  the 
rock  belts  of  this  region. — Stratigraphical  succession  of  the  Minnesota  coast. — The  several 
subordinate  groups  described  in  detail;  the  Saint  Louis  gabbros;  the  Duluth  Group;  the  Lester 
Eiver  Group ;  the  Agate  Bay  Group ;  the  Duluth,  Lester  River,  and  Agate  Bay  Groups  at  the  east 
end  of  the  Minnesota  coast;  the  Beaver  Bay  Group;  the  Temperance  Eiver  Group. 

Section  II.  Isle  Eoyale  and  the  neighboring  mainland  to  Nipigon  Bay. — Eelation  of  the  Isle 
Eoyale  rocks  to  those  of  the  Minnesota  coast. — Sources  of  information  with  regard  to  this 
region. — Isle  Eoyale  and  its  geology. — The  ICeweeBawan  rocks  of  the  mainland  between  Thunder 
Bay  and  Nipigon  Bay. — The  Nipigon  Lake  basin  and  its  geology. 

Section  III.  Michipicoten  Island  and  the  east  coast. — Michipicoten  Island;  accounts  of  its 
geology,  by  Logan  and  Macfarlane. — Microscopic  examinations  of  its  rocks. — The  east  coast; 
accounts  by  Logan  and  Macfarlane. — Cape  Choyye. — Pointe  aux  Mines. — Mamainse. — Batche- 
wanung  Bay. — Gros  Cap. 

The  north  and  east  coasts  of  Lake  Superior  together  form  one  of  the 
iinest  lines  of  rock  exposure  in  the  world.  From  Duluth  to  the  Sault — 
a  distance  of  over  600  miles,  without  taking  into  account  any  but  the 
greatest  indentations  —  the  rocks  are  in  nearly  continuous  exposure. 
Short  pebble  beaches,  usually  not  more  than  a  few  rods  in  extent,  and  very 
rarely  over  a  mile,  here  and  there  interrupt  the  absolute  continuity  of  the 
exposure  ;  but  even  in  these  places  the  rocks  on  either  side  of  the  gap  may 
often  be  connected  by  outcrops  in  the  woods  behind,  by  islands  in  front,  or 
by  continuous  rock  surfaces  not  too  far  beneath  the  water.  Frequently  the 
exposures  are  abrupt  cliffs  rising  from  the  water's  edge  to  a  height  more 
commonly  from  20  to  60  feet,  less  commonly  from  50  to  1,000  feet.  Only 
rarely  are  the  exposures  of  soft  rocks  like  sandstone  ;  and  for  the  most  part 
they  are  of  some  sort  of  crystalline  rock. 

The  North  Shore  thus  stands  sharply  contrasted  with  the  South  in  its 

260 


EOCKS  OP  THE  NOETH  AND  EAST  SHOEES.        261 

scenic  characteristics.  Except  over  short  distances  between  Marquette  and 
Keweenaw  Bay,  on  the  north  shore  of  Bete  Grise  Bay,  and  on  the  north 
side  of  Keweenaw  Point,  the  south  coast  of  Lake  Superior  shows  only 
sandstone  or  conglomerate  as  the  shore  rock,  while  even  these  rocks  are 
absent  for  fully  200  miles  of  the  distance  between  Duluth  and  the  Sault, 
counting  only  those  interruptions  to  rock  exposures  which  are  more  than 
two  or  three  miles  in  length.  Low  cliffs  of  sandstone  are  met  with  at 
several  points  on  the  South  Shore,  but  the  largest  do  not  exceed  75  feet  in 
height.  Moreover,  except  on  the  eastern  part  of  Keweenaw  Point,  and  in 
the  Huron  and  Porcupine  Mountains,  there  is  either  no  high  ground  or  it  is 
so  far  inland  as  to  have  little  influence  on  the  scenery  of  the  shore. 

From  the  head  of  the  lake  to  Grand  Portage  Bay,  or  nearly  to  the 
national  boundary  line,  a  distance  of  about  150  miles,  typical  Keweenawan 
strata  form  the  coast.  At  Grand  Portage,  slates,  with  interbedded  and 
intersecting  diabases  and  gabbros,  rise  from  beneath  the  Keweenawan  beds. 
These  rocks  make  up  the  so-called  Animikie  Group  or  "Lower  Division  of 
the  Copper-Bearing  Series,"  which  I  take,  however,  to  be  undoubtedly 
Huronian.  These  slates  form  Pigeon  Point,  the  north  and  west  sides  of 
Thunder  Bay,  and  the  islands  at  its  mouth,  including  Thunder  Cape.  Isle 
Royale  is  composed  of  Keweenawan  strata,  undoubtedly  in  part  the  con- 
tinuation of  beds  seen  on  the  Minnesota  coast.  The  peninsula  between 
Black  and  Nipigon  Bays  is  composed  chiefly  of  sandstones  belonging  at  the 
base  of  the  Keweenaw  Series,  while  typical  Keweenawan  diabases,  amyg- 
daloids  and  interbedded  sandstones  and  porphyry-conglomerates,  with  the 
usual  massive  porphyries,  form  the  coast  and  its  numerous  flanking  islands 
from  Black  Bay  to  the  east  end  of  the  Battle  Islands,  a  distance  of  some 
75  miles.  Beyond  the  Battle  Islands  the  north  and  east  coast  is  formed 
chiefly  of  ancient  gneisses  and  crystalline  schists,  part  of  which  possibly 
belong  with  the  Huronian,  although,  as  indicated  in  a  subsequent  chapter, 
this  still  remains  a  matter  of  some  doubt.  Along  the  eastern  coast,  how- 
ever, as  far  as  the  Sault,  Keweenawan  beds  form  now  and  then  projecting 
headlands — in  the  case  of  the  Cape  of  Mamainse  reaching  a  very  consid- 
erable development.  Michipicoten  Island  is  also  made  up  of  the  same 
formation. 


(;K<)I.f)C;i(AI.  MAP  <)i  Tin;    NOKTII    WIOHTKI  i.\"     COAsr    .)]     I.AKIl      S  ITI'-K' lOI.' . 


THE  MINNESOTA  COAST.  263 

porphyry.     In  the  gap  between  this  gabbro  and  the  slate  on  the  Saint  Louis, 
the  base  of  the  Keweenaw  Series  Hes  concealed. 

The  Saint  Louis  River  slates  at  Thompson,  eight  miles  west  of  Fond  du 
Lac,  trend  N.  85°  E.,  and  have  a  dip  of  some  30^  to  40°  southward.  Far- 
ther down  stream,  towards  the  junction  with  the  overlying  sandstone,  they 
trend  somewhat  more  to  the  nortlieast.  The  gabbro  does  not  exhibit  any 
sign  of  bedding.  However,  the  trend  of  the  hills  which  it  forms  in  the 
southeast  part  of  T.  49,  R.  15  W.,  is  about  N.  46°  E.,  and  near  Duluth  still 
more  to  the  north.  Eighteen  miles  north,  of  Duluth,  on  the  Clo^uet  River, 
in  the  southern  part  of  T.  53,  R.  14  W.,  the  same  gabbro  reappears.  It 
seems  to  form  a  belt  running  at  first  northeast  and  then  more  and  more  to 
the  north  until  it  finally  takes  a  nearly  northerly  course.  So  far  as  these 
facts  go  there  is  no  definite  evidence  of  unconformity  between  the  gabbro 
and  the  Saint  Louis  slates.     The  appearance  is  rather  the  other  way. 

In  the  eastern  part  of  the  city  of  Duluth  we  begin  to  find  plainly  bed- 
ded rocks  flanking  the  coarse  gabbro  on  the  east,  and  for  the  entire  dis- 
tance to  Grand  Portage  Bay  these  bedded  rocks  prevail,  as  also  back  in 
the  country  for  many  miles.  They  are  diabases  of  several  kinds;  amyg- 
daloids;  typical  luster-mottled  raelaphyrs,  or  fine-grained  oli vine-diabases ; 
coarse-grained  gabbros,  belonging  chiefly  xo  the  orthoclase-free  kinds,  but 
including  also  orthoclase-gabbros;  anorthite-rock;  felsites;  quartz-porphy- 
ries; granitic  porphyries;  porphyry-conglomerates;  and  red  sandstones 
and  shales.  In  other  words,  we  find  here  precisely  the  same  rocks  that 
characterize  the  Keweenaw  Series  on  the  South  Shore.  Detrital  beds  are 
here  relatively  rare,  and  the  layers  thin,  as  compared  with  those  of  the  Ke- 
weenaw Point  series,  but  we  have  here  to  do  with  quite  low  horizons  which 
are  in  general  comparatively  free  from  detrital  layers. 

At  Duluth,  as  already  said,  the  trend  of  the  layers  in  sight  on  the  coast 
is  at  first  even  slightly  west  of  north,  with  an  easterly  dip  of  45°.  These 
are  rapidly  changed  for  a  due  northerly  trend,  and  25°  easterly  dip,  and 
these  again,  by  the  time  the  mouth  of  the  Lester  River,  which  is  5J  miles 
below  Duluth,  is  reached,  for  a  N.  30°  E.  trend,  and  a  15°  S.  E.  dip.  Be- 
tween Lester  and  French  rivers,  the  strike  directions  make  more  and  more 
easting,  becoming  finally,  on  French  River,  12  J  miles  below  Duluth,  N.  50°  E., 


264       COPPEK-BEAEING  EOCKS  OP  LAKE  SUPEEIOE. 

the  dip  remaining  at  15°  S.  E.  From  Duluth  to  French  River  the  coastline 
trends  about  N.  50*^  E.,  so  that  in  this  distance  the  rock  beds,  running  more 
to  the  north  than  the  coast  line,  intersect  it  at  an  angle  which  varies  from 
55°  near  Duluth  to  nearly  0°  at  French  River.  One  interesting  result  of 
this  relation  between  the  trend  of  the  sti-ata  and  that  of  the  coast  line  is  the 
production  of  points  projecting  southwestward  and  formed  of  the  harder  beds. 
It  is  these  projecting  points,  with  other  smaller  ones  along  the  Minnesota  coast, 
that  have  been  represented  by  Norwood  as  formed  of  a  series  of  dikes.  As 
shown  below,  dikes  exist  here,  but  they  are  relatively  very  infrequent,  and 
nearly  always  of  small  thickness.  From  Duluth  to  French  River,  then, 
there  is  a  constant  ascent  in  geological  horizon,  and  the  thickness  crossed 
cannot  be  much  less  than  from  8,000  to  9,000  feet. 

From  French  River  to  Burlington  Bay  there  is  again  a  somewhat 
more  northerly  trend  in  the  rock  beds,  but  since  the  coast  line  here  also 
runs  more  around  to  the  north,  the  dips  at  the  same  time  flattening  to  10° 
and  even  6°,  there  is  not  much  added  in  this  distance  to  the  thickness  above 
given.  After  Burlington  Bay  is  passed,  the  strike  begins  to  cut  the  coast 
more  sharply,  and  by  the  time  Split  Rock  River  is  reached,  45  miles  below 
Duluth,  fully  10,000  feet  of  thickness  have  been  crossed. 

In  the  vicinity  of  Split  Rock  River  the  layers  strike  nearly  due  north, 
cutting  the  coast  at  an  angle  of  35°;  but  half  way  between  Split  Rock 
River  and  Beaver  Bay,  coast  and  strata  are  again  trending  together,  at 
about  40°  east  of  north.  Below  Beaver  Bay,  again,  the  strata  turn  away 
from  the  coast  to  the  northward,  and  for  some  two  miles  below  Baptism 
River  strike  only  a  very  few  degrees  east  of  north,  and  by  this  time  the 
coast  must  have  crossed  fully  16,000  feet  in  thickness  of  rock  beds.  Be- 
yond the  last  point,  however,  both  coast  and  strata  begin  curving  more  and 
more  around  to  the  east,  the  two  coinciding  at  N.  50°  E.,  somewhere 
between  Petit  Marais  and  Two  Islands  River.  In  the  vicinity  of  Two 
Islands,  Cross,  and  Temperance  rivers,  are  the  highest  strata  met  with 
anywhere  on  the  Minnesota  coast,  or,  indeed,  on  the  entire  north  shore  of 
the  lake,  with  the  exception  of  Isle  Royale.     In  the  80  miles  between 


EELATIONS  OP  TEENDS  OF  COAST  LINE  AND  STEATA.        265 

Duluth  and  Temperance  River,  the  coast  line  has  crossed  some  1 7,000  feet 
of  strata. 

Two  miles  below  Temperance  River,  in  Sec.  28,  T.  59,  R  4  W.,  a 
descent  of  the  coast  line  in  geological  horizon  begins  to  be  perceptible. 
This  descent  continues  without  interruption  for  a  distance  of  70  miles,  or  to 
the  end  of  the  Minnesota  coast,  at  Pigeon  Point.  From  Temperance  River 
to  Grand  Portage  both  coast  line  and  strata  curve  more  and  more  to  the 
eastward,  but  the  strata  change  direction  more  rapidly  than  the  coast  line, 
so  that  they  cut  it  at  a  small  angle  all  the  way,  producing  points  like  those 
described  as  characterizing  the  coast  line  west  of  Temperance  River,  but 
with  the  difference  that  the  points  now  project  eastward,  instead  of  to  the 
southwest  At  Grand  Portage,  the  Keweenawan  beds  striking  out  under 
the  lake,  the  Huronian  or  Animikie  slates  appear  from  beneath. 

The  Minnesota  coast  line,  looked  at  as  a  whole,  presents  a  sort  of  flat 
crescentic  shape,  with  the  concavity  towards  the  lake.  The  same  is  true  of 
the  courses  of  the  strata,  but  the  crescents  formed  by  them  have  a  much 
smaller  radius,  and  hence  intersect  that  formed  by  the  coast  line,  trending 
more  to  the  north  at  the  Duluth  end,  and  more  to  tlie  east  at  the  Grand 
Portage  end.  In  following  the  coast,  then,  from  the  slates  of  the  Saint  Louis 
River  to  Grand  Portage,  we  ascend  in  geological  horizon  to  a  point  near  Two 
Islands  River,  and  from  a  point  just  east  of  Temperance  River  descend  again 
to  the  same  slates  at  Grand  Portage.  Since  the  exposures  are  almost  con- 
tinuous, the  coast  line  thus  gives  a  complete  cross-section  of  the  whole 
thickness  of  Keweenawan  beds  present  in  northeastern  Minnesota.  Since 
the  junction  line  between  these  Keweenawan  strata  and  the  underlying 
slates  makes  quite  a  large  angle  with  the  lake  shore  at  both  ends,  and  since 
the  prevailing  dips  are  so  flat,  it  follows  that  the  first-named  rocks  spread 
far  back  into  the  country.  At  the  mouth  of  the  Bruld  River  they  lie  some 
12  miles  back;  at  Grand  Marais  18  to  20;  at  the  middle  of  the  crescent, 
near  Manitou  River,  30  miles,  at  about  which  distance  they  remain  until 
near  Duluth  and  the  Saint  Louis  River.  The  slates  themselves  have  about 
the  same  flat  position,  so  that  they,  in  their  turn,  spread  over  a  wide  belt  of 
country. 

Equally  simple  with  the  general  structure  as  thus  laid  down,  is  the 


2Q6  COPPEEBBAEmG  EOCKS  OP  LAKE  SUPEEIOE. 

general  stratigraphical  succession  displayed  on  the  Minnesota  coast.  Cer- 
tain groups  of  beds  are  plainly  to  be  made  out,  and  in  many  cases  mimite 
stratigraphical  measurements  could  be  made  in  detail  through  thicknesses 
of  thousands  of  feet.  Of  course,  the  greater  the  detail  attempted,  the  greater 
would  be  the  obstacles  met  with,  in  the  way  of  faults — which  are  numerous 
along  the  Minnesota  coast — thinning  out  of  beds,  corrugations  of  beds,  and 
similarity  of  lithological  composition  between  diflferent  layers. 

All  of  the  Keweenawan  beds  of  the  Minnesota  coast  belong  to  the 
Lower  Division  of  the  series.  The  same  statement  applies  to  all  of  the  Ke- 
weenawan rocks  of  the  North  Shore,  except  a  small  area  at  the  southeast 
corner  of  Isle  Eoyale. 

The  following  are  the  subordinate  groups  of  beds  into  which  I  have 
subdivided  the  Keweenawan  rocks  of  the  Minnesota  coast,  with  a  total  thick- 
ness of  upwards  of  20,000  feet.  The  thickness  of  the  first  group  is  so  un- 
certain, and  indeed  irregular,  that  it  is  difficult  to  give  an  approximately  cor- 
rect estimate  of  the  total  thickness.  Above  the  lowest  group,  as  already 
said,  the  thickness  appears  to  lie  between  17,000  and  18,000  feet.  In  all 
probability,  22,000  to  24,000  feet  would  not  be  very  far  from  the  truth  as 
an  estimate  of  the  total  thickness. 

I.  The  Saint  Louis  River  gabbro  and  associated  red  porphyries. — 
These  rocks  are  chiefly  coarse  orthoclase-gabbro,  but  include  also  ortho- 
clase-free  gabbro,  and  a  very  few  beds  of  fine-grained  diabase.  Red 
augite-syenite  and  granitic  porphyry  occur  in  large  areas,  constituting  at 
times  the  entire  mass  of  hills.  Felsitic  porphyries  occur,  but  more  rarely. 
Similar  rocks,  similarly  associated,  occur  at  the  same  horizon  about  the 
headwaters  of  Poplar,  Cascade,  and  Brul^  rivers,  but  are  not  found  where 
they  should  appear  at  the  Grand  Portage  end  of  the  coast,  though  it  is 
quite  possible,  and  even  probable,  that  some  of  the  overflows  of  coarse 
gabbro  found  capping  the  slates  of  the  Thunder  Bay  country  belong  here. 
The  thickness  of  this  group  is  difficult  to  estimate,  but  is  probably  not 
overstated  at  6,000  feet. 

II.  The  Duluth  Group. — This  group  is  a  succession  of  heavy  but 
sharply-defined  beds  of  very  fine-grained  but  aphanitic  rocks,  belonging  to 
the  ashbed  type  of  diabases,  and  to  the  diabase-porphyrites.     A  very  few 


SUBOEDINATE  GROUPS  OP  THE  MINNESOTA  COAST,  267 

beds  of  rather  coarse-grained  orthoclase-free  gabbro  are  included;  and 
there  is  a  little  interleaved  detrital  matter.  Thin  amygdaloids  of  peculiar 
character  cap  many  of  the  beds  of  the  upper  two-thirds  of  the  group,  but 
the  amygdaloidal  character  never  reaches  so  great  a  development  as  in 
some  of  the  succeeding  groups.  This  group  is  distinctly  recognizable  at 
both  ends  of  the  coast,  and  at  points  in  the  interior,  wherever  its  course 
has  been  crossed.  Its  thickness  lessens  as  it  is  followed  eastward;  but 
at  Duluth  it  is  not  far  from  5,000  feet. 

III.  The  Lester  River  Group. — This  is  a  succession  of  heavy,  distinct 
beds  of  fine-grained  brown  rocks,  largely  of  the  ashbed  type.  Diabase-por- 
phyrites,  some  of  the  ordinary  diabases,  rare  beds  of  coarse-grained  gabbro, 
and  two  or  three  belts  of  granitic  porphyry  are  also  included.  Amygda- 
loids are  almost  unknown,  and  no  detrital  material  has  been  observed.  The 
rocks  of  this  group  are  known  at  both  ends  of  the  coast,  and  at  intervening 
points  in  the  interior.     The  thickness  is  about  2,600  feet. 

IV.  The  Agate  Bay  Group. — This  is  a  succession  of  relatively  very 
thin  beds  with  very  highly  vesicular,  stratiform  amygdaloids,  which  must 
make  up  two-thirds  of  the  thickness  of  the  group.  The  prevalent  non- 
amygdaloidal  rock  is  a  fine-grained,  olivine-bearing  diabase  or  melaphyr. 
Towards  the  base  are  a  number  of  layers  of  diabase-porphyrite,  also  with 
strongly  developed  amygdaloids.  Thin  seams  of  reddish  sandstones  and 
conglomerate  are  also  included.  This  group  forms  the  coast  line  for  a 
distance  of  some  35  miles  below  the  mouth  of  Lester  River,  and  has  been 
traced  some  miles  farther  east  by  exposures  in  the  back  country,  but  it 
does  not  appear  at  the  eastern  end  of  the  Minnesota  coast,  having  appar- 
ently quite  thinned  out.  This  fact  is  in  accordance  with  the  general  law 
of  thinning  towards  the  east,  which  is  obeyed  by  all  three  groups  below, 
and  by  the  one  above.     The  thickness  of  the  group  is  about  1,500  feet. 

V.  The  Beaver  Bay  Group. — This  group  is  especially  characterized 
by  a  predominance  of  black,  coarse-grained,  olivine-bearing  gabbros  in 
very  heavy  layers  without  amygdaloids,  and  by  the  great  abundance  and 
prominence  of  its  included  red  felsitic  porphyries  and  granite-like  rocks. 
There  are,  however,  very  considerable  thicknesses  included  of  fine-grained 
ashbed-diabases,  with  and  without  amygdaloids,  while  the  ordinary  fine- 


268       COPPEE-BEARING  EOCKS  OF  LAKE  SUPERIOE. 

grained  diabases  with  amygdaloids  are  not  excluded — though  the}^  are  rarely" 
met  with.  No  detrital  material  has  been  observed.  In  following  the  coast 
line  eastward  the  beds  of  this  group  are  crossed  in  ascending  order  between 
SplitRock  and  Baptism  rivers, a  distance  of  some  18  miles;  and  in  descending 
order  in  the  28  miles  below  Grand  Marais;  besides  which  are  also  exposures 
of  the  same  beds  in  the  intervening  country  back  of  the  lake  shore.  In 
its  eastern  extension  this  group  does  not  exceed  4,000  to  5,000  feet  in  thick- 
ness, but  to  the  west  it  must  fully  reach,  if  it  does  not  exceed,  6,000  feet. 

VI.  The  Tempeeance  River  Group. — This  is  a  succession  of  very  dis- 
distinctly  and  thinly  bedded  fine-grained  diabases  and  melaphyrs,  with 
strongly  developed  amygdaloids,  and  several  seams  of  detrital  matter,  in 
the  shape  of  red  shaly  sandstone  and  conglomerate,  one  sandstone  layer 
exceeding  200  feet  in  thickness.  Towards  the  base  of  the  group  are  some 
layers  of  dense  ashbed-diabase  and  diabase-porphyrite.  The  rocks  of  this 
group  form  the  coast  line  from  a  point  two  miles  below  Baptism  River  to 
Grand  Marais,  a  distance  of  50  miles.  They  are  the  highest  rocks  seen  on 
the  Minnesota  coast,  and  have  a  thickness  in  sight  of  some  2,500  to  3,000  feet. 

In  giving  more  detailed  accounts  of  these  several  groups,  it  will  be 
most  convenient  to  take  them  up  in  ascending  order,  considering  in  each 
case  first  the  more  western  exposures,  and  then  the  eastern  extensions. 

The  Saint  Louis  gabbros  and  porphyries. — The  rocks  of  this  group  form 
a  bold  range  of  hills,  extending  from  Duluth  in  a  S.  46°  W.  direction,  seven 
miles  on  the  north  side  of  the  Saint  Louis  River.  They  have  also  been  car- 
ried northward  from  Duluth  to  the  Cloquet  River,  and  up  that  stream  nearly 
to  township  55,  a  distance  of  over  25  miles;  the  belt,  as  a  whole,  having 
apparently  at  first  a  northeasterly,  then  a  northerly,  and  again  a  north- 
easterly trend,  where  left  on  the  upper  Cloquet  River.  How  wide  the  belt 
is  remains  quite  uncertain,  a  broad  ai'ea  of  country  without  exposures  lying 
west  and  north  of  it;  but,  judging  from  the  relative  positions  of  the  west- 
ernmost exposures  of  the  gabbro,  and  the  easternmost  of  the  underlying 
slates  in  the  neighborhood  of  Fond  du  Lac,  the  width  cannot  there  exceed 
two  miles,  if  it  reaches  that  distance.-^    Equally  a  matter  of  inference  is  the 

'Directly  north,  or  even  uortliwesi  from  Dulutli,  one  can  travel  on  gabbro  and  intersecting  red 
rock  for  some  miles,  but  this  is  because  the  belt  here  trends  northward. 


COAESB  GABBEOS  OF  THE  ST.  LOUIS  ElVEE.  269 

inclination  of  the  mass  as  a  whole;  judging  from  the  adjacent  rock  beds 
this  is  some  45°  southeastward  from  the  western  extremity  to  near  Duluth; 
about  the  same  amount  eastward  at  Duluth,  and  a  good  deal  less  than  this 
to  the  south  of  east  on  the  Cloquet.  No  sign  of  anything  like  subordinate 
bedding  can  be  seen  in  the  rock  itself  It  is  massive  and  irregularly 
jointed,  making  great  ledges  facing  in  different  directions,  and  furnishing 
bare,  rounded  summits  to  the  hills  which  it  composes. 

The  prevalent  type  of  the  gabbro  of  this  belt  and  the  kind  constituting 
the  hills  at  Duluth  is  of  a  hght-gray  color,  and  very  coarse-grained,  single 
feldspar  crystals  sometimes  reaching  even  an  inch  or  two  in  length.  The 
augitic  ingredient  is  plainly  in  greatly  subordinate  quantity,  and  often  on  a 
fresh  surface  its  presence  cannot  be  detected  at  all.  On  exposed  surfaces, 
however,  the  weathering  generally  brings  it  out,  and  then  it  can  be  plainly 
seen  to  fill  the  spaces  left  between  the  feldspars.  Titaniferous  magnetite  is 
also  often  perceptible  to  the  naked  eye  in  large  particles. 

Less  commonly  the  grain  is  finer  and  the  color  darker,  the  augitic  ingre- 
dient at  the  same  time  becoming  more  plentiful.  In  the  thin  section  the 
predominant  feldspar  is  seen  to  be  a  plagioclase  belonging  near  the  oligo- 
clase  end  of  the  series.  There  appears  also  to  be  always  a  younger  feld- 
spar present,  which  has  the  character  of  orthoclase  and  fills  corners  between 
the  plagioclase  crystals,  around  whose  contours  it  moulds  itself  sharply. 
Streng  and  Kloos  found  1.61  per  cent,  of  potash  in  the  rock,  which  they 
very  properly  regarded  as  belonging  to  orthoclase.  The  spaces  between 
the  feldspars  are  filled  with  a  diallage  which  is  always  more  or  less  altered  to 
greenish  uralite.  The  alteration  in  many  sections  is  carried  beyond  uralite, 
to  chlorite.  The  magnetite  is  very  large,  abundant  and  titaniferous.  Apa- 
tites of  large  size  are  found  in  all  sections.  Biotite  is  a  not  uncommon 
accessory.  Olivine  is  absent  from  all  sections.  A  large-sized  figure  of  the 
thin  section  of  the  Duluth  rock  is  given  on  Plate  VI.^ 


1  Microscopic  descriptions  of  the  Duluth  gabbro  have  been  hitherto  published  by  Streng  (Ueber 
die  Krystallinischen  Gesteeine  von  Minnesota  in  Nordamerika— Leonhard  u.  Geinitz,  Neues  Jahr- 
buch  tiir  Miueralogie,  Geologie  und  Paleontologie,  1877,  p.  113),  and  N.  H.  Winchell  (Eighth  An- 
nual Eeport  of  the  Geological  and  Natural  History  Survey  of  Minnesota,  1880,  p.  22).  Streng  calls 
the  rook  "  hornblende-gabbro,"  regarding  the  hornblende  as  primary.  His  conclusions  are  summed 
ap  as  follows:  "The  borublende-gabbro  of  the  Saint  Louis  River  at  Duluth  consists  of  a  greatly  pre- 


270  COPPEE-BEARIlsG  EOCKS  OP  LAKE  SUPEEIOE. 

Farther  west  on  this  gabbi'o  belt,  as  for  instance  on  King's  Creek, 
N.  E.  ^,  Sec.  12,  T.  49,  R.  15  W.,  the  rocks  appear  largely  to  belong  to  the 
olivine-bearing,  orthoclase-free  kinds,  having  at  the  same  time  a  very  basic 
feldspar. 

The  I'ed  rocks  that  occur  so  largely  associated  with  the  gabbro  of 
Duluth  lie  in  it  very  irregularly,  and  form  nothing  like  distinct  belts,  so 
far  as  I  could  make  out.  They  may  be  seen  in  great  patches,  hundreds  of 
feet  square,  and  surrounded  on  all  sides  by  the  gabbro,  and  again,  as  at  the 
quarry  near  Rice's  Point,  in  irregular  veins  from  two  to  three  inches  to  sev- 
eral feet  in  width.  Much  the  most  abundant  kind  of  these  red  rocks  is  one 
which  presents  macroscopically  a  wholly  crystalline  texture,  and  a  pinkish 
color  mottled  with  green.  Pink  feldspar  facets,  now  and  then  striated, 
quartz,  and  a  greenish  mineral  may  all  be  made  out  with  the  naked  eye. 
Under  the  microscope  the  rock  is  seen  to  be  chiefly  composed  of  reddened 
orthoclase,  but  similai'ly  reddened  oligoclase,  greenish  hornblende,  quartz 
and  magnetite  are  also  present.     The  quartz  occurs  both  in  quite  large 

dominating  plagioclase  (labradoiite),  little  orthoclase,  some  hornblende,  diallage,  magnetite,  and 
titanic  iron,  and  holds  also  very  minute  quantities  of  chalcopyrite  and  epidote,  which  is  at  times  asso- 
ciated with  quartz."  My  study  leads  me  to  agree  in  the  main  with  these  conclusions.  I  take,  how- 
ever, the  hornblende  to  be  wholly  uralite,  the  plagioclase  to  be  more  often  nearer  oligoclase  than 
labradorite,  and  the  iron  oxide  ingredient  to  be  wholly  titaniferous  magnetite  rather  than  both  mag- 
netite and  titanic  iron.  The  epidote,  quartz,  and  chalcopyrite  are  so  rare  as  not  to  deserve  mention 
in  any  general  description  of  the  rock.     The  following  analysis  is  given  by  Streng : 

Si02 49.15 

AljOa 21.90 

FesOa 6.60 

FeO 4.54 

CaO , 8.22 

MgO  3.03 

KsO  1.61 

NajO 3.83 

H2O 1.92 

100.  80 

P2O6 0.33 

TiOs 0.18 

Winchell  finds  that  the  "  chief  Ingredients,  which  are  always  present,  are  plagioclase  and  pyrox- 
ene, but  the  latter  is  sometimes  very  small  in  amount."  He  finds  "also  titaniferous  iron,  generally 
magnetic,  almost  always  present,  and  sometimes  in  quantity  sufficient  to  render  it  an  iron  ore  of  low 
grade.  *  *  *  Pyrito,  calcite,  epidote  and  chlorite  also  exist  in  some  parts,  *  *  *  especially  as  geodes, 
nests,  and  vein-fillings,  or  as  products  of  change.  The  plagioclase  "  is  provisionally  taken  for  labrador- 
ite," but  in  some  places  it  is  said  to  appear  ' '  more  like  anorthite."  The  jjyroxene  is  taken  to  be  diallage, 
much  of  it  "fibrous  from  incipient  change,  the  products  being  ferrite  and  viridite."  Hornblende  and 
nralite  are  not  mentioned.  The  iron  oxide  is  considered  to  be  titanic  iron,  although  magnetic,  because 
of  the  presence  of  the  white  decomposition  product. 


EBD  EOOKS  OF  DULUTH,  271 

patches  and  again  in  little  strings  running  through  and  through  the  feld- 
spars, in  the  usual  manner  of  secondary  quartz.  A  number  of  these  small 
patches  of  quartz  lying  near  each  other  will  polarize  together,  showing  that 
they  are  part  of  one  individual.  Moreover,  the  same  is  true  of  the  larger 
quartz  areas,  and  numbers  of  small  particles  lying  near  them,  so  that  all 
of  the  quartz  is  considered  to  be  secondary.  This  secondary  quartz  is  fre- 
quently scattered  through  the  feldspars  in  such  a  manner  as  to  present  the 
appearance  of  graphic  granite,  and  again  it  is  arranged  in  irregularly  radi- 
ating lines.  Chlorite  is  often  present  as  an  alteration-product  of  both  feld- 
spars and  hornblende.  No  base  finer  than  the  rest  of  the  rock  was  observed, 
so  that  the  name  should  apparently  be  syenite,  the  quartz  being  taken  as 
secondary.  Since  the  hornblende  is  probably  uralite,  as  in  the  similar  rocks 
of  other  parts  of  the  extent  of  the  formation,  the  rock  is  probably  an  augite- 
syenite.  We  have  in  this  rock  precisely  the  same  as  is  found  in  many  peb- 
bles of  the  porphyry-conglomerates  of  Keweenaw  Point,  and  such  as  is  found 
massive  again  along  other  portions  of  the  Minnesota  coast. 

Another  kind,  less  common  than  the  foregoing,  presents  a  red  matrix 
with  little  green  or  black  in  it,  but  numerous  facets  of  red  feldspar  difficult 
to  distinguish  from  the  matrix,  save  in  certain  positions.  This  is  a  true 
"granitic  porphyry,"  standing  between  the  granites  and  felsites.  Under 
the  microscope  it  appears  originally  to  have  been  composed  of  a  minutely 
crystalline  base;  but  the  whole  is  now  saturated  through  and  through  with 
secondary  quartz 

Less  common  than  either  of  the  foregoing,  but  still  forming  quite  large 
patches  in  the  gabbro,  is  another  red  rock  which  presents  to  the  naked  eye 
an  aphanitic  light-red  matrix,  in  which  minute  orthoclases  are  very  sparsely 
scattered.  Underneath  the  microscope  it  shows  a  nearly  white  matrix  so 
thoroughly  saturated  with  secondary  quartz  that  it  is  often  difficult  to  tell 
its  exact  original  nature.  The  quartz  is  arranged  in  arborescent  clusters 
and  in  crossing  forms,  and  all  of  a  cluster  will  polarize  together.  Numer- 
ous black  particles,  some  of  them  undoubtedly  magnetite,  others  more  mi- 
nute and  hair-like,  are  contained;  also  porphyritic  crystals  of  orthoclase  and 
augite.  The  whole  section  is  dotted  with  minute  brownish  particles.  One 
or  two  minute  porphyritic  quartzes  were  observed.     The  rock  is  plainly 


272       COPPEE-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 

enough  a  felsitic  porphyry,  with  various  products  of  devitrification  in  the 
base. 

These  three  varieties  of  red  rock,  thus  described  as  occurring  at  Duluth, 
are  evidently  but  difi'erent  phases  of  the  same  rock,  and  without  much 
doubt  are  connected  with  each  other  in  the  mass,  though  this  was  not 
proved  in  the  field. 

The  exposures  of  gabbro  on  the  Cloquet  River  regarded  as  belonging 
to  this  belt  are  on  Sec.  10,  T.  54,  R.  13  W.  (270  N.,  1,500  W.);  on  Sec.  5, 
T.  53,  R.  13  W.  (1,700  N.,  2,000  W.);  at  the  falls  on  the  N.  E.  J,  Sec.  18, 
T.  53,  R.  13  W.  (1,650  N.,  700  W.);  at  a  number  of  points  through  Sec. 
36,  T.  53,  R.  14  W.;  in  the  S.  E.  i  Sec.  35,  T.  53,  R.  14  W.  (75  N.,  600  W.); 
and  about  the  falls  in  the  S.  E.  J,  Sec.  34,  T.  53,  R.  14  W.,  where  the  show- 
ing is  a  very  large  one.  The  rock  seen  at  these  points  is  pretty  uniform  in 
character,  and  is  a  very  fresh  olivine-gabbro.  It  is  light-gray  in  color,  very 
coarse-grained,  and  composed  chiefly  of  very  fresh  plagioclase  (anorthite). 
Quite  fresh  diallage  fills  in  the  spaces  between  the  feldspars.  A  few  large, 
fresh  olivines  occur  here  and  there  in  the  section.  Titaniferous  magnetite 
is  abundant  and  large-sized,  and  biotite  occurs  in  a  few  small  scales. 

From  the  last  exposure  examined  on  the  Cloquet,  in  Sec.  10,  T.  54,  R. 
13  W.,  it  is  some  eighty  miles  in  a  N.  50°  E.  direction  to  the  vicinity  of 
Brul^  Lake,  where  Mr.  Chauvenet  found  again  a  large  development  of 
coarse  gabbros  and  red  granitic  porphyries.  The  intermediate  country  has 
not  yet  been  surveyed,  and  is  well-nigh  unknown,  save  to  the  Indians. 
There  can  be  little  doubt,  however,  that  the  same  belt  runs  through.  I  am 
informed  by  Professor  N.  H.  Winchell  that  he  found  such  rocks  on  what  would 
be  the  line  of  this  belt  in  making  a  northwesterly  traverse  from  the  mouth 
of  Poplar  River.  Some  sixteen  miles  back  from  the  mouth  of  Baptism 
River  in  a  northwesterly  direction,  Messrs.  Campbell  and  McKinlay  found 
a  granitic  porphyry,  which  may  belong  to  the  same  belt,  but  the  country 
was  low  and  swampy,  and  no  other  exposures  were  in  sight. 

The  exposures  about  BruM  Lake,  and  the  headwaters  of  Bruld  and 
Cascade  rivers  are  on  a  grand  scale,  and  of  great  interest.  To  reach  them 
my  assistants,  Messrs.  Chauvenet  and  McKinlay,  started  from  the  lake  shore 
at  Grrand  Marais,  Sec.  21,  T.  61,  R.  1  E.;  went  thence  northwest  to  Devil's 


EOCKS  OF  BEULE  LAKE  AND  EAGLE  MOUNTAIN.      273 

Track  Lake,  in  the  southern  part  of  T.  62,  R.  1  W.;  canoed  this  lake  to  the 
western  end;  portaged  to  Cascade  River  in  Sec.  26,T.62,R.2W.;  and  thence 
ascended  Cascade  River  to  its  source  in  a  series  of  small  lakes,  in  the  south- 
ern part  of  what  would  be  T.  63,  R.  2  "W.,  if  the  country  had  been  surveyed. 
They  crossed  in  this  ascent  a  series  of  distinctly  bedded  diabases  and  amyg- 
daloids,  dipping  southward  at  a  low  angle.  The  lake  which  forms  the 
source  of  Cascade  River  lies  in  what  would  be  about  Sec.  22,  T.  63,  R.  2 
W.  Thence  they  took  a  W.  N.  W.  course  to  Brul^  Lake,  which  lies  east 
and  west,  with  a  length  and  width  respectively  of  about  ten  and  two  miles. 
The  shores  of  the  lake  are  bold  and  rocky,  and  exceedingly  irregular  in 
outline.  The  rocks  assigned  to  the  horizon  now  under  description  were 
found  first  on  and  about  Eagle  Mountain,  which  lies  ten  miles  north  and  two 
west  of  the  northeast  corner  of  T.  62,  R.  2  W.,  or  somewhere  about  the 
S.  E.  i  of  Sec.  22,  T.  63,  R.  2  W.  It  rises  abruptly  on  the  east  side  of  a 
small  lake  to  a  height  of  450  feet,  or  to  upwards  of  1,500  feet  above  Lake 
Superior. 

The  mountain  is  a  bold  mass  of  bright  red  rock,  and  from  its  summit 
may  be  seen  numbers  of  other  elevations  composed  of  the  same  red  rock. 
This  rock  is  a  granitic  porphyry,  and  over  most  of  the  mountain  presents 
an  appearance  closely  resembling  that  of  the  second  kind  of  red  rock  men- 
tioned as  occurring  at  Duluth,  while  it  is  precisely  similar  to  rocks  seen 
cutting  coarse  gabbro  at  the  same  low  horizon  in  the  Bad  River  country  of 
Wisconsin.  It  presents  to  the  naked  eye  the  appearance  of  being  chiefly 
made  up  of  small  red  feldspars,  but  there  are  areas  which  will  not  reflect 
any  light.  No  other  mineral  is  to  be  detected.  Under  the  microscope  the 
section  presents  precisely  the  same  appearance  as  that  of  the  Duluth  rock, 
being  made  up  of  reddened  orthoclases  and  matrix,  the  latter  now  so  thor- 
oughly saturated  with  secondary  quartz  arranged  in  bunches  of  radiating 
lines,  that  its  original  nature  is  difficult  to  decide.  The  quartz  also  pene- 
trates some  of  the  recognizable  orthoclases,  many  of  which  are,  however, 
without  it.  One  or  two  particles  of  greenish  hornblende  were  observable; 
minute  black  particles  also  occur.  Near  the  top  of  the  mountain  a  more 
dense  kind  than  usual  was  noticed,  which  turned  out  to  be  a  true  felsitic  por- 
phyry. No  line  of  demarkation  was  noticed  between  the  kinds.  All  about 
18  L  s 


274       COPPER-BEARING  ROCKS  OF  LAKE  SUPERIOR. 

the  mountain  the  rock  appears  as  if  massively  bedded,  with  an  east  and  west 
trend,  and  a  10°  to  lb°  southward  dip.  East^vard  from  Eagle  Mountain 
three  or  four  miles  the  red  granitic  porphyries  were  again  found  exposed. 
On  the  foot  of  the  mountain  on  the  south,  moderately  coarse  dark-gray  ortho- 
clase-gabbro  was  in  place,  appearing  to  underlie  the  granitic  porphyry. 

Leaving  Eagle  Mountain,  the  course  of  the  party  lay  in  a  northerly 
direction,  through  a  string  of  four  lakes.  In  the  second  of  these.  Pike 
Lake,  massive  ledges  of  gray  gabbro  10  to  30  feet  high  run  along  both 
sides  of  the  lake.  This  gabbro  is  very  coarse,  much  weathered  and  iron- 
stained,  and  shows  a  great  deal  of  very  coarse  titaniferous  magnetite.  Under 
the  microscope  it  is  seen  to  be  closely  allied  to  the  Duluth  gabbros,  from 
which  it  differs,  however,  in  having  more  orthoclase,  a  less-altered  diallage, 
and  in  containing  quite  a  little  secondary  quartz.  The  same  rock  continues 
largely  exposed  to  the  end  of  the  last  lake  in  the  series  in  about  what  would 
be  Sec.  15,  T.  63,  R.  2  W.  From  the  last  lake  the  trail  leads  through  a 
gorge  10  miles  in  a  direction  slightly  north  of  west  to  Bruld  Lake.  The 
walls  of  the  gorge  average  some  50  feet  in  height,  now  and  then  rising  into 
cones  of  bare  rock  100  to  150  feet  high.  They  are  often  not  more  than  a 
few  rods  apart.  That  on  the  north  is  for  most  of  the  distance  red  granitic 
porphyry,  and  that  on  the  south  at  first  the  coarse  gray  gabbro  of  the  lakes 
below,  while  nearer  Bruld  Lake  it  is  composed  of  a  finer-grained  brownish- 
gray  orthoclase-gabbro.  In  this  rock  there  is  a  good  deal  of  augite  with 
crystalline  outlines,  it  having  formed  before  the  feldspars,  besides  diallage 
with  the  usual  relation  to  the  feldspars,  which  are  both  oligoclase  and  ortho- 
clase. There  is  contained  a  great  deal  of  titaniferous  magnetite  in  black 
rods. 

Brul^  Lake  is  a  sheet  of  water  some  ten  miles  in  length  by  two  in  great- 
est width,  with  an  exceedingly  irregular  outline,  and  numerous  rocky  islands. 
The  lake  lies  in  a  rock  basin,  and  its  shores,  especially  the  northern,  rise 
into  bold  cliffs,  and  the  whole  landscape  is  unequalled  for  beauty  anywhere 
in  the  Northwest.  There  was  not  enough  time  spent  here  to  work  out  any 
details,  but  enough  was  seen  to  learn  that  the  rocks  lie  in  distinct  belts 
trending  slightly  south  of  west.  At  the  northwest  corner  a  red  granitic 
porphyry  like  that  of  Eagle  Mountain  has  a  great  development.     South 


THE  DULUTH  BEDS.  275 

from  here  are  belts  of  fine-grained  ashbed-diabase  and  of  a  diabase -porphyry 
with  a  fine-grained  to  aphanitic  gray  matrix  and  large  red  crystals  of  a  tri- 
clinic  feldspar.  Still  south  of  these  belts  are  others  of  a  medium-  to  very 
coarse-grained  gray  gabbro.  One  of  these  belts,  consisting  of  a  medium- 
grained  kind,  with  much  whitened  feldspar,  and  much  magnetite,  can  be 
traced  for  several  miles  from  the  end  of  the  lake,  through  a  line  of  islands 
to  the  north  shore.  Still  south  of  these  belts,  and  appearing  both  in  the 
islands  and  on  the  northeast  shore,  are  again  others  of  a  red  granitic  por- 
phyry, running  into  a  true  quartz-porphyry ;  of  fine-grained  ashbed-diabase, 
and  of  a  very  coarse  orthoclase- bearing  gabbro  like  that  seen  farther  south 
towards  Eagle  Mountain. 

The  gabbro  sheets  overlying  the  slates  of  Pigeon  River  and  Thunder 
Bay,  and  above  alluded  to  as  possibly  belonging  to  the  same  general  hori- 
zon with  the  Duluth  gabbros,  are  described  in  connection  with  the  slates.^ 

The  Duluth  Group. — The  rocks  in  the  neighborhood  of  Duhith  which 
I  assign  to  this  group  were  found  exposed  in  the  streets  of  the  town  itself, 
along  the  lake  shore  to  the  mouth  of  Chester  Creek,  and  on  the  hillside  in 
the  triangular  area  between  the  latter  creek  and  the  lake  shore.  As  ex- 
plained on  a  previous  page,  these  rocks  ti-end  at  first  west  of  north,  and  thfen 
about  north  to  and  beyond  Chester  Creek,  dipping  at  first  45°  eastward, 
but  near  Chester  Creek  not  more  than  20°.  The  whole  thickness  displayed 
is  not  far  short  of  5,000  feet.  With  the  exception  of  two  thin  beds  of  a 
moderately  coarse,  black  gabbro,  and  a  little  interleaved  detrital  matter,  all 
of  this  thickness  is  made  up  of  a  succession  of  very  fine-grained  to  apha- 
nitic, gray  to  brown  rocks,  which  are  frequently  porphyritically  developed, 
with  red  and  more  rarely  white  feldspars  as  porphyritic  ingredients;  and 
which,  in  the  upper  two-thirds  of  the  thickness  often  present  amygdaloids 
as  the  upper  portions  of  the  flows.  These  amygdaloids  have  commonly  a 
light-brown  or  reddish-brown  matrix,  and  amygdules  which  are  prevailingly 
epidote;  but  amygdules  of  epidote  and  quartz,  of  epidote  and  calcite,  and 
of  a  green  earthy  substance,  evidently  a  decomposition-product,  also  occur. 
A  general  epidotic  decay  is  often  presented  in  the  shape  of  reticulated 
strings  and  blotches  of  epidote  through  the  amygdaloids,  and  even,  to  some 

I  See  also  Chapter  VIII. 


276       COPPEE-BEAEIKG  EOCKS  OF  LAKE  SUPEEIOE. 

extent,  in  the  more  compact  portions  of  the  beds.  The  amygdules  are  rela- 
tively small  and  not  very  thickly  strewn.  They  are  frequently  elongated 
in  a  common  direction  and  in  a  very  striking  manner.  The  same  amygda- 
loids  which  exhibit  this  elongation  of  the  amygdules  in  a  common  direction 
show  also  other  signs  of  having  flowed  as  lava,  such  as  flowage  lines,  ropy 
texture  in  a  common  direction,  etc.  The  most  interesting  of  these  phenom- 
ena is,  however,  an  appearance  of  stratification  presented  on  a  weathered 
surface,  which  is  evidently  directly  connected  with  the  viscous  flow  of  the 
original  lava. 

As  to  the  thickness  of  the  layers  of  this  series  at  Duluth,  it  may  be  said 
that  the  thickness  of  the  lower  layers  is  exceedingly  difficult  to  determine, 
the  rock  exposures  showing  the  same  material  of  great  widths,  and  there 
being  no  amygdaloids.  Evidently  the  thicknesses  are  very  considerable, 
probably  measured  even  by  hundreds  of  feet.  Higher  up,  however,  the 
layers  become  plainly  thinned,  and  alternate  more  rapidly,  and  in  the  upper 
third  of  the  series  amygdaloids  and  compact  portions  alternate  with  each 
other  quite  rapidly,  as  is  well  seen  along  the  shore  west  of  Chester  Creek. 

An  examination  of  a  large  number  of  sections  of  these  rocks  under 
the  microscope  showed  them  all  to  be  closely  the  same,  varying  only  in 
texture  and  fineness  of  grain.  The  main  ingredients  in  all  are  plagioclase, 
augite  and  magnetite.  The  triclinic  feldspar  occurs  both  in  the  ground- 
mass  and  as  the  chief  porphyritic  ingredient,  the  reddish  crystals  showing 
frequently  and  plainly  the  striation  to  the  naked  eye.  In  both,  measure- 
ments give  the  low  angles  indicative  of  oligoclase.  The  porphyritic  crys- 
tals are  commonly  filled  with  brownish  particles  of  ferrite,  and  are  nearly 
always  more  or  less  thoroughly  dulled  by  alteration.  The  augite  is  only 
rarely  fresh,  generally  showing  more  or  less  of  a  change  to  a  greenish,  non- 
polarizing,  viriditic  substance,  with  which  change  there  is  also  connected  the 
formation  of  much  magnetite  in  small  particles.  It  exists  both  as  a  filling 
to  the  spaces  between  the  feldspars,  and  in  little  rounded  gramiles,  the  latter 
phase  being  especially  characteristic  of  the  finer-grained  kinds.  Magnetite 
is  present,  however,  not  only  as  an  alteration-product  of  the  augite,  but 
also  occurs  abundantly  in  all  sections  as  an  original  constituent.  Besides 
these  main  ingredients,  there  are  also  to  be  observed  apatite,  which  occurs 


THE  DULUTH  BEDS.  277 

in  many  sections  in  the  usual  slender  needles ;  epidote,  which  in  many  sec- 
tions is  abundantly  pi-esent  in  the  groundmass,  at  times  to  such  an  extent 
as  to  form  pseud-amygdules,  besides  occurring  as  a  true  amygdule  in  the 
amygdaloids ;  quartz,  also  as  an  alteration-product,  associated  with  the  epi- 
dote, and  also  in  some  of  the  excessively  fine-grained  kinds  as  true  infil- 
trating secondary  quartz.^  Olivine  is  absent  throughout.  These  rocks 
then  are  to  be  called,  according  to  their  texture  and  degree  of  crystalline 
development,  fine-grained  diabase,  porphyritic  diabase,  diabase-porphyrite, 
and  diabase-amygdaloid. 

The  two  beds  of  gabbro  above  alluded  to  are  in  strong  contrast  with 
the  rest  of  the  rocks  of  this  group.  They  show  a  black,  rather  coarse, 
highly  crystalline,  rough-textured  rock,  which  in  the  thin  section  is  seen  to 
be  made  up  of  anorthite  ;  diallagic  augite,  very  coarse  and  abundant,  partly 
fresh  and  partly  altered  to  viridite  and  uralite ;  and  very  coarse  magnetite 
or  titanic  iron. 

With  the  exception  of  the  last  rocks  described,  Professor  N.  H.  Winchell, 
if  I  understand  him  correctly,  would  regard  all  of  these  fine-grained  rocks, 
and  especially  the  amygdaloidal  and  porphyritic  phases,  as  metamorphosed 
shales  and  sandstones,  and  as  altered  from  the  red  sandstone  of  Fond  du 
Lac,  to  which,  as  a  result  of  alteration,  he  also  refers  the  granite,  granitic 
porphyry,  and  felsite  of  the  Duluth  gabbros,  and  indeed  of  the  whole 
Minnesota  coast.  The  usual  proofs  are  present  that  the  rocks  now  under 
description  are  entirely  original  and  have  flowed  as  lavas.  These  are,  in 
brief,  completely  crystalline  texture  in  most  kinds ;  presence  of  some  origi- 
nal non-polarizing  base  in  some  of  the  porphyritic  kinds  ;  complete  absence 
in  all  of  any  traces  of  fragmental  texture;  true  gas  vesicles  in  the  amygda- 
loids;   elongation  ot  these  vesicles  in  a  common  direction;    flowage  lines 

1  One  variety  of  tlie  Duluth  fine-grained  rocks  A.  Streng  has  described  in  some  detail  under  the 
name  of  "melaphyr-porphyry,"  the  term  melaphyr  being  used  for  any  older  plagioclase-augite  rock. 
(A.  Streng  und  J.  H.  Kloos:  "Ueber  die  Krystalliniachen  Gesteine  von  Minnesota  in  Nord  Amerika," 
in  Neues  Jahrbuch  f.  Mineralogie,  etc.,  1877,  p.  42.)  It  is  evidently  the  rock  which  is  largely  exposed 
near  the  elevator  in  East  Duluth,  N.E.  i  of  the  S.E.  i  Sec.  27,  T.  .50,  E.  14  W.,  and  again— the  same 
Ijelt— on  Brewery  Creek,  at  250  paces  nortb  and  100  west  of  the  southeast  corner  of  Sec.  22,  T.  50,  R. 
14  W.  It  presents  a  very  dense  groundmass,  with  very  thickly  scattered  porphyritic  crystals  of  red 
feldspar,  mostly  triclinic.  The  following  is  the  analysis  given  by  Streng:  SiOj,  50.03;  AI2O3, 
15.38;  FesOj,  11.78;  FeO,  3.90;  CaO,  5.39;  MgO,3.G0;  K,0, 1.14;  Na,O,5.01;  HA2.73;  CO,,  0.98=99.94; 
PiOs,  0.33.  His  analysis  shows  the  essentially  basic  nature  of  this  porphyry,  which  Messrs.  Streng  and 
KlooH  class  with  the  somewhat  similar  rock  from  Saint  Croix  Falls,  Wisconsiu. 


278       COPPEE-BEARmG  EOOKS  OF  LAKE  SUPBEIOE. 

and  ropy  texture  in  the  amygdaloids;  and  division  of  the  beds  into  lower 
compact  and  upper  vesicular  portions.  To  these  we  need  only  add  that 
should  we  look  on  them  as  of  detrital  origin,  we  must  do  the  same  for  all 
the  amygdaloids  of  the  Lake  Superior  basin,  and  for  all  of  the  crystalline 
rocks  with  which  they  occur.  This  would  leave  us  only  a  few  insignificant 
dikes  to  regard  as  of  eruptive  origin.  Between  the  Keweenaw  Point  dia- 
bases and  amygdaloids  and  those  of  Duluth  there  is  no  difference  as  to  the 
general  nature — the  origin  of  the  one  is  the  origin  of  the  other. 

True  detrital  material  was,  however,  observed  interbedded  in  the  Du- 
luth series  at  two  points;  one  of  these  was  low  down  in  the  series,  on  the 
hillside  above  the  Catholic  church  at  Duluth,  where  a  small  exposure  of  a 
very  finely  laminated  argillaceous  slaty  rock  is  to  be  seen;  the  other  is  on 
the  lake  shore  between  Brewery  and  Chester  Creeks,  where  may  be  seen  a 
light-brownish,  quite  plainly  bedded  fine-grained  sandstone  composed  of  a 
mixture  of  basic  and  acidic  detritus,  the  former  predominating. 

Several  small  dikes  were  observed  cutting  the  beds  of  this  mem- 
ber on  the  lake  shore  between  Chester  Creek  and  Duluth.  These  dikes 
trend  with  the  strata,  but  cut  across  them  at  right  angles  to  the  bedding. 
They  are  composed  of  a  fine-grained  black  rock,  which,  near  the  middle  of 
the  dike,  is  plainly  crystalline,  while  towards  the  sides  it  is  aphanitic.  This 
rock  has  not  been  examined  under  the  microscope,  but  is  precisely  the  same 
raacroscopically  as  that  of  some  similar  dikes  occurring  five  miles  further 
down  the  coast,  below  the  mouth  of  Lester  River,  which  is  a  very  highly 
augitic  diabase,  without  olivine,  and  with  but  little  magnetite. 

Since  these  beds  make  so  large  an  angle  with  the  coast  near  Duluth, 
they  depart  rapidly  inland.  So  far  as  our  limited  explorations  went  inland, 
the  belt  of  country  under  which  they  are  supposed  to  lie  is  largely  low  and 
without  exposures,  lying  back  of  the  first  or  lake  range  of  bold  hills.  Fol- 
lowing the  range  line  between  ranges  11  and  12  north  seven  miles  from  the 
crossing  of  Knife  River  in  the  S.  W.  J,  Sec.  6,  T.  52,  R.  11  W.,  Mr.  McKinlay 
found  no  exposures,  but  eastward  from  the  last  point  three  miles,  in  the  N.W.^ 
of  the  S.  W.  5,  Sec.  4,  T.  53,  R.  1 1  W.,  he  found  a  large  exposure  of  a  very 
fine-grained  massive  gray  rock,  with  porphyritic  triclinic  feldspars,  which 
both  to  the  naked  eye  and  under  the  microscope  resembles  closely  the  gray 


THE  LESTEE  EIVEE  BEDS.  279 

Duluth  rocks,  to  whose  horizon  it  may  reasonably  be  referred.  The  rocks 
of  the  upper  Cascade  River,  and  the  east  end  of  the  Minnesota  coast,  which 
may  belong  in  part  to  the  member  now  under  description,  are  considered 
after  the  next  two  members  are  described. 

The  Lester  Biver  Group. — The  third  member  of  the  succession  on 
the  Minnesota  coast  I  have  divided  from  the  second  by  a  rather  arbitrary 
line.  Its  beds  were  seen  best  exposed  on  the  lake  shore  from  a  point  be- 
tween Chester  and  Tischer's  Creeks,  in  Sec.  24,  T.  50,  R  14  W.,  to  a  point 
about  two  miles  beloW  the  mouth  of  Lester  River,  in  Sec.  34,  T.  51,  R.  13 
W. ;  along  Lester  River,  and  in  the  woods  west  of  the  river,  in  Sees.  4  and 
6,  T.  50,  R.  13  W.,  and  Sees.  29  and  33,  T.  51,  R.  13  W.;  along  French 
River,  in  Sees.  6  and  7,  T.  51,  R.  12  W.;  in  scattering  ledges  in  the  woods 
in  the  southern  part  of  T.  53,  R.  11  W.,  north  of  Knife  River;  and  again 
along  Encampment  River,  in  Sees.  3,  10,  and  11,  of  T.  53,  R.  10  W. 

This  group  is  made  up  chiefly  of  beds  of  fine-grained  to  nearly  black, 
dark-gray,  brown,  or  reddish-brown  compact  rocks,  occasionally  porphy- 
ritic  with  reddish  plagioclases ;  but  this  character  is  not  so  marked  as  in 
the  preceding  group.  No  detrital  material  was  observed,  and  only  one  or 
two  amj^gdaloids,  although  feebly  developed  pseud-amygdaloids  occur  more 
frequently.  A  few  coarse-grained  beds  are  included,  and  there  are  two  or 
three  belts  or  areas  of  i-ed  granitic  porphyry.  Several  narrow  dikes  were 
seen  on  the  coast,  like  those  of  the  Duluth  Group,  and,  like  them,  trending 
with  the  general  direction  of  the  beds,  now  altered  from  what  it  was  near 
Duluth. 

The  prevalent  fine-grained  beds  were  best  seen  on  Lester  and  French 
Rivers.  At  the  mouth  of  Lester  River  and  for  some  distance  below,  and 
again  up  the  river  for  half  a  mile,  are  large  exposures  of  a  very  fine- 
grained, dark-brownish  to  dark-greenish,  compact,  very  heavy  rock,  with 
a  few  minute  porphyritic  feldspars.  The  chief  constituent  is  a  plagioclase, 
in  minute  tabular  crystals,  which  never  give  higher  angles  than  for  oligo- 
clase.  Quite  subordinate  in  quantity  are  the  minute  particles  of  augite, 
many  of  which  are  largely  altered  to  a  greenish  substance,  with  which  is 
also  associated  more  or  less  magnetite,  evidently  as  a  product  of  alteration. 


280       COPPER-BEARING  ROCKS  OP  LAKE  SUPERIOR. 

Original  magnetite  and  rare  and  small  porphyritic  oligoclases  complete 
the  list  of  constituents.  The  rock  belongs  to  Pumpelly's  ashbed  type. 
The  southeast  dip,  at  10°,  can  be  plainly  made  out  in  this  rock  at  the  mouth 
of  the  river.  Ascending  Lester  Eiver,  lower  layers  come  to  view.  Near 
the  middle  of  the  west  line  of  the  N.  W.  |  of  Sec.  4,  T.  50,  R.  13  W.,  the 
rock  is  very  fine-grained,  of  a  dark  brownish-gray  color,  and  under  the 
microscope  presents  the  typical  appearance  of  Pumpelly's  melaphyrs, 
namely,  large  augites,  including  numerous  tabular  plagioclases  (oligoclase), 
and  with  numerous  olivines,  altered  to  greenish  and  brownish  substances, 
magnetite,  and  much  red  ferrite  in  the  spaces  between  the  augites.  Near 
the  northwest  corner  of  the  section  a  coarse  band  is  crossed.  Next,  through 
the  greater  part  of  Sec.  33,  where  the  river  makes  four  falls,  whose 
height  aggregates  some  80  feet,  the  rocks  are  very  dense,  dark  brownish- 
gray  diabases,  alternating  with  red-  and  dark-green-mottled  varieties.  These 
are  seen  in  the  thin  section  to  be  the  usual  pseud-amygdaloidal  diabase, 
holding  augite,  often  fresh,  but  often  altered  to  a  dark-green  substance, 
much  reddened  plagioclase,  magnetite,  pseud-amygdaloidal  chlorite,  and 
occasional  porphyritic  oligoclases. 

In  Sees.  6  and  7  of  T.  51,  R  12  W.,  French  River  makes  nine  falls 
over  the  rocks  of  this  group,  the  individual  falls  running  from  6  to  70  feet 
in  height.  Between  these  falls  the  river  rushes  down  an  inclined  plane 
nearly  on  the  slope  of  the  S.  E.  dip.  The  rocks  in  sight  are  largely 
fine-grained,  rough-textured,  luster-mottled  kinds,  ranging  from  black  to 
bright  red  in  color,  according  to  the  amount  of  secondary  peroxide  of  iron 
present.  They  are  very  highly  augitic,  with  much  magnetite  and  olivine 
in  minute  particles,  wholly  altered  to  a  green  substance,  between  the  aiigite 
grains.  All  specimens  show  rare  and  small  porphyritic  oligoclase,  and 
there  is  often  pseud-amygdaloidal  chlorite.  The  rock  at  the  falls  in  the  N. 
"W.  \  of  Sec.  6,  beyond  which  the  river  was  not  ascended,  is  very  dense, 
with  conchoidal  fracture  and  of  a  dark-brown  to  nearly  black  color.  Its 
thin  section  shows  the  common  characters  of  this  variety,  viz,  predominant 
plagioclase  (oligoclase)  in  small  tabular  crystals,  and  the  augite  in  minute 
rounded  particles.  One  amygdaloid  was  noticed  on  French  River,  near 
the  north  line  of  Sec.  7,  and  730  steps  west  from  the  northeast  corner  of 


THE  LESTBE  EIVEE  BEDS.  281 

the  section.  The  amygdules  are  thickly  crowded,  small,  often  elongated, 
and  chiefly  composed  of  radiating  laumontite  in  the  specimen  brought  away. 
The  fine-grained,  conchoidal-fracturing,  brownish  rock,  with  accompanying 
laumontitic  amygdaloid,  seen  on  Encampment  River,  in  the  S.  E.  ^  of  Sec. 
10,  T.  53,  R  10  W.,  is  probably  to  be  placed  with  the  fine-grained  rocks 
of  the  Lester  River  Group. 

It  was  not  possible  to  determine  whether  all  of  the  coarse-grained 
rocks  of  the  Lester  River  Group  are  interbedded  flows  and  not  dikes,  but 
most  of  them  are  plainly  the  former,  and  the  rocks  of  those  exposures 
whose  relations  were  doubtful  are  in  all  respects  identical  with  those  of  the 
undoubted  beds.      As  an  example  may  be  mentioned  the  rock  quarried 
below  the  mouth  of  Chester  Creek.     The  thin  section  of  this  rock  is  figured 
at  Figs.  3  and  4  of  Plate  II,  and  is  further  described  in  the  table  on  page 
46.      It  is  a  medium-grained,  highly   crystalline,  black,  rough-textured, 
olivine-gabbro  or  diabase,  consisting  chiefly  of  anorthite  and  diallagic  augite, 
and  containing  also  large  particles  of  olivine  and  titaniferous  magnetite. 
Externally  it  presents  a  luster-mottling,  such  as  is  seen  in  the  finer  rocks, 
and  from  the  same  cause.     It  is  the  same  rock  that  forms  the  few  coarse 
beds  of  the  Duluth  Group,  the  uppermost  amygdaloids  of  which  group  it 
closely  overlies.     It  forms  a  bed  of  very  considerable  thickness,  and  can 
be  followed  along  the  lake  shore  for  many  rods,  varying  somewhat  in 
coarseness  of  grain.     A  similar  rock  closely  overlies  the  fine-grained  dia- 
base of  the  mouth  of  Lester  River.     In  it  the  olivines  are  more  highly 
altered,  being  almost  wholly  changed  to  a  brownish  ferruginous  substance. 
This  layer  can  also  be  traced  for  a  long  distance  on  the  shore.     Similar 
rocks  show  again  on  French  River,  near  the  north  line  of  Sec.  7,  T.  51,  R. 
12  "W.,  and  in  a  great  ledge  200  feet  high  on  the  west  line  of  the  S.  "W.  ^ 
of  Sec.  26,  T.  53,  R.  11  W.     The  rock  on  French  River  occurs  plainly  in- 
terbedded with  the  fine-grained  rocks  already  described.     It  is  moderately 
coarse  in  grain,  gray,  minutely  spotted  with  red,  and  of  a  rough  texture.     It 
consists  chiefly  of  anorthite,  diallagic,  very  fresh  augite  in  large  crystals, 
each  one  of  which  includes  several  detached  areas,  and  olivine,  which 
occurs  in  large  patches  crossed  by  black,  brown,  and  red  bands  of  iron- 
oxide.     The  thin  section  is  represented  in  Figs.  1,  2,  and  4  of  Plate  III. 


282       OOPPBE-BBAEING  EOOKS  OP  LAKB  SUPEEIOE. 

The  rock  in  Sec.  26,  T.  53,  R.  11  W.,  forms  a  ridge  which  presents  a  bold 
cli£P  to  the  northwest  and  a  gradual  slope  to  the  southeast.  Macroscopic- 
ally  it  is  somewhat  different  from  any  of  the  foregoing,  presenting  a  very 
light-gray  color,  mottled  with  darker  shades,  but  the  thin  section  shows 
that  it  is  essentially  the  same  rock,  and  that  the  differences  are  due  to 
smaller  amounts  of  augite  and  to  the  great  freshness  of  the  rock.  Even 
the  very  large  olivines  are  unusually  fresh,  being  traversed  only  by  a  few 
rifts  bordered  by  a  greenish  alteration-product.  Coarse-grained,  rough- 
textured  black  olivine-gabbro  belonging  to  the  Lester  River  Group  forms 
the  barrier  rock  of  the  falls  of  Encampment  River  in  the  N.  E.  ^  of  Sec.  10, 
T.  53,  R.  10  W. 

Another  kind  of  coarse-grained  rock  is  presented  in  the  ledges  on  the 
west  line  of  Sec.  28,  T.  54,  R.  13  W.  This  is  an  orthoclase-gabbro,  carry- 
ing orthoclase,  oligoclase,  diallage,  augite  in  long-twinned  blades,  apatite 
and  a  good  deal  of  secondary  quartz.  Externally  it  is  brownish-black  and 
resinous-looking  from  alteration,  and  peculiar  for  its  long-bladed  augite 
crystals.    The  thin  section  of  this  rock  is  pictured  in  Figs.  1  and  2  of  Plate  V. 

The  red  porphyries  of  the  Lester  River  Group  can  be  best  seen  on  the 
lake  shore  both  above  and  below  Lester  River.  The  importance  of  the  place 
not  being  realized  at  the  time,  sufficient  attention  was  not  given  to  it  to 
determine  the  relation  of  these  red  rocks  to  those  adjoining  them.  Some 
of  the  specimens  brought  away  show  a  medium-grained,  highly  crystalline 
rock,  which  in  the  thin  section  looks  as  if  it  might  be  a  very  greatly  altered 
orthoclase-gabbro.  Quite  a  little  augite,  much  of  which  is  fresh,  is  con- 
tained, and  the  reddened  feldspars  are  filled  with  secondary  quartz.  Other 
specimens  show  a  rock  more  like  the  granitic  porphyry  of  "Duluth,  and 
others  again  are  distinctly  felsitic  porphyries.  Even  the  latter  kinds  are 
saturated  with  secondary  quartz,  and  all  kinds  so  much  altered  that  the 
original  condition  of  the  matrix  is  not  easy  to  determine.  At  one  point 
below  the  mouth  of  Lester  River  a  coarse,  black  olivine-gabbro  includes 
patches  and  vein-like  bands  of  the  red  rock,  in  this  case  one  of  the  more 
distinctly  crystalline  and  augite-bearing  kinds,  while  a  few  rods  farther 
along  the  shore  the  same  red  rock  forms  the  whole  face  of  the  exposure.* 
Some  of  the  bands  of  red  in  the  black  gabbro  are  only  a  few  inches  wide 


RED  EOCKS  AND  DIKES  OF  THE  LESTEE  EIVEE  GEOUP.      283 

and  have  serpentine  courses,  intersecting  one  another.  There  can  be  no 
doubt  that  the  rock  and  conditions  are  the  same  as  observed  in  the  case  of 
the  red  rock  penetrating  the  gabbro  at  Duluth.  The  red  rock  below  Lester 
River  becomes  more  and  more  fine-grained  as  it  is  followed  down  the  coast, 
until  it  presents  the  appearance  of  a  felsite,  with  distinct  red  orthoclases. 
Still  farther  it  is  much  weathered  and  earthy,  with  seams  of  calcite  and 
lai'ge  sized  "vugs,"  lined  with  fine  crystals  of  the  same  mineral.  In  the 
same  vicinity  it  is  thickly  dotted  with  amygdule-like  spots  of  white  calcite, 
one-fourth  inch  in  diameter.  Whether  these  are  true  amygdules  or  replace- 
ments has  not  been  determined.  True  quartz  porphyry  and  granitic  por- 
phyry are  exposed  along  Encampment  River  in  the  N.  W.  \  of  Sec.  11,  T. 
53,  R.  1  W.,  with  a  width  of  400  paces.  This  belt  lies  at  or  near  the  sum- 
mit of  the  Lester  River  Group 

Below  Lester  River  these  red  rocks  were  observed  to  be  cut  by  sev- 
eral narrow  dikes,  10  to  20  feet  wide,  and  trending  N.  45°  to  50°  E.,  or 
with  the  strata.  These  dikes  were  marked  by  a  very  strong  cross-jointing, 
and  near  the  walls  by  a  close-jointing  parallel  to  the  walls.  In  the  middle 
of  the  dikes  the  rock  is  black,  fine  grained,  but  highly  crystalline,  and 
rough  in  texture.  Towards  the  sides  where  the  jointing  parallel  to  the  walls 
comes  in  it  is  aphanitic,  dark-green  in  color,  and  greasy  from  the  presence 
of  chlorite.  A  section  of  the  rock  from  the  middle  portion  shows  augite 
predominating,  partly  fresh,  and  partly  altered  to  a  brownish  substance,  in 
areas  enveloping  numbers  of  minute  plagioclases  (labradorite),  just  as  in 
the  "luster-mottled"  melaphyrs  of  Pumpelly.  Magnetite  is  present  in 
small  particles,  and  besides  the  viriditic  and  ocherous  material,  evidently 
resulting  from  a  change  of  the  augite,  there  are  other  particles  of  some- 
what similar  material,  usually  of  a  deeper  tint,  lying  between  the  augites 
in  small  rounded  forms  which  show  no  tendency  to  polarize  together.  These 
are  evidently  altered  olivines,  and  the  resemblance  to  Pumpelly 's  luster- 
mottled  rocks,  save  in  unusual  fineness  of  grain,  is  thus  complete.  The 
section  of  the  finer-grained  rock  from  the  side  of  the  dike  shows  it  to  be 
the  same,  except  that  it  is  in  an  excessively  fine  condition,  has  its  augite 
largely  changed  to  a  chloritic  substance,  and  contains  some  non-polarizing 
base. 


284       COPPBE-BEAEIFG  EOCKS  OF  LAKE  SUPEEIOE. 

The  Agate  Bay  Group. — The  Agate  Bay  Group  of  beds  is  finely  dis- 
played in  its  entire  thickness  of  some  1,500  to  2,000  feet,  along  the  lake 
shore  for  a  distance  of  some  34  miles,  between  a  point  in  the  S.  W.  \  of 
Sec.  34,  T.  51,  R.  13  W.,  a  mile  and  a  half  below  the  mouth  of  Lester 
River,  and  one  in  the  S.  E.  \  of  Sec.  14,  T.  54,  R.  9  W.,  two  miles  above 
the  mouth  of  Encampment  River.  These  beds  ai-e  also  to  be  seen  exposed 
on  French,  Knife,  Encampment,  and  Gooseberry  rivers,  for  short  distances 
from  their  mouths. 

The  most  striking  external  characteristics  of  this  group,  as  compared 
with  those  previously  described,  are  the  relative  thinness  and  distinctness 
of  its  beds;  the  great  number  of  highly  vesicular  amygdaloids,  which 
must  make  up  more  than  half  the  entire  thickness  of  the  group;  the  pe- 
culiar appearance  of  subordinate  stratification  presented  by  both  amygda- 
loidal  and  compact  portions  of  the  layers,  when  weathered;  the  prevalent 
fine  grain  of  all  save  one  or  two  of  the  beds,  and  the  presence  of  two  or 
three  thin  layers  of  red  sandstone,  shale  and  conglomerate. 

The  lower  beds  of  this  group,  which  are  to  be  seen  along  the  shore  in 
Sees.  34,  35,  24,  and  2(5,  T.  51,  R.  13  W.,  trending  more  to  the  northward 
than  the  coast  and  dipping  southeast  about  15°,  are  somewhat  peculiar. 
The  rocks  exposed  about  the  lower  part  of  Encampment  River  in  Sec.  1 1 , 
T.  53,  R.  10  W.,  appear  also  to  belong  here,  as  do,  in  part,  those  along 
the  shore  for  two  or  three  miles  above  the  mouth  of  the  same  river,  the 
broad  bay  into  which  this  river  empties  setting  back  far  enough  to  reach 
these  lower  layers.  The  non-amygdaloidal  portions  of  these  lower  beds 
are  composed  largely  of  very  dense  conchoidal-fracturing  diabases  of  the 
ashbed  type,  some  having  a  dark-greenish  to  black  color,  while  others  have 
a  more  reddish-brown  color,  when  the  grain  is  excessively  fine.  There  is 
often  more  or  less  unindividualized  material,  when  the  rock  becomes  a 
diabase-porphyrite.  Other  layers  again  have  the  compact  portions  a  coarser 
rock,  often  much  altered  and  crumbly,  and  of  various  purple  and  brown 
shades.  These  are  the  usvial  Keweenawan  fine-grained  olivine-free  diabases. 
The  amygdaloids  of  all  these  beds  are  plainly  marked.  They  carry  chiefly 
laumontite,  calcite  and  quartz  in  the  cavities,  which  are  often  of  large  size 
(^  to  J  inch),  smooth-walled,  and  elongated  in  a  common  direction.  Many 
are  empty,  and  being  thickly  strewn,  the  result  is  a  completely  honey- 


THE  AGATE  BAY  BEDS.  285 

combed  rock.  In  one  place,  about  two  miles  below  Lester  River,  one  of 
the  hard  reddish-brown  dense  beds  above  referred  to  was  furnished  not  only 
with  an  upper,  but  with  a  basal  amygdaloid,  in  which  the  cavities  were 
large,  smooth-walled,  and  quite  regularly  oval,  often  empty,  or  lined  with 
drusy  quartz. 

A  fine  showing  of  one  of  these  hard  beds  is  to  be  met  with  on  the 
coast  near  the  center  of  Sec.  22,  T.  53,  R.  10  W.,  about  ten  miles  above 
the  mouth  of  Encampment  River.  Here  is  a  bold  cliff  25  feet  high,  of 
the  hard,  dense,  light-brown  rock,  which  below  is  without  amygdules,  but 
which  as  it  is  traced  up  the  cliff  becomes  more  and  more  amygdaloidal, 
finally  becoming  a  highly  vesicular  amygdaloid,  with  large  cavities  elon- 
gated in  a  common  direction,  and  carrying  saponite,  calcite  and  laumontite. 
Under  the  microscope  the  non-amygdaloidal  portion  of  this  rock  is  seen  to 
be  chiefly  composed  of  tabular  plagioclases  (oligoclase)  set  in  a  brownish, 
iron-infiltrated  matrix  which  will  not  affect  the  polarized  light.  Augite  is 
present  only  in  very  rare  minute  rounded  grains  and  in  a  few  porphyritic 
crystals.  Particles  of  magnetite  and  porphyritic  oligoclases  complete  the 
list  of  ingredients.  The  rock  is  a  diabase- porphy rite  with  the  augite  nearly 
or  wholly  wanting. 

Some  of  the  less  dense  beds  showed  pseud-amygdaloidal  phases,  with 
pseud-amygdules  chiefly  of  a  dark-greenish  chlorite,  and  at  several  points 
on  the  shore  of  Sec.  25,  T.  51,  R.  13  "W.,  a  crumbling  pseud-amygdaloid 
was  found  with  a  light  reddish  color,  mottled  with  dark-green  chlorite 
pseud-amygdules,  and  presenting  at  first  sight,  especially  on  cross  fract- 
ure, a  strong  resemblance  to  a  reddish  sandstone,  with  angular  grains. 
This  resemblance  is  heightened  by  the  fact  that  the  rock  has  a  bedded 
appearance.  A  careful  examination,  however,  of  the  weathered  surface  of 
the  rock  shows  that  it  probably  has  a  completely  crystalline  texture,  and 
this  is  abundantly  proved  to  be  the  case  by  the  thin  section,  which  shows 
-that  the  rock  is  nothing  but  one  of  the  usual  pseud-amygdaloids,  of  inter- 
locking crystalline  texture,  and  with  no  trace  of  fragmental  origin.  This 
rock  would  undoubtedly  be  taken  for  a  sandstone  by  most  observers  at  first 
sight. 


286       COPPEE-BEAEmG  EOCKS  OF  LAKE  SUPEEIOE. 

.  The  remaining  two-thirds  of  the  Agate  Bay  Group  forms  the  most  of 
the  coast  from  Talmage  River  to  beyond  Gooseberry  River,  in  which  dis- 
tance there  is,  on  the  whole,  an  ascent  of  the  coast  line  in  geological  horizon; 
but  there  are  minor  descents  and  ascents  according  to  the  relations  between 
the  irregularities  of  the  coast  and  the  trends  of  the  strata.  The  dips  arfe 
flatter  than  further  east,  never  exceeding  10°,  and  sometimes  sinking  to  5° 
for  considerable  distances.  The  prevailing  rock  of  the  non-amygdaloidal 
portions  of  the  beds  is  fine-grained  and  olivine-bearing,  having  nearly  always 
the  characters  of  Pumpelly's  melaphyrs,  i.  e.,  large  augites  including  num- 
bers of  minute  plagioclases,  and  much  olivine  and  magnetite  crowded  into 
the  spaces  between  the  augites,  the  olivine  generally  altered  into  a  reddish 
or  greenish  material.  Though  generally  much  finer-grained  than  the  typ- 
ical melaphyr  of  the  Greenstone  of  Keweenaw  Point,  these  rocks  often 
show,  in  the  less  altered  portions,  which  are  then  quite  black  in  color,  a 
distinct  luster-mottling.  In  less  fresh  kinds  there  is  a  tendency  to  weather 
to  a  semi-nodular  surface,  the  augite  resisting  decomposition  better  than 
the  interspaces.  These  more  altered  kinds  run  through  various  shades  of 
brown  and  red,  more  or  less  mottled  with  green.  There  is  a  good  deal  of 
variation  as  to  coarseness  of  grain  in  different  beds,  and  an  extreme  coarse- 
ness carries  the  rock  into  a  true  olivine-gabbro,  which  plainly  enough,  as 
may  be  seen  even  with  the  naked  eye,  is  a  phase  of  the  fine-grained  mel- 
aphyr. Such  a  rock  presents  itself  on  a  large  scale,  with  bedding  surfaces 
hundreds  of  feet  in  length  and  width  shelving  into  the  lake  at  a  low  angle, 
along  the  coast  between  Sucker  River  Bay  and  Knife  River,  in  the  north- 
east part  of  T.  51,  R.  12  W.  Sections  of  this  rock  are  figured  on  Plate  III. 
Encampment  Island,  a  mile  east  of  Encampment  River,  is  again  formed  of 
one  of  these  coai'ser  kinds,  which  also  can  be  seen  forming  a  distinct  layer 
between  finer-grained  beds,  on  a  cliff  side  in  the  north  half  of  Sec.  22,  T. 
53,  R.  10  W,,  50  to  75  feet  above  the  lake. 

The  amygdaloids  of  the  upper  two-thirds  of  the  Agate  Bay  Group  are 
very  strongly  characterized.  In  the  first  place  they  are  very  highly  vesic- 
ular; the  vesicles  are  always  small  and  often  so  closely  crowded  that  when 
the  amygdules  are  dissolved  from  them  the  rock  is  almost  as  open  as  well- 
raised  bread.     The  common  amygdules  are  laumontite,  saponite,  and  cal- 


THE  AGATE  BAT  BEDS.  287 

cite.  This  seems  the  order  of  abundance  for  the  whole  group,  but  in  sep- 
arate layers  either  one  or  the  other  of  the  first  two  may  predominate.  The 
small,  rounded,  white  or  greenish-white  spots  of  saponite  are  very  char- 
acteristic. Prehnite  is  a  much  rarer  amygdule,  and  agate  still  rarer.  An- 
other equally  important  characteristic  is  the  stratiform  appearance  taken  on 
by  these  amygdaloids.  This  appearance,  which  has  received  some  notice 
on  a  previous  page,  is  also  found,  though  to  a  less  extent,  in  the  interbedded 
massive  layers,  or  rather  in  the  massive  lower  portions  of  the  beds  of  which 
the  amygdaloids  form  the  upper  portions.  In  both  amygdaloids  and  compact 
portions  this  stratiform  appearance  is  especially  brought  out  by  weathering. 
This  was  seen  beautifully  illustrated  at  the  mouth  of  a  creek  on  Sec.  15,  T. 
52,  R  11  E.,  about  two  and  a  half  miles  above  Agate  Bay.  The  creek 
enters  the  bay  over  the  low  shore  cliff,  into  which  it  has  worn  its  way  back 
for  some  distance.  Just  where  the  water  flows  over  it  the  rock  is  hard  and 
massive,  without  trace  of  stratiform  appearance,  but  on  either  side  it  may 
be  traced  distinctly  into  the  usual  obscurely  stratiform  material. 

At  a  Httle  distance  an  exposed  cliff  presents  much  the  appearance  of  a 
series  of  sedimentary  beds,  as  for  instance  a  set  of  rather  heavily  bedded 
sandstones  alternating  with  shales;  indeed,  these  rocks,  as  already  indicated, 
were  long  ago  called  "metamorphic  sandstone  and  shale"  by  Norwood,  and 
are  now  regarded  as  such  by  N.  H.  Winchell.  But  a  closer  study  shows 
that  here,  as  everywhere,  both  heavier  and  thinner  layers  are  made  up  of 
rocks  identical  with  the  amygdaloids  and  diabases  of  Keweenaw  Point, 
whose  completely  interlocked  crystalline  condition  in  the  lower  portions, 
and  highly  vesicular  condition  in  the  upper  portions — the  massive  and  vesic- 
ular parts  grading  into  one  another — abundantly  prove  their  origin  as  lava 
flows.  The  lower  portions  of  the  beds  have,  too,  very  frequently,  a  well- 
marked  columnar  structure,  another  characteristic  of  the  Keweenaw  Point 
flows,  and  of  flows  of  eruptive  rocks  generally.  As  already  said,  microscop- 
ically, these  rocks  are  oli vine-bearing  augite-plagioclase  kinds  identical 
with  the  Greenstone  of  Keweenaw  Point,  even  to  the  peculiar  crowding 
between  the  augites  of  the  ohvine  and  magnetite  particles.  They  are 
also  the  same  rocks,  only  finer  in  grain,  as  the  coarse  black  diabases  which 
Norwood  and  Winchell  themselves  regard  as  eruptive,  and — which  will  be 


288       COPPBE-BEAEIFG  EOCKS  OF  LAKE  SUPEEIOE. 

taken  by  some  as  still  more  conclusive  proof  of  eruptive  origin — they  are 
identical,  even  to  the  same  crowding  of  the  olivine  in  the  interspaces,  with 
the  rocks  of  the  narrow  dikes  found  along  the  coast  westward  to  Duluth. 
As  shown  below,  the  rock  of  the  dikes  observed  cutting  the  Agate  Bay  beds 
themselves  is  a  true  luster-mottled  melaphyr. 

The  subordinate  layers,  of  which  both  amygdaloids  and  compact  por- 
tions appear  to  be  made  up,  are  never  regular  or  persistent,  and  in  this 
respect  there  is  a  contrast  with  the  subordinate  layers  of  true  sedimentary 
rocks,  the  most  irregular  of  which  are  never  so  irregular  as  these.  When 
in  the  field  it  was  thought  that  the  peculiar  combination  of  resemblances  to 
sedimentary  beds  and  to  the  usual  eruptive  rocks  of  the  region  shown  in 
these  stratiform  amygdaloids  and  associated  massive  layers,  might  find  ex- 
planation in  the  origin  of  the  amygdaloids  as  volcanic  ashes,^  i.  e.,  frag- 
mental  volcanic  material  stratified  by  water.  In  this  case  the  amygdules 
might  be  pseud-amygdules,  which  could  of  course  originate  as  well  from 
fragmental  basic  material  as  from  the  same  material  in  the  original  massive 
condition.  But  the  study  of  specimens,  and  more  especially  the  mici'O- 
scopic  study  of  thin  sections,  which  develops  the  wholly  non-fragmental 
character  of  the  material,  and  the  true  vesicular  nature  of  the  amygdaloids, 
show  that  such  an  idea  is  wholly  untenable. 

The  following  section,  made  out  along  the  clifis  just  west  of  Agate  Bay, 
serves  well  to  show  the  sort  of  succession  everywhere  to  be  observed  in  these 
stratiform  beds.  The  section  could  have  easily  been  extended  both  up  and 
down,  but  is  sufficient  as  an  illustration.  The  layers  dip  some  6°  to  8°  to 
the  sou.theast.     The  order  is  an  ascending  one. 

Feet. 
lA.  Massive,  vertically  columnar  layer  of  a  medium-grained,  distinctly 
crystalline,  purplish  rock,  mottled  dark  and  light  on  a  weathered 
surface.  Sparsely  scattered  pseud-amygdules  of  calcite  and 
laumontite,  and  of  chlorite,  are  contained.  The  thin  section  shows 
plagioclase;  augite,  inclosing  plagioclases ;  magnetite;  olivine, 
wholly  altered  to  red  oxide  of  iron  and  a  green  substance,  and 
crowded  with  the  magnetite  between  the  augites;  and  pseud- 
amygdules  of  a  pale  greenish  substance.  Grading  into  the  over- 
lying rocks  by  increase  of  abundance  of  amygdules.  Thickness 
seen  above  water 5 

'Norwood  seems  to  have  had  this  idea  with  regard  to  some  of  these  beds,  though  most  of  them 
he  regarded  as  "metamorphic  shales."  See  Owen's  Geological  Survey  of  Wisconsiu,  Iowa,  and  Min- 
nesota, p.  351. 


THE  AGATE  BAY  BEDS.  289 

IB.  Stratiform  laumontitic  amygdaloid,  tlie  irregular  and  non-continuous 
layers  less  than  six  inches  in  thickness.  Matrix  much  as  in  fore- 
going;  finer  grained.    Amygdules  small,  not  exceeding  J  inch, 

thickly  crowded,  of  laumontite,  calcite,  saponite.    Thickness 10 

15 

IIA.  Massive,  vertically  columnar  layer,  of  a  rock  similar  to  that  of  lA.,  but 
finer  grained.  In  places  laumontite  amygdules  or  pseud-amyg- 
dules  run  through  the  whole  thickness.  Subordinately  stratiform 
when  weathered.    Thickness 3 

IIB.  Stratiform  amygdaloid  like  IB.    Thickness 6 

9 

IIIA.  Massive,  vertically  columnar  layer  of  a  rock  closely  resembling  that  of 

lA.,  but  showing  under  the  microscope  larger  olivines  and  more 
pseud-amygdaloidal  chlorite.  Exceedingly  irregular  in  thickness, 
expanding  and  contracting  suddenly  from  a  few  inches  to  several 

feet,  and  vice  versa.    Thickness  from  4  inches  to 3 J 

IIIB.  Stratiform  laumontitic  and  calcitic  amygdaloid;  subordinate  layers  a 

few  inches  thick.     Thickness 6 

H 

IVA.  Massive,   vertically  columnar  layer  like  lA.,   IIA.,   iiiA.;   exceedingly 

irregular  in  thickness,  running  from  nothing  to 2 

IVB.  Stratiform  laumontitic  amygdaloid,  the  amygdules  larger  than  usual, 

reaching  J  inch,  and  even  1  inch.    Thickness 20 

22 

VA.  Massive,  vertically  columnar  layer;  rock  like  that  of  lA.;  very  irreg- 
ular.   Thickness,  nothing  to 2 

YB.  Stratiform  laumontitic  and  calcite  amygdaloid;    layers  so  thin  as  to 
appear  shaly.    Thickness 3 

5 

VI.  Massive,  vertically  columnar  layer,  somewhat  amygdaloidal  towards 

top ;  rock  like  lA.    Thickness 15 

VII.  Eed  shaly  sandstone ;  very  irregular  in  thickness;  running  away  down 
into  crevices  in  the  underlying  massive  rock,  and  in  places  curi- 
ously intermingled  with  the  overlying  amygdaloid.    Thickness 2  to  3 

VIII.  Stratiform  laumontitic  amygdaloid,  layers  at  times  very  thin.  Thick- 
ness   ; , .  .         10 

IX  A.  Massive  layer,  with  the  vertically  columnar  structure  strongly  marked, 
of  a  fine-grained  dark-gray  to  nearly  black  rook,  which  under  the 
microscope  is  seen  to  be  one  of  the  usual  olivine-bearing  mela- 
phyrs,  peculiar  only  in  unusual  fineness  of  grain.  This  layer  forms 
the  greater  jiart  of  the  face  of  the  west  point  of  Agate  Bay.  It  is 
exposed  in  an  immense  surface,  sloping  towards  the  lake,  as  much 
as  a  quarter  of  a  mile  in  length,  and  at  times  several  hundred  feet 
vride.  As  one  walks  over  this  great  surface  the  ends  of  the  col- 
umns, which  show  in  cross-section,  are  finely  displayed,  and  are  seen 
to  lack,  as  usual,  the  regularity  of  the  basaltic  columns  of  some 
regions.  They  are  made  by  the  intersections  of  several  systems 
of  joints,  here  quite  close,  and  have  varying  numbers  of  sides 
19  L  S 


290 


COPPER-BEARING  EOCKS  OF  LAKE  SUPEEIOE. 


andTarying  sizes.  Many  of  the  columns  are 
triangular.  A  fine  cross-section  of  this  layer 
is  seen  on  the  southwest  side  of  Agate  Bay. 

Thickness 10 

IXB.  Stratiform  amygdaloid 8 


-18 


lOlJ 

The  above  section  well  illustrates  the  usual  alterna- 
tions, but  the  layers  are  not  always  so  thin.  The  upper- 
most layers,  seen  farther  down  the  coast,  appear  to  be 
much  heavier.  This  may  be  seen  in  the  vicinity  of 
Castle  Danger,  Sec.  27,  T.  64,  R.  9  W.,  and  at  the  Falls 
of  the  Gooseberry,  section  2 1  of  the  same  township,  where 
the  massive  beds  at  times  exceed  50  feet  in  thickness. 

While  the  Agate  Bay  Group,  throughout  its  whole 
extent,  is  so  plainly  made  up  of  a  series  of  thin  layers  of 
alternating  massive  and  vesicular  material,  and  while  the 
general  flat  lakeward  dip  is  very  evident,  there  are 
numerous  minor  irregularities,  such  as  running  out  of 
individual  layers,  warping  of  the  layers,  and  faulting. 
All  of  these  render  it  very  difficult  to  trace  out  the  sub- 
ordinate succession  for  any  considerable  thickness.  The 
warping  of  the  layers  is  very  interesting.  As  one  fol- 
lows the  lake  shore  single  beds  may  be  seen  disappearing 
beneath  the  lake,  and  reappearing  within  a  few  hundred 
feet,  while  in  some  cases  the  warping  is  much  more  vio- 
lent, the  rocks  for  short  distances  presenting  the  appear- 
ance of  steeply  inclined  strata,  but  soon  recovering  their 
usual  position.  The  general  flat  lakeward  dip,  the  slight 
deviation  of  the  general  trend  of  the  strata  from  that  of 
the  lake  coast,  the  warping  just  alluded  to,  and  the  minor 
irregularities  of  the  coast  line  all  combine  to  produce 
many  peculiar  effects  in  the  appearance  of  the  stratifi- 
cation, which  at  first  sight  are  often  puzzling. 

Faults,  apparently  of  small  extent, — i.  e.,  under  100 


::t 


'=r-/ 


■f-^ 


■u 


!?! 


-V 


0^. 


(\»'f" 


"R. 


?^fT=—5>«i 


■sin 


'\~- 


— v?vv-\ 


Vi  I 


i-^i 


THE  AGATE  BAT  BEDS.  291 

feet — were  seen  at  a  mimber  of  points,  and  it  is  quite  possible  that  greater 
ones  exist,  though  hardly  such  as  could  affect  very  greatly  the  estimate 
given  for  the  total  thickness  of  the  group. 

Some  peculiar  appearances  noted  in  the  massive  rocks  in  the  upper 
part  of  the  Agate  Bay  Group  deserve  further  attention.  At  several  points 
east  of  the  mouth  of  Encampment  River,  bright-red  bands  an  inch  or  two 
in  width  were  noticed  standing  out  on  weathered  surfaces.  The  bands  are 
serpentine,  making  all  sorts  of  irregular  curls,  and  intersecting  one  another 
without  any  system.  To  the  naked  eye  the  bands  and  inclosing  rock 
appear  different  only  in  color,  even  this  difference  being  much  less  notice- 
able on  a  fresh  surface.  A  thin  section  of  one  of  these  bands  with  some 
of  the  adhering  wall  rock,  from  the  shore  of  the  N.  E.  ^  of  Sec.  12,  T.  53, 
R.  10  W.,  shows  plainly  under  the  microscope  that  both  band  and  inclosing 
rock  are  the  same,  save  for  the  numerous  red  hematite  particles  and  greater 
abundance  of  altered  olivines  in  the  former.  This  peculiar  appearance  is 
probably  a  flowage  result. 

Another  peculiar  appearance  is  the  resemblance  to  a  boAvlder-conglom- 
erate  presented  in  a  few  places  by  some  of  the  more  massive  layers.  For 
instance,  on  the  shore  of  the  S.  E.  ^  of  Sec.  14,  T.  54,  R.  9  E.,  about  two 
miles  above  the  mouth  of  Split  Rock  River,  the  shore  cliff  is  formed  of  a 
compact,  purplish,  fine-grained  rock,  which  on  a  weathered  surface  presents 
the  peculiar  light  and  dai'k  mottling  so  characteristic  of  the  fine-grained 
olivine-diabases  or  melaphyrs.  In  other  words,  it  is  the  usual  massive  rock 
of  the  Agate  Bay  Group.  At  one  point,  near  the  water's  edge,  this  very 
massive  dense  rock  presented  a  marked  stratiform  appearance,  though 
plainly  grading  on  either  side  and  upward  into  a  rock  without  this  structure. 
At  the  same  time  there  appeared  to  be  contained  large,  rounded,  protruding 
bowlders.  Close  inspection  showed  these  ball-like  masses  to  be  exactly  the 
same  as  the  rock  inclosing  them,  into  which  they  graded  insensibly.  The 
thin  section  of  both  rock  and  apparent  bowlder  proves  to  be  a  fine-grained, 
typical,  luster-mottled  melaphyr,  with  olivine  and  very  abundant  magnetite 
in  the  interspaces  of  the  large  augites.  The  structure  is  apparently  referable 
to  the  spheroidal  form  so  often  taken  on  by  basaltic  rocks  in  cooling. 

Yet  another,  but  quite  different  appearance,  as  of  foreign  masses  con- 


292       COPPER-BEARING  ROOKS  OF  LAKE  SUPERIOR. 

tained  in  the  eruptive  rocks,  was  noted  on  the  shore  near  the  east  line  of 
Sec.  1,  T.  53,  R.  10  E.  Here  the  shore  cliff  for  some  distance  is  a  black 
rock,  coarser  than  the  most  of  the  massive  rocks  of  the  Agate  Bay  Group. 
This  rock  contains  angular  masses  of  a  very  coarse  white-weathering  an- 
orthite-rock,  of  all  sizes  from  fragments  a  few  inches  across  to  others  three 
or  four  feet  in  length.  These  masses  are  irregularly  and  never  abundantly 
distributed.  The  appearance  is  like  what  is  often  met  with  in  the  overlying 
or  Beaver  Bay  Group  of  beds,  and  is  further  described  in  connection  with 
that  group. 

The  sandstone  layers  observed  in  the  Agate  Bay  Group  are  all  very 
-thin,  never  exceeding  three  or  four  feet  in  thickness.  They  were  met  with 
at  five  different  points  along  the  coast  between  Lester  and  Gooseberry 
rivei-s,  and  probably  at  three  different  horizons.  About  three  miles  below 
Lester  River,  on  Sec.  34,  T.  51,  R.  13  W.,  large  water-worn  surfaces  of 
one  of  the  dense,  hard,  brown,  laumontitic  amygdaloids,  above  described  as 
characterizing  the  lower  portions  of  the  Agate  Bay  Group,  are  seen  to  be 
traversed  by  intersecting  seams  of  bright-red  sandstone,  from  a  fraction  of 
an  inch  to  several  inches  in  width  Besides  the  larger  areas  of  amygda- 
loid contained  by  the  intersecting  sandstone  seams,  there  are  smaller  pieces 
completely  surrounded  by  the  sandstone  of  one  seam.     Fig.  13  represents 

these   intersecting   seams  as 
.i/j,  t.  v..  A-^  ■':-J^^^^^^^^^^^:^I-'^,-?'-^^V^^JW?:>  seen  m  an  area  20  by  10  feet. 


^^^>^^^'M^>^^i^^^^^^'^W9^^W^M'>  The  sandstone  of  these  seams 


■.) 


o'^  is  fine-grained,  dark-reddish 


^tiM^<^SM^MM^cMj^5SI^^!MM  and  argillaceous.     The  thin 

section  shows  it  to  be  wholly 


'  'a  -^. 


r'5ir;H-i^ir  ?. riP  s  ~  C\' 


'^Sf^^-U ^^if^^WU^M^^  fragmental,  and  to  be  com- 
*--c€i.^^^^a>.;s^^^^g,^ti^c^s^cdlh^^'^:^  posed  of  minute  particles  of 

Fig.  13. -Sandstone  "veins,"  Minnesota  coast-plan.  quartz   with   Other   porphyry 

detritus,  together  with  here  and  there  dark-colored  particles  and  particles 
of  tricUnic  feldspar  which  probably  are  from  some  of  the  basic  eruptives. 
When  first  seen  these  apparent  veins  of  sandstone  showed  no  connection 
with  any  other  sandstone,  but  a  few  rods  farther  down  the  coast  the  amyg- 
daloid in  which  they  occur  was  found  overlain  by  the  remnant  of  a  thin 


SANDSTONES  AND  DIKES  OF  AGATE  BAY  GEOUP. 


293 


red  sandstone  layer,  or  rather  conglomerate,  the  pebbles  being  wholly  o£ 
amygdaloid.  Proceeding  down  the  coast,  the  next  sandstone  met  with  is 
the  seam  already  described  in  the  section  given  of  the  succession  at  Agate 
Bay.  This  sandstone  is  much  like  the  one  last  mentioned,  descending  in 
the  same  way  far  down  into  the  crevices  of  the  underlying  I'ock,  and  inter- 
mingled curiously  with  amygdaloid.  It  is  only  two  or  three  feet  thick. 
Still  farther  down  the  coast,  on  the  N.  E.  J,  Sec.  32,  T.  5',  R.  10  E.,  red 
shaly  sandstone  was  noticed  occurring  with  stratiform  laumontitic  amygda- 
loid, as  represented  in 

Fig.   14.      The    sand-    /^-?^ii£S:t^^.-3^?^5l?rS^M^.'"S'^4S|^^^ 
stone  layer  with  which 
this  must  connect  was 
not  seen  here.    At  the  -?»<■ 


projecting  point   near 

the  S.  E.  corner  of  Sec. 

1,  T.  53,  R.10  W.,  thin       "^^ 

sandstone    seams    are 

to  bp  sppn   intprlpavprl  ^'*''  ^^' — Sketch  of  cliif  on  Minnesota  coast,  showing  penetration  of 

fissures  of  amygdaloid  by  sandstone. 

with  the  melaphyr,  and 

on  the  shore  of  Sec.  32,  T.  54,  R.  9  W.,  two  and  a  half  miles  above  the  mouth 
of  Gooseberry  River,  is  a  remnant  of  a  porphyry-conglomerate — the  pebbles 
small,  usually  less  than  one-eighth  of  an  inch  across,  and  subangular — with 
abundant  calcareous  cement. 

A  very  few  narrow  dikes,  much  like  those  of  the  Lester  River  Group, 
were  observed  cutting  the  layers  of  the  Agate  Bay  Group.  One  of  these  is 
to  be  seen  on  the  shore  of  Sec.  34,  T.  51,  R.  13  W.,  two  miles  below  Lester 
River.  It  corresponds  in  trend  with  the  bedded  rocks  which  it  cuts,  but 
lies  at  right  angles  to  the  dip.  It  is  composed  of  a  dense  black  rock,  which 
has  not  been  examined  microscopically,  is  very  strongly  and  closely  cross- 
jointed,  and  is  only  a  foot  wide.  A  very  prominent  dike,  five  feet  wide,  is 
seen  in  the  cliff  on  the  shore  of  Sec.  15,  T.  53,  R.  10  W.  Fig.  15  shows 
the  occurrence  of  this  dike,  which  is  composed  of  a  very  fine-grained,  black, 
luster-mottled  melaphyr,  the  thin  section  of  which  looks  much  like  that 


294 


COPPER-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 


Tt^sle^  Zz?z-^ 


of  the  Greenstone  of  Keweenaw  Point.    It  consists  chiefly  of  very  fresh  augite 

in  relatively  large  areas,  inclosing 


numbers  of  tabular  plagioclases  (an- 
orthite),  and  having  in  the  inter- 
spaces, which  are  chiefly  occupied 
by  the  same  tabular  plagioclases, 
many  small  altered  olivines  and 
particles  of  magnetite.  The  dike  is 
strongly  cross-jointed,  save  at  the 
edges,  which  are  traversed  by  joints 
parallel  to  the  walls,  and  are  com- 
posed of  an  aphanitic  rock,  much 
altered  to  chlorite.  The  rocks  tra- 
versed    are    the    usual     stratiform 

Fig.  15.— Dike  traversing  stratiform  amygdaloid  and  amygdaloid     and     Columnar     mola- 
columnar  melaphyr,  two  miles  below  Lester  Eiver,  r-     i        a  -r>         r~i 

Minnesota  coast.  phyr  ol  the  Agate  liay  Uroup. 

Equivalents  of  the  Duluth,  Lester  Biver,  and  Agate  Bay  Groups  at  the  east 
end  of  the  Minnesota  coast. — At  the  eastern  end  of  the  Minnesota  coast  there 
intervenes,  between  the  Huronian  slates  and  the  base  of  the  Beaver  Bay 
Group,  which  next  overlies  the  Agate  Bay  beds,  a  space  only  three  and  a  half 
miles  wide,  measured  at  right  angles  to  the  east  and  west  strike.  With  the 
flat  dip  prevalent  in  this  region  this  width  cannot  include  a  total  thickness  of 
more  than  3,000  feet,  while  at  the  Duluth  end  of  the  coast  there  lie  between 
the  same  horizons  the  whole  of  the  Duluth,  Lester  Eiver,  and  Agate  Bay 
groups,  a  thickness  of  some  9,000  feet;  not  to  speak  of  the  Duluth  gabbros, 
which  must  add  several  thousand  feet  more.  Forty  miles  west  of  Grand 
Portage,  however,  about  Brul^  Lake,  in  T.  63,  R.  2  W".  and  R.  3  W.,  the 
gabbros  are  present  in  full  force,  while  between  them  and  the  lower  limit 
of  the  Beaver  Bay  Group  there  is  a  width  of  10  miles,  within  which  space, 
supposing  the  dip  to  be  not  more  than  10°,  a  figure  which  the  observations 
along  Cascade  River  show  to  be  closely  right,  there  is  room  for  a  much 
greater  thickness  of  the  Duluth,  Lester  River,  and  Agate  Bay  groups.  The 
small  thickness  on  the  lake  shore  near  Grand  Portage  is  probably  to  be 


KOCKS  OF  CASCADE  EIVEE.  295 

assigned  in  part  to  actual  thinning,  such  sudden  thinnings  being  easily 
explicable  when  the  rock  layers  are  nearly  altogether  of  eruptive  origin; 
but  also,  in  part,  to  some  non-conformity  with  the  underlying  slates.  How- 
ever, the  whole  question  of  the  way  in  which  the  Duluth,  Lester  River, 
and  Agate  Bay  groups  extend  to  the  eastward  is  one  which  will  have  to 
be  further  studied  in  the  interior  along  the  various  rivers  entering  the  lake — 
especially  along  Brul^,  Cascade,  Poplar,  and  Temperance  rivers. 

Cascade  River  was  examined  by  Messrs.  Chauvenet  and  McKinlay 
above  Sec.  26,  T.  62,  R.  2  W.,  but  this  was  not  far  enough  down  stream 
to  determine  the  existence  or  non-existence  here  of  the  Agate  Bay  beds. 
The  first  rocks  met  with  by  Mr.  Chauvenet  on  the  Cascade  were  found  just 
where  he  first  struck  the  river  in  Sec.  26,  T.  62,  R.  2  W.  Here,  for  about 
half  a  mile,  there  are  exposed  in  the  bed  and  on  the  sides  of  the  river,  beds 
of  a  dark-gray  to  reddish-brown  aphanitic  rock  with  conchoidal  fracture, 
much  like  some  of  the  dense  brown  rocks  of  the  Lester  River  Group.  The 
only  thin  section  made  showed  a  diabase-porphyrite,  composed  chiefly  of 
minute  tabular  plagioclases  with  a  good  deal  of  a  matrix  which  shows  but  a 
feeble,  flickering  light  when  revolved  between  the  crossed  nicols.  These 
rocks  are  quite  plainly  bedded,  dipping  southward  at  about  9°.  In  places 
they  seem  to  show  a  subordinate  structure  parallel  to  the  bedding,  while 
miost  exposed  surfaces  are  so  weathered  as  to  fall  in  showers  of  small  frag- 
ments when  struck  with  a  hammer.  One  or  two  vesicular  layers  were 
noted,  the  vesicles  smooth-surfaced,  elongated,  one-sixth  to  one-fourth  inch 
in  length,  and  either  empty  or  lined  with  drusy  quartz. 

Continuing  the  ascent  of  Cascade  River,  no  more  exposures  were  found 
until  reaching  the  falls  in  section  10.  In  the  vicinity  of  these  falls  the  rocks 
succeed  one  another  in  the  following  descending  order:  (1)  dark-gray  very 
fine-grained  rock,  resembling  the  fine-grained  diabases  of  the  Duluth  and 
Lester  River  groups,  not  examined  under  the  microscope;  (2)  brick-red 
quartz-porphyry;  (3)  medium-grained,  black  gabbro  or  luster-mottled  mel- 
aphyr.  The  last  rock  forms  the  barrier  over  which  the  river  falls.  Five 
hundred  feet  above  the  falls  comes  in  a  medium-grained,  brownish-gray 
orthoclase-gabbro,  the  thin  section  of  which  resembles  closely  that  of  the 
peculiar  orthoclase-gabbro  of  the  Lester  River  Group  seen  largely  exposed 


296       COPPEE-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 

on  the  west  line  of  Sec.  28,  T.  51,  R.  13  W.  It  shows  labradorite,  orthoclase, 
pale-greenish-brown  augite — often  in  twinned  blades — titaniferous  mag- 
netite, apatite  and  a  good  deal  of  secondary  quartz.  The  next  rock  noted 
was  half  a  mile  up  stream  at  the  40-foot  fall,  which  is  somewhere  near  the 
southern  part  of  Sec.  11,  T.  62,  R.  2  W.  These  falls  are  over  a  very  fine- 
grained, grayish-brown  rock,  resembling  the  predominant  gray  diabases  of 
the  Duluth  Group.  There  are  strong  appearances  here  of  a  southern  dip  at 
an  angle  between  10°  and  15°.  Above  these  falls  for  three-fourths  of  a 
mile  the  river  is  expanded  into  a  lake;  then  comes  another  fall  over  the 
same  compact,  gray  rock,  which  is  exposed  also  above  the  falls  on  a  large 
scale.  The  thin  section  of  a  specimen  taken  from  the  bed  of  the  river  above 
the  falls  bears  out  completely  the  external  resemblance  to  the  Duluth  gray 
diabases.  For  a  mile  above  this  fall  the  river  is  again  a  lake,  at  whose 
upper  end  are  again  exposures  of  a  fine-grained,  gray  to  black  rock,  with 
luster-mottlings  one-eighth  inch  across. 

The  prevalence  among  the  rocks  of  the  upper  Cascade  of  fine-grained 
diabases  closely  resembling  those  near  Duluth  and  Lester  River;  the  occur- 
rence among  them  also  of  quartz-porphyry,  coarse-grained  black  gabbro, 
and  of  the  peculiar  orthoclase-gabbro  with  augite  twins;  the  nearly  com- 
plete absence,  so  far  as  observed,  of  amygdaloids;  the  geographical  posi- 
tions of  the  exposures  with  regard  to  the  older  rocks  about  Brul^  Lake  and 
Eagle  Mountain,  and  the  newer  ones  on  the  lake  coast — all  combine  to 
render  it  highly  probable  that  we  have  here  to  do  with  the  eastern  exten- 
sions of  the  Lester  River  and  Duluth  groups. 

The  exposures  at  the  eastern  end  of  the  Minnesota  coast  between  the 
base  of  the  Beaver  Bay  Group  and  the  Animikie  slates  are  unfortunately 
not  continuous,  having  between  them  long  beaches.  The  base  of  the 
Beaver  Bay  Group  intersects  the  shore  about  six  miles  above  Poi'tage  Bay 
Island.  Below  here  for  about  a  mile  were  observed  numerous  exposures  of 
a  very  dense  brown,  conchoidally  fracturing  rock,  which  is  plainly  bedded, 
and  dips  8°  to  10°  south,  with  a  due  east  and  west  trend,  the  rock  appear- 
ing in  a  series  of  eastward  projecting  points,  which  make  an  angle  with  the 
lake  shore  of  about  45°.  The  layers  are  distinctly  columnar,  and  are  often 
much  shattered  by  close  jointing,  which  in  places  is  almost  like  a  slaty 


EOCKS  OF  POETAGE  BAY  ISLAND.  297 

structure,  the  rock  coming  out  in  thin  slabs.  The  intervening  beaches  are 
presumably  occupied  by  amygdaloids.  This  compact  rock  resembles,  both 
externally  and  beneath  the  microscope,  some  of  the  conchoidally  fracturing 
layers  of  the  Agate  Bay  Group  (as  for  instance  that  above  described  as 
occurring  on  the  shore  near  Silver  Creek,  Sec.  22,  T.  53,  R.  10  E.),  and 
also  some  of  the  layers  of  the  beds  of  the  Duluth  and  Lester  River  groups. 

Below  these  brown  rocks  no  exposures  were  found  for  about  a  mile, 
when,  four  miles  above  Portage  Bay  Island,  dark-gray,  fine-grained  diabase 
pseud-amygdaloids  alternate  with  amygdaloids  in  which  the  matrix  is  dark 
gray  and  the  amygdules  rather  sparse  and  large  and  chiefly  of  stilbite'  and 
calcite.  Similar  rocks  are  seen  again  after  a  long  beach,  at  about  three  miles 
above  Portage  Bay  Island.  After  this  another  long  beach  intervenes, 
beyond  which  there  is  first  a  low,  in-egular  exposure  of  a  coarse-grained 
black  gabbro,  which  is  possibly  a  dike,  and  then,  just  at  the  west  point  of 
Grand  Portage  Bay,  dark-gray,  medium-grained  diabases  with  amygdaloids 
like  those  just  described.  These  gray  diabases  and  amygdaloids  are  not 
much  like  anything  else  seen  on  the  Minnesota  coast. 

On  Portage  Bay  Island  still  lower  rocks  are  visible.  On  the  northeast 
corner  of  the  island  are  the  uppermost  beds  of  the  Animikie  Group,  seen 
near  the  water's  edge.  Overlying  these  beds,  and  forming  the  mass  of  the 
island,  is  a  considerable  thickness  of  an  aphanitic  black  rock,  which  under 
the  microscope  appears  to  be  made  up  chiefly  of  augite  in  aggregations  of 
rounded  grains  and  magnetite  particles.  The  plagioclases  are  subordinated 
to  the  feldspars  -in  quantity,  the  proportion  varying  in  different  sections. 


^nar/ziie  and,  gaoT-ii-sZa^, 

Fig.  16. — Generalized  section  of  Portage  Bay  Island,  Minnesota  coast. 

Some  sections  show  more  or  less  of  an  isotropic  material  penetrated  by 
minute  plagioclases,  as  in  the  matrices  of  many  of  the  amygdaloids.  Other 
sections  show  numerous  chlorite  pseud-amygdules.     In  one  place  near  the 


298       COPPEE-BBAEING  BOOKS  OF  LAKE  SUPEEIOE. 

eastern  point  of  the  island  this  rock  was  seen  weathering  out  into  little 
spheroids  asually  under  half  an  inch  in  diameter.  These  black  rocks  are 
somewhat  peculiar,  and  although  allied  to  the  ashbed-diabases  of  the  Du- 
luth  Grroup,  they  diflfer  in  the  very  high  content  of  rounded  augite  particles, 
in  which  respect  this  rock  differs,  indeed,  from  any  other  as  yet  examined 
from  the  entire  extent  of  the  Keweenaw  Series.  The  uppermost  rocks  of 
the  island  are  coarser  grained,  and  include  rather  coarse  olivine-diabases, 
and  at  least  one  bed  of  an  orthoclase-gabbro  which,  in  the  thin  section,  looks 
just  like  that  of  Lester  River. 

It  is  difficult  to  decide  just  how  to  correlate  these  eastern  rocks  with 
those  near  Duluth.  It  is  evident  that  in  a  general  way  they  resemble  the 
rocks  of  the  Duluth  and  Lester  River  groups.  The  resemblance  to  these 
groups  of  the  brown  and  gray  diabases  of  the  coast  above  Portage  Bay 
is  very  strong.  On  the  whole,  since  the  peculiarly  characteristic  stratiform 
olivine-bearing  beds  and  associated  shaly  amygdaloids  of  the  Agate  Bay 
Group  are  not  seen  here  at  all,  I  am  disposed  to  regard  that  group  as 
having  thinned  out  to  nothing,  and  to  divide  the  rocks  that  are  seen  here 
between  the  Lester  River  and  Duluth  groups,  which  must  also,  of  course, 
have  greatly  thinned,  if  this  reference  is  a  correct  one. 

Beaver  Bay  Group. — The  Beaver  Bay  Group  was  made  out  from  the 
coast  cliffs  between  Split  Rock  River  and  a  point  about  two  miles  below 
Baptism  River;  from  numerous  inland  exposures  for  five  to  eight  miles  back 
from  the  mouths  of  Beaver,  Baptism,  and  Temperance  rivers,  and  from  the 
coast  cliffs  between  Grand  Marais  and  a  point  in  the  Indian  reservation, 
about  four  and  a  half  miles  above  the  western  point  of  Grand  Portage  Bay. 
This  group  is  characterized  by  the  great  predominance  of  coarse-grained 
rocks,  next  in  abundance  to  which  are  felsitic  and  quartziferous  porphyries 
and  granite-like  rocks.  Dense  brown  and  gray  diabases  of  conchoidal 
fracture  occur  to  some  extent,  while  amygdaloids  and  fine-grained  diabases 
of  the  ordinary  type,  though  rare,  are  not  excluded. 

While  the  prevalent  coarse-grained  rocks  of  the  group  are  plainly 
enough  flows,  exhibiting  the  usual  flat  lakeward  dip,  and  while  the  same  is 
clearly  true  of  much  of  the  porphyry,  there  are  other  places  where  there  is 


beaVer  bay  group.  299 

mucli  difficulty  in  determining  the  positions  which  these  rocks  occupy. 
Some  of  these  places  are  difficult  because,  between  the  porphyries  and  the 
nearest  bedded  basic  rocks,  there  are  gaps  without  exposure,  such  as  shingle 
beaches  along  the  coast,  and  these  gaps  are  either  so  long  that  the  relations 
of  the  rocks  can  only  be  guessed  at,  or  else  they  are  so  short  that  it  is  hard 
to  see  how  one  rock  can  pass  imder  the  other.  So  many  of  these  cases 
proved  tractable  under  further  study  that  there  remains  but  little  doubt  that 
in  nearly  all  the  porphyry  would  be  found  plainly  enough  to  overlie  or 
underlie  the  other  rocks  in  a  regular  way.  In  yet  other  cases,  however, 
vertical  contacts  between  the  two  rocks  were  found,  and  while  these  con- 
tacts are  in  some  cases  demonsti-ably  due  to  faulting,  in  others  this  is 
not  so  evident,  and  there  are  certainly  places  where  the  coarse-grained 
basic  rocks  look  much  as  if  intrusive,  while  the  red  granite-like  rocks 
always  present  this  appearance.  Undoubted  dikes  occur  cutting  the  por- 
phyries, and  that  much  more  frequently  than  in  any  of  the  preceding 
groups,  but  •  they  are  always  of  fine-grained  rocks,  unlike  the  associated 
coarse-grained  kinds.  The  aphanitic,  brown  and  gray  rocks  above  referred 
to,  as  well  as  the  rare  amygdaloids  and  associated  fine-grained  diabases,  are 
always  plainly  bedded.  No  rocks  distinctly  of  detrital  origin  were  observed 
in  the  group. 

On  the  whole,  the  group  may  be  briefly  described  as  made  up  of  bedded 
coarse  basic  rocks,  with  interbedded  fine-grained  basic  rocks,  only  rarely 
amygdaloidal,  and  also  of  interbedded  acid  porphyries  in  very  irregular 
areas,  which  are  individually  limited  in  extent,  contracting  suddenly  from 
several  hundred  feet  in  thickness  to  nothing.  The  whole  group  is  much 
faulted;  fine-grained  diabase  dikes  are  not  uncommon,  while  gabbros  and 
coarse-grained  granite-like  rocks  are  present  in  intersecting  masses.  The 
total  thickness  is  probably  understated  at  6,000  feet. 

In  describing  this  group  more  in  detail,  I  find  it  most  convenient  to  fol- 
low its  exposures  from  west  to  east  along  the  coast,  taking  the  different  kinds 
of  rocks  as  they  come.  Beginning  on  the  west,  the  basal  bed  of  the  group  is 
a  felsite,  forming  the  bold  point  one  and  one-half  miles  above  the  mouth  of 
Split  Rock  River.  For  some  distance  both  east  and  west  of  the  mouth  of  this 
river  the  rocks  trend  far  around  to  the  north,  west  of  the  river  lying  nearly 


300 


COPPEE-BEARING  EOCKS  OP  LAKE  SUPEEIOE. 


due  north,  and  to  the  east  of  it  changing  gradually  more  towards  the  N.  N.  E. 
The  coast  line  is  trending  here  as  a  whole  just  about  N.  E.,  but  subordinately 
it  gives  a  number  of  long  trends  corresponding  to  the  more  northerly  trend 
of  the  rocks,  as  shown  in  Fig.  17.     Until  this  unusual  amount  of  northing 


Fig.  17. — Sketch-map  of  rock  exposures  near  Split  Rock  River,  Minnesota  coast,  T.  54,  R.  H  W. 

in  the  strike  is  realized  the  exposures  in  this  vicinity  are  confusing.  The 
following  is  the  succession  of  strata  roughly  made  out  for  some  four  miles 
along  the  coast  in  the  neighborhood  of  Split  Rock  River.  There  are  prob- 
ably some  gaps  in  the  succession,  and  the  thicknesses  given  are  only  the 
roughest  approximations.  Nevertheless,  the  section  in  its  general  features 
is  correct,  and  will  well  illustrate  the  nature  of  the  lower  beds  of  this  group 


SECTION  NEAE  THE  MOUTH  OF  SPLIT  KOCK  EIVEE.  301 

Feet 

I.  Red  felsitic  porphyry. — This  rock  shows  finely  on  the  coast  two  miles  above 

the  mouth  of  Split  Eock  Eiver,  where  it  forms  a  very  striking  red 
cliff  30  to  50  feet  high.  Close  inspection  of  the  cliff  shows  run- 
ning through  it  a  sort  of  banding  which  is  brought  out  by  narrow 
whitish  strings,  and  also  by  lighter  and  dai'ker  shadings  in  the 
general  red  color  of  the  mass.  These  bands  curve  up  and  down  in 
the  most  irregular  manner;  for  a  while  they  will  seem  to  be  nearly 
horizontal,  and  then  will  suddenly  change  to  vertical,  and  indeed 
to  all  sorts  of  directions  within  a  few  feet.  That  they  represent  a 
fluidal  structure  there  can  be  little  doubt.  There  is  no  tendency 
to  split  parallel  to  this  banding.  Under  the  microscope  the  thin 
sections  show  a  matrix  closely  resembling  that  of  the  quartz-por- 
phyry of  the  pebbles  of  the  Calumet  conglomerate  of  Keweenaw 
Point,  figured  on  Plate  XII,  Fig.  2.  It  presents  a  reddish  splotchy 
appearance,  the  minute  ferrite  particles  which  give  the  color  being 
very  iri'egularly  distributed ;  and  appears  to  be  made  up  largely  of 
what  Eosenbusch  calls  crypto-crystalline  matter.  With  this  are  par- 
ticles which  may  be  orthoclase,  and  a  good  deal  of  quartz,  arranged 
in  the  peculiar  ramifying  way  that  characterizes  the  secondary 
quartz  of  the  granitic  porphyries.  Scattered  through  this  ground 
mass  are  numerous  minute  black  particles.  The  porphyritic  in- 
gredients noted  are  comparatively  small  crystals  of  orthoclase  and 
oligoclase,  no  quartzes  having  been  observed.  On  the  lake  shore 
this  porphyry  has  not  been  observed  in  contact  with  either  the 
underlying  or  the  overlying  rocks,  being  separated  from  them  by 
beaches.  Two  miles  north,  however,  on  Split  Eock  Eiver,  the  con- 
tact with  the  underlying  melaphyr  is  very  nicely  exposed.  The 
dip  here  is  to  the  east  18°  to  20°,  and  several  other  observations 
along  Split  Eock  Eiver  show  the  same  eastern  dip.  At  one  point 
on  the  Split  Eock  Eiver,  about  one  and  a  half  miles  from  the  lake, 
the  pori^hyry  is  cut  by  a  dike,  six  feet  wide,  of  a  very  fine  dark- 
gray  rock.  The  dike  trends  with  the  strata,  nearly  north  and 
south,  and  dips  at  right  angles  to  them,  or  70°  west.  Thickness 
of  this  porphyry 600-700 

II.  Ashhed-diabase  and  diabase-amygdaloid. — Several  layers  (not  more  than 

four)  each  with  an  amygdaloid,  of  a  very  dense,  light-gray  to  dark 
brownish-gray,  conchoidally  fracturing  rock,  which,  in  the  thin  sec- 
tion, shows  tabular  oligoclase  (measurements  on  four  different 
slices  failing  to  find  any  angle  above  25°) ;  augite  in  aggregates  of 
rounded  particles;  and  magnetite,  as  the  chief  ingredients.  In 
sections  of  the  densest  kinds  there  are  areas  which  have  little  or 
no  action  between  the  nicols,  and  which  appear  therefore  to  be 
glass.  More  or  less  brown  ocherous  matter  appears  in  the  sections, 
the  amount  varying  directly  with  the  amount  of  brown  tinge  pre- 
sented by  the  rock  macroscopically.    Of  the  two  amygdaloids  seen, 


302       OOPPER-BEAEING  EOCKS  OF  LAKE  SUPERIOR. 

one,  which  is  exposed  ou  the  strike  for  several  hundred  feet  at  the 
foot  of  a  cliff,  on  the  first  point  above  the  mouth  of  Split  Rock 
River,  shows  a  weathered,  greenish,  earthy  matrix,  and  rather 
sparsely  scattered  amygdules  of  laumontite.  The  other  amygda- 
loid, seen  on  the  face  of  the  first  point  below  Split  Rock  River,  has 
a  brownish,  aphanitic  matrix,  and  is  so  highly  vesicular  that  the 
vesicles  touch  one  another.  These  vesicles  are  small,  very  smooth- 
walled,  elongated  in  a  common  direction,  partly  empty,  but  for  the 
most  part  filled  with  amygdules  of  chalcedonic  quartz,  laumontite, 
or  chlorite.  These  layers  occupy  a  surface  width,  measured  from 
west  to  east,  of  over  half  a  mile,  and  must  have  a  thickness,  there- 
fore, of  at  least  800  feet 800 

in.  DarTi-gray  to  hlaek  gdhhro. — This  laj'er  or  series  of  layers  forms  the  coast 
line  for  about  a  mile  in  the  S.  E.  ^  of  Sec.  6  and  S.  W.  J  of  Sec.  5, 
T.  54,  R.  8  W.  The  lower  layers  are  rather  fine-grained  to  medium- 
grained,  dark-gray  to  nearly  black,  rough-textured,  and  marked  by 
strong  luster-mottliugs.  The  thin  section  shows  labradorites  ar- 
ranged in  curving  lines,  which  are  evidently  the  result  of  flowage. 
The  augites  are  relatively  large,  and  inclose  each  countless  minute 
feldspars.  The  magnetite  is  unusually  abundant,  being  so  thickly 
crowded  between  the  augites  as  to  render  these  interspaces  nearly 
black.  Quite  large  fragments  of  the  rock  are  raised  by  the  magnet. 
Though  olivines  are  generally  present  in  these  luster-mottled  rocks, 
none  could  be  detected  here.  This  section  is  figured  on  Plate  VIII, 
Fig.  4.  Higher  layers  become  coarser  in  grain,  and  of  a  rougher 
texture,  with  less  marked  luster-mottlings.  In  the  thin  section  of 
this  coarser  rock  the  augites  are  strongly  diallagic  and  still  larger 
than  the  plagioclases,  but  not  nearly  so  much  so  as  in  the  previously 
described  rock.  They  are  usually  fresh,  though  occasionally  altered 
to  chlorite.  Good-sized  olivines,  partly  fresh,  but  generally  traversed 
by  broad  brown  and  green  bands  of  alteration,  are  here  abundantly 
present.  Still  higher  up,  near  the  top  of  these  layers,  the  grain 
becomes  quite  fine  again,  but  no  thin  sections  have  been  examined. 
Wear  the  middle  of  the  S.  E.  i  of  Sec.  5,  T.  54,  R.  8  E.,  this  gabbro 
is  interrupted  by  a  vertically  placed  mass  of  excessively  coarse- 
grained anorthite-rock.  The  cutting  mass  is  from  50  to  75  feet 
wide,  and  bears  north  and  south.  It  shows  on  both  sides  of  a  little 
square-angled,  rock-walled  bay,  on  the  south  point  of  which  it  rises 
as  much  as  a  liundred  feet  above  the  lake.  On  both  sides  of  the 
cutting  mass  the  black  gabbro  is  filled  with  large  angular  masses  ' 
of  the  same  coarse  anorthite-rock.  The  included  masses  reach 
sometimes  many  tons  in  weight,  and  in  some  places  predominate 
over  the  iucluding  gabbro,  which  then  appears  as  if  veining  the 
coarser  rock.  At  the  west  angle  of  the  bay  the  included  masses 
are  nearly  absent,  and  the  gabbro  resumes  its  usual  vertically  col- 
umnar appearance.     At  the  north  angle  of  the  bay  the  anorthite- 


SECTION  NEAE  THE  MOUTH  OF  SPLIT  ROCK  EIVEE.  303 

rock  rises  again  to  a  height  of  over  150  feet.  The  inclusions  of 
angular  masses  of  the  anorthite-rock  in  the  gabbro  indicate  the 
more  recent  origin  of  the  latter,  and  this  conclusion  is  borne  out  by 
the  thin  section  made  from  a  specimen  taken  at  its  contact  with 
the  gabbro,  in  which  the  relatively  fine  gabbro  surrounds  the  ends 
of  the  anorthite  crystals,  as  the  base  of  any  porphyry  does  the  por- 
phyritic  crystals  which  lie  imbedded  in  it.  Since  the  strike  is  now 
trending  somewhat  more  around  to  the  northeast,  and  the  dip  at 
the  same  time  flattening  somewhat,  this  gabbro  probably  does  not 
exceed  in  thickness  some 900 

IV.  Ashbed-diabase  and  diabase-amygdaloid. — The  compact  portions  of  this 

layer,  which  is  a  dark-gray  or  brownish-gray  compact  rock,  macro- 
scopically  like  the  rock  of  II,  is  largely  exposed  along  the  coast, 
in  the  N.  E.  i  of  Sec.  5,  T.  54,  E.  8  W.,  and  Sec.  33,  T.  55,  E.  8  W. 
The  overlying  amygdaloid,  which  is  seen  at  several  points,  has  an 
aphanitic,  dark-gray  matrix,  not  unlike  much  of  the  comijact  rock 
below,  while  the  amygdules  are  wholly  of  light  pink  laumontite 
lying  in  elongated,  smooth-walled,  closely  crowded  vesicles,  aver- 
aging from  ^  to  f  of  an  inch  in  greatest  length.  The  amygdaloid 
is  26  feet  thick,  and  the  whole  thickness  of  the  layer  probably  not 
more  than 100 

V.  Olivine-gabbro. — This  gabbro  is  in  sight,  directly  overlying  the  above  de- 

scribed amygdaloid  at  several  places  along  the  shore  of  Sec.  33,  T. 
55,  E.  8  W.  It  is  a  medium-grained  to  rather  fine-grained,  dark- 
gray  to  black  rock,  much  like  that  of  III.  The  thin  section  shows 
numerous  rather  fresh  olivines;  large  diallagic  augites,  often  includ- 
ing many  plagioclases  (labradorite) ;  and  titaniferous  magnetite. 
This  layer  shows  a  strongly  marked  columnar  structure  at  right 
angles  to  the  bedding,  and  in  places  the  columns  are  even  cross- 
jointed,  so  as  to  show  a  rude  ball-and-socket  jointing.  In  places 
also  weathering  brings  out  distinctly  a  spheroidal  structure.  At 
a  projecting  point  in  the  N.  W.  J  of  the  H".  E.  ^  of  Sec.  5,  T.  54, 
E.  8  W.,  this  layer  is  crossed  by  a  mass  of  coarse  white  -anorthite- 
rock  similar  to  that  above  described,  and  in  the  vicinity  the  gabbro 
holds  numerous  angular  masses  of  the  anorthite-rock.  Thickness 
of  the  layer,  about - 100 

VI.  Bed  quartsiferous  porphyry. — The  rock  of  this  layer  is  to  be  seen  at  the 

point  in  the  northern  part  of  Sec.  33,  T.  55,  E.  8  W.    The  thickness 
■  of  this  layer  is  about 100 

Total  thickness  of  the  section 2, 600 

The  granite-like  rock  forming  the  end  of  this  point  may  also  be  part  of 
the  last  layer,  and   faulted  down  into  its  present  position,  but  from  its 


304       COPPEE-BEAEING  BOOKS  OF  LAKE  SUPEEIOE. 

crystalline  character  is  rather  to  be  regarded  as  an  intersecting  mass.  This 
rock  resembles  closely  in  the  hand  specimen  a  moderately  coarse  flesh- 
colored  granite,  much  more  closely  than  the  similar  rock  at  Duluth.  Large 
cleavage  facets  of  pale  flesh- colored  feldspar  make  up  most  of  the  specimen, 
which  shows  also  quite  distinctly  large  quartz  areas.  There  are  also  indefi- 
nite dark-colored  patches  of  small  size.  Under  the  microscope,  however, 
the  rock  is  seen  to  be  essentially  the  same  as  that  at  Duluth,  from  which  it 
differs  chiefly  in  its  relatively  small  amount  of  ferric  oxide.  The  section  is 
almost  entirely  made  up  of  feldspars  (orthoclase  with  a  little  oligoclase)  and 
quartz,  which  occur  both  in  quite  small  and  quite  large  areas  relatively  to 
the  feldspars.  All  of  these  areas  are,  however,  included  within  the  feldspars, 
never  filling  corners  between  them  as  with  true  granite;  and  since  many 
areas  polarize  together  within  the  mass  of  one  or  more  feldspar  crystals,  it 
is  evident  that  the  quartz  is  all  later  than  the  feldspars,  i.  e.,  either  secondary 
to  them,  or  filling  spaces  left  in  them  by  some  solving  process. 

From  the  last  point  noted  in  the  foregoing  section  (near  south  line  of 
Sec.  28,  T.  55,  E.  8  W.),  to  the  red  rocks  of  the  south  point  of  Beaver 
Bay  (S.  E.  ^,  Sec.  12,  T.  55,  R  8  W.),  a  distance  of  eight  and  a  half  miles, 
the  only  rock  noted  was  a  very  dark-gray  to  black  diabase  or  gabbro,  now 
olivine-bearing,  now  not,  always  very  highly  augitic,  and  often  showing  a 
very  coarsely  nodular  weathered  surface,  resulting  from  the  resistance  to 
weathering  of  the  very  large  augites.  Usually  the  exposui'es  are  low  lake- 
ward-dipping  surfaces;  but  in  the  N.  W.  ^,  Sec.  27  are  cliffs  of  the  black  rock 
150  feet  high.  A  rude  columnar  structure  is  often  visible.  For  a  while  the 
high  dip,  18°  to  20°,  is  continued,  but  more  to  the  south  of  east  than  before, 
and  soon  it  flattens,  and  the  trend  becomes  more  nearly  parallel  to  the 
shore-line. 

At  Beaver  Bay  a  red  granite-like  rock  and  a  quartziferous  porphyry 
suddenly  appear  again  among  the  black  rocks.  The  occurrences  are  very 
interesting,  and  much  like  those  described  as  presenting  themselves  at  the 
close  of  the  above  detailed  section  (N.  E.  ^,  Sec.  33,  T.  55,  R.  8  W.),  but  more 
difficult  to  reduce  to  order.  Fig,  18  is  a  sketch  map  of  Beaver  Bay,  show- 
ing the  exposures  of  the  different  rocks. 

The  common  black  rock  of  the  vicinity,  as  seen  for  instance  along  the 


EOCKS  OF  BEAVER  BAY. 


305 


bed  of  Beaver  River,  and  on  the  south  point  of  Beaver  Bay,  is  the  same 
as  that  already  described  as  showing  for  some  miles  along  the  coast  above 
Beaver  Bay.  It  is  a  moderately  coarse,  very  dark-colored,  and  often  nearly 
black,  very  highly  crystalline  rock,  in  which  the  three  common  ingredients — 
augite,  anorthite,  and  titaniferous  magnetite — can  often  be  seen  with  the 
naked  eye.  The  very  dark  color  is  due  to  the  great  abundance  of  the 
augitic  ingredient,  which,  while  commonly  subordinate  to  the  plagioclase 


Fig.  18. — Sketch-map  of  rock  exposures  in  the  vicinity  of  Beaver  Bay,  Minnesota  coast,  T.  55,  E.  8  E 


in  this  class  of  rocks,  here  greatly  predominates.  It  shows  all  gradations 
between  augite  without  foliation,  and  very  highly  foliated  diallage.  It  is 
commonly  quite  fresh,  but  is  also  found  more  or  less  completely  altered  to 
a  greenish  material.  It  is  not  only  the  most  abundant  ingredient,  but  is 
also  much  the  coarsest.  Olivine  is  absent  from  some  sections,  and  is  never 
abundant,  but  when  present  is  seen  in  very  large  areas,  in  which  an  alter- 
ation to  a  reddish-brown  ocherous  substance  has  always  progressed  very 

20  LS 


306       COPPEE-BEARING  EOCKS  OP  LAKE  SUPEEIOE. 

far.  The  brownish  alteration-product  often  gives  a  resinous  look  to  the 
hand  specimen. 

On  Beaver  River  this  black  rock  in  one  or  two  places  shows  indications 
of  a  southeasterly  dip  of  some  18°  to  20°,  but  on  the  north  point  of  Beaver 
Bay,  where  it  is  to  be  seen  in  bold  ledges,  with  a  finely  developed  columnar 
structure,  the  dip  is  flatter. 

At  several  points  along  Beaver  River  this  black  gabbro  carries  large 
masses  of  an  anorthite-rock  similar  to  that  described  as  occurring  further  up 
the  coast.  The  bowlder-like  character  of  the  anorthite-rock,  though  often 
pronounced,  is  not  always  so  plain,  and  in  some  places  it  looks  more  like  a 
dependency  of  the  prevailing  black  gabbro.  Thin  sections  of  this  rock 
occasionally  show  rare  particles  of  augite  or  diallage  between  the  feldspar 
grains. 

The  acid  rocks  of  this  vicinity  are  found  forming  the  south  point  of 
Beaver  Bay,  along  the  shore  of  the  bay  at  the  points  D  E  F,  where  they 
are  confusedly  intermingled  with  the  basic  rocks,  on  the  small  island  on 
the  north  side  of  the  bay,  and  on  Cedar  Island,  which  lies  some  500  yards 
east  of  the  north  point  of  the  bay.  Both  of  these  islands  are  composed  of 
quartzose  porphyry. 

The  bold  point  on  the  south  side  of  Beaver  Bay  is  a  mass  of  pink 
granite-like  rock,  bounded  on  all  sides  by  precipices  about  100  feet  high, 
and  rising  in  the  center  to  an  elevation  of  130  feet  above  the  lake.  The 
rock  of  which  this  point  is  composed  has  a  pale  flesh  color,  and  presents  to 
the  naked  eye  the  appearance  of  a  highly  feldspathic  granite  of  medium  to 
fine  grain.  Shining  feldspar  facets  appear  to  make  up  most  of  the  rock. 
The  thin  sections  seem  also  to  be  almost  entirely  made  up  of  feldspars,  but 
always  much  altered  and  changed,  with  much  secondary  quartz.  In  some 
sections  where  this  replacement  has  been  carried  far,  it  is  impossible  to  tell 
whether  we  may  not  have  also  some  replaced  matrix.  Black  and  opaque 
particles  of  magnetite,  ferrite,  and  augite  are  the  accessories.  Some  of  the 
clusters  of  black  particles  appear  as  if  due  to  the  alteration  of  augite  crys- 
tals. At  the  point  C  this  red  rock  is  seen  surrounded  on  three  sides  by  a 
mass  of  rather  fine-grained  diabase,  in  which  the  anorthite  crystals  are  often 
inclosed  in  numbers  by  single  augites. 


EOCKS  OP  BBAVEE  BAT.  307 

In  the  same  vicinity,  a  narrow  dike  of  very  dense  greenish-black  dia- 
base zigzags  about  in  the  red  rock.  The  islets  I  are  composed  of  a  coarse 
anorthite-rock,  whilst  the  larger  islet  H  is  again  of  very  fine-grained,  dark- 
gray  diabase. 

Extending  along  the  north  side  of  the  neck  of  the  point,  at  the  mouth 
of  Beaver  River,  C  C,  for  a  distance  of  over  200  paces,  as  indicated  on  the 
accompanying  sketch,  is  a  black-tinted,  banded  felsite.  The  thin  sections 
of  this  rock  show  an  excessively  fine  granular  mixture  of  quartz  and  ortho- 
clase  particles,  which  in  some  sections  appears  to  include  a  good  deal  of 
isotropic  matter.  Opaque  black  and  brown  particles  are  abundant,  and  here 
and  there  an  augite  crystal  is  seen.  The  banding  is  produced  by  short 
seams  of  lighter-colored  material,  which  under  the  microscope  show  more 
coarsely  crystalline  matter.  The  contact  of  this  felsite  with  the  crystalline 
rocks  of  the  point  was  not  observed. 

The  small  island  on  the  north  side  of  Beaver  Bay,  and  again  the 
larger  island  to  the  northeast  (Cedar  Island),  are  made  up  of  a  quartz-por- 
phyry with  an  exceedingly  dense  and  conchoidally  fracturing,  purple-tinted 
matrix,  in  which  are  rather  thickly  scattered  pink  feldspars,  averaging  an 
eighth  of  an  inch  in  length,  and  black  quartzes  one-fiftieth  to  one-thirtieth 
of  an  inch  across.  The  feldspar  crystals  are  often  very  distinctly  arranged 
in  curving  lines  indicating  flowage,  which  is  also  shown  by  the  similar  lines  of 
lighter  shading  in  the  matrix.  Under  the  microscope  the  thin  section  of  this 
rock  shows  much  of  a  substance  that  does  not  affect  the  polarized  light  at  all. 
Through  the  unindividualized  base  are  excessively  minute  particles  and  ar- 
borescent clusters  of  quartz.  The  blackish  and  pinkish  particles  generally 
seen  in  these  porphyries  are  here  much  sparser  and  smaller  than  usual,  and 
the  rock  as  a  whole  is  one  of  the  nearest  to  a  glassy  condition  of  any  of  the 
felsitic  porphyries  examined.  The  silica  content  is  76.83  per  cent.  On 
the  outer  island  the  quartz-porphyry  has  a  very  marked  columnar  structure, 
and  presents  at  the  same  time  a  strong  appearance  of  the  usual  flat  lake- 
ward  dip. 

At  the  point  A  of  the  map,  the  common  coarse  gabbro  is  involved  with  a 
fine-grained  black  rock,  as  indicated  in  the  accompanying  figure.  The  face 
represented  is  the  east  wall  of  the  gorge  of  Beaver  Eiver,  near  the  bridge, 


308 


COPPEE-BEAEING  EOCKS  OF  LAKE  SUPEEIOR. 


and  cuts  obliquely  across  the  strike.  The  fine-grained  rock  shows  in  the 
thin  section  a  mixture  of  rounded  augite  grains  and  tabular  oligoclases, 
with  some  magnetite.  It  is  close  to  the  so-called  ashbed-diabases.  At  B 
of  the  map  the  rock  exposed  is  a  reddish-weathering,  medium-grained  ortho- 
clase-gabbro,  with  all  the  characteristics  of  the  orthoclase-gabbros,  as  de- 
scribed in  Chapter  III,  strongly  developed,  viz :  presence  of  much  ortho- 


FiG.  19. — Section  on  Beaver  Kiver,  Minnesota. 

clase;  the  plagioclase  oligoclase ;  much  secondary  quartz;  much  coarse 
titaniferous  magnetite ;  diallage  and  augite  with  a  viriditic  and  ocherous 
alteration,  and  large  apatite  crystals.  The  rock  is  strongly  contrasted  with 
the  neighboring  olivine-gabbros.  Similar  rocks  show  at  K  and  J  and  again 
at  D.  At  the  latter  point  a  fine-grained  black  diabase  with  augite  filling 
the  interspaces  of  the  labradorites,  and  not  in  rounded  grains,  and  con- 
taining small  olivines,  is  involved  with  the  orthoclase-gabbro,  coarse  anor- 
thite-rock  and  pink  felsite  in  the  most  confusing  manner.  The  same  con- 
fused appearance  is  met  with  at  E,  where  fine-grained  olivine-diabase 
has  with  it  red  granitic  porphyry  in  a  thin  seam,  and  coarse  anorthite-rock 
in  irregular  masses.  At  F  an  excessively  fine-grained  ashbed-like  diabase 
is  intermingled  with  granitic  porphyry. 

As  indicated  above,  it  is  very  difficult  to  reduce  these  involved  expos- 
ures to  an  intelligible  order.  The  prevailing  black  rock  of  the  vicinity  is 
plainly  enough  one  of  the  usual  lakeward-dipping  flows.  The  quartz- 
porphyry  of  Cedar  Island  seems  as  plainly  part  of  a  similar  flow.  Possibly 
all  of  the  acid  rocks  belong  to  one  great  belt  overlying  and  in  part  involved 


EOCKS  NORTH  OF  BEAVEK  BAY.  309 

■with  the  coarse  gabbros,  since  they  appear  to  lie  in  a  zone  parallel  to  the 
general  trend,  and  since  they  form  unmistakable  flows  elsewhere  along  this 
coast.  The  fine-grained  diabases  at  the  contact  of  the  two  zones  are  then 
to  be  taken  as  subsequent  intrusions,  as  those  cutting  the  red  rocks  of  the 
south  point  of  the  bay  undoubtedly  are.  But  this  explanation  is  not  entirely 
satisfactory.  Probably  a  better  one  is  reached  by  regarding  the  red  crys- 
talline rock  at  the  south  point  of  the  bay  as  the  intersecting  mass  from 
which  came  the  flows  of  quartziferous  porphyry. 

In  the  country  north  of  Beaver  Bay,  as  far  as  the  north  line  of  T.  56, 
R.  8  W.,  Messrs.  McKinlay  and  Campbell  found  numerous  exposures  of 
coarse-grained  gabbro,  frequently  forming  bold  ridges  trending  N.  50  to  N. 
60  E.,  and  presenting  to  the  northwest  precipitous  rocky  faces  sometimes 
upwards  of  one  hundred  feet  in  height,  and  to  the  southeast  gradual  slopes. 
Commonly  these  exposures  show  coarse-grained  dark-gray  to  black  olivine- 
gabbros,  such  as  characterize  the  Beaver  Bay  Group  generally,  but  among 
the  specimens  brought  back  are  some  of  orthoclase-gabbro  in  all  respects 
like  those  figured  on  Plate  V  at  Figs.  1  and  2.  One  orthoclase-gabbro  is 
somewhat  peculiar,  and  is  interesting  as  exhibiting  in  an  extreme  way  some 
of  the  characteristics  of  this  class  of  rocks.  It  is  found  on  the  north  line 
of  Sec.  2,  T.  56,  R.  8  W.,  760  paces  west  of  the  northeast  corner.  Exter- 
nally it  is  quite  coarse,  mottled  white  and  green,  titaniferous  magnetite  mak- 
ing up  fully  one-third  of  the  rock.  Under  the  microscope  the  twinned  augites 
are  occasionally  fresh,  but  are  commonly  wholly  altered  to  greenish  uralite. 

Among  these  ledges  of  coarse-grained  rocks  are  much  rarer  beds  of 
flne-grained  kinds,  while  Mr.  McKinlay  found,  in  the  northeast  part  of  T. 
55,  R.  8  W.,  quite  a  zone  of  fine-grained  brownish  diabases,  occasionally 
showing  some  tendency  to  an  amygdaloidal  character,  and  including  beds 
of  the  typical  melaphyr  or  fine-grained,  lustre-mottled  oli vine-diabase.  These 
beds  are  to  be  seen  well  exposed  along  the  north  branch  of  Beaver  River 
in  Sec.  35,  T.  55,  R.  8  W.,  and  in  Sec.  2,  T.  56,  R.  8  W. 

On  the  coast  below  Beaver  Bay,  for  two  miles  and  a  half,  coarse  black 
gabbro,  like  that  at  and  above  Beaver  Bay,  is  constantly  in  sight.  It  also 
forms  the  rocky  island  near  the  east  line  of  Sec  6,  T.  55,  R.  7  W.,  where, 
as  also  frequently  along  the  shore,  it  exhibits  the  usual  rather  flat  lakeward 


310 


COPPEE-BEAEING  EOOKS  OF  LAKE  SUPEEIOE. 


dip,  and  strong  cross-columnar  structure.  All  along  here  the  rock  tends  to  be 
very  rich  in  diallage  and  consequently  dark  in  color.  The  diallage  is  always 
very  much  coarser  than  any  other  ingredient  of  the  rock,  inclosing  a 
number  of  the  feldspars,  which,  according  to  measurement,  are  always  at 
the  anorthite  end  of  the  series.  Some  of  the  diallages  are  two  to  three 
inches  across,  and  show  a  brilliant  brassy  luster  on  the  cleavage.  This  was 
noticed  in  a  number  of  places,  but  at  one  point  on  the  N.  E.  ^  of  Sec.  6 
it  was  especially  noticeable.  Here  one  can  walk  across  a  pavement  hun- 
dreds of  square  feet  in  area,  made  of  the  ends  of  the  basaltic  columns.  The 
surface  is  weathered  to  a  general  brown  hue,  but  in  every  direction  flashes 
back  the  sunlight  to  the  eye  from  the  brilliant  brassy  diallages. 

At  several  points  in  this  vicinity  the  black  rock  was  observed  to  include 
masses  of  coarse  anorthite-rock.     The  latter  did  not  appear  to  occur  here 

in  bowlder-like  masses,  but  rather  in  large, 
irregularly  outlined  areas.  At  one  point  on 
the  shore  of  Sec.  6,  directly  west  of  the  island 
above  referred  to,  the  nearly  white  anorthite- 
rock  rises  like  a  dome  in  the  black  gabbro 
which  is  seen  above  and  on  both  sides  of  it. 
The  southern  point  of  the  island  is  formed  of 
anorthite-rock;  and  due  north  from  this  point, 
on  the  mainland,  is  another  area  of  white  rock, 
apparently  trending  north  and  south. 

On  the  shore  of  Sec.  32,  T.  56,  R  7  W., 

Fig.  20. — Sketch-map  of  expos-    ,  ,  iii  ii  n  iii 

ures,  Minnesota  coast,  Sec.  33,  T.  56,  R.  the  Columnar  black  rock  bows  down  suddenly 
"^  ^'  in  the  direction  of  the  general  trend  and  disap- 

pears under  a  mass  of  a  pink  granite-like  rock  which  forms  a  bold  point 
much  like  that  of  the  south  side  of  Beaver  Bay.  This  point  projects 
into  the  lake  in  a  nearly  due  east  direction.  On  its  north  side  is  a  bay, 
which  is  bounded  on  the  north  by  another  wall  of  red  rock,  while  behind 
and  on  the  beach  are  exposures  as  indicated  in  the  following  figure.  The 
point  A  forms  a  vertical  wall  on  the  south  side  of  the  bay;  the  red  rock  of 
which  it  is  composed  looks  much  like  that  of  the  south  point  of  Beaver  Bay, 
but  differs  in  having  a  large  amount  of  quite  coarse  quartz.    The  thin  section 


J  e&ZoceZ  wU^  secerns  ofre^ 
JeJsite 


THE  BEAVER  BAT  GEOUP. 


311 


shows  a  rock  composed  chiefly  of  clouded  orthoclase  and  oligoclase,  and 
large  quartz  areas.  A  few  clusters  of  rounded  particles  of  augite  are  seen. 
The  augite  is  older  than  the  feldspars  and  the  feldspars  older  than  the 
quartz.  The  latter  ingredient  does  not  saturate  the  feldspars  after  the 
usual  manner  of  secondary  quartz,  but  molds  itself  around  them  or  invades 
them  in  large  areas,  or  even  includes  broken  pieces  of  feldspars. 

At  B,  of  Fig.  20,  the  rock  is  a  fine-grained,  brownish-black,  conchoid- 
ally  fracturing  diabase  of  the  ordinary  type,  with  chlorite  and  quartz  pseud- 
amygdules,  rising  in  a  low  ledge  above  the  shingle  of  the  beach.  Expos- 
ures C  C,  also  low,  are  of  the  same  kind  of  rock,  mingled  with  which  are 
seams  and  patches  of  brick-red  felsite,  the  black  and  red  presenting  the 
appearance  of  having  been  in  a  semi-fused  condition  together.  The  red 
felsite  in  the  thin  section  shows  a  red  matrix,  saturated  with  exceedingly 
thin  fibers  of  secondary  quartz  arranged  in  sheaf-like  bundles.  Quite  large 
quartz  areas  are  also  contained,  which  are  not  single  individuals  as  in  the 
quartzes  of  a  quartz-porphyry.  It  is  possibly  only  a  phase  of  the  pink  rock 
of  the  south  wall  of  the  bay. 

Beyond  the  point  C  there  are  80  paces  of  a  beach  without  ledge,  be- 
yond which  again  rises  the  north  wall  of  the  bay,  D  E.  This  wall  is  composed 
as  indicated  in  the  following  figure.     The  west  part  of  the  wall  is  made  up 


Fig.  21.— Section  of  wall  DE  of  Fig.  20. 


of  plainly  bedded  fine-grained  diabase,  laumontitic  amygdaloid,  and  luster- 
mottled  melaphyr.  These  are  terminated  by  a  narrow  ravine  15  feet  wide 
and  100  feet  deep,  the  east  face  of  which  is  made  up  of  the  pink  felsite 
which  forms  the  rest  of  the  point.  This  face  is  beautifully  slicken-sided, 
being  polished  in  some  places  to  a  glassy  surface,  and  marked  from  top  to 


312       COPPEE-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 

bottom  by  grooves  and  striae  inclining  5°  to  15°  away  from  the  vertical, 
towards  the  north.  The  red  rock  of  this  wall  and  of  the  rest  of  the  point 
is  a  pink  to  purple  felsite,  often  showing  many  lines  of  a  lighter  color  than 
the  rest,  and  seamed  with  strings  and  veins  of  calcite.  The  bedded  dia- 
bases and  amygdaloids  of  the  bay  seem  to  have  been  let  down  by  faulting 
into  their  present  position  between  the  two  walls  of  red  rock. 

Beyond  this  bay  to  the  northeast  the  felsite  of  its  northern  point  forms 
the  lake  coast  for  about  a  mile,  in  which  direction  it  presents  a  very  dis- 
tinct and  rather  low  southeastward  dip  (15'^)  with  a  trend  more  around  to 
the  north  than  that  of  the  lake  coast,  so  that  new  layers  succeed  each  other 
somewhat  quickly,  and  the  total  thickness  of  the  felsite  mass  must  be  very 
considerable.  The  general  southeastward  dip  has  at  times  superinduced 
upon  it  a  bowing,  by  which  for  short  distances  the  rock  will  appear  to  plunge 
underneath  the  water  at  a  high  angle.  Calcitic  veins,  and  at  times  a  gen- 
eral calcitic  decay,  affect  the  rock  in  many  places,  but  much  of  it  is  with- 
out the  calcite.  Where  the  flat  lakeward  dip  is  plainest  there  is  often  a  well- 
marked  columnar  structure  at  right  angles  to  the  bedding. 

At  one  point  on  the  shore  of  the  S.  W.  ^,  Sec,  28,  T.  56,  R.  7  W.,  this 
felsite  presents  somewhat  interesting  appearances.  The  ledges  here  are 
very  large,  forming  a  cliff  20  feet  high  for  a  distance  of  many  hundred  feet, 
with  broad  surfaces  shelving  into  the  water  at  an  angle  of  about  15°,  and 
affected  by  a  strong  columnar  cross-jointing.  The  rock  of  the  upper  lay- 
ers is  aphanitic,  but  of  a  rough  texture  and  a  flesh  color.  Thickly  dotting 
it  are  very  fine,  dark-colored,  hair-like  lines,  forming  curves  and  curls  of 
various  forms,  the  whole  appearance  suggesting  strongly  that  of  a  thin  sec- 
tion of  some  modern  rhyolite  or  of  some  glassy  rock  with  hair-like  bodies.^ 
Under  the  microscope  this  rock  is  seen  to  have  a  matrix  which  is  completely 
saturated  with  quartz,  in  the  ramifying  and  network  forms  which  indicate  a 
secondary  origin.  The  original  matrix  appears  to  have  had  some  minute 
feldspars,  but  much  of  it  seems  to  have  been  without  crystalline  structure. 
The  hair-like  bodies  resolve  themselves  into  linear  clusters  of  red  and  black 
particles  of  ferrite,  which  I  take  to  be  the  alteration-product  of  some  orig- 
inal constituent,  either  ciystalline  or  unindividualized.     They  are  certainly 

'  Compare  Zirkel,  Microscopical  Petrography,  Plate  VII,  Fig.  1,  and  Plate  IX,  Fig.  1. 


THE  BEAVEE  BAY  GEOUP. 


313 


not  products  of  infiltration  like  the  so-called  dendrites,  since  they  run 
through  and  through  the  mass  of  the  rock.  They  appear  to  me  to  furnish 
one  more  point  of  resemblance  between  the  ancient  felsites  and  quartz- 
porphyries  and  the  modern  rhyolites,  and  are  another  proof  of  the  erup- 
tive origin  of  these  rocks. 

Yet  another  proof  of  an  eruptive  origin  is  furnished  by  the  rock  of  the 


jifif 


Fig.  22. — Flowage  structure  in  felsite,  Minnesota  coast. 

lower  layers  of  this  cliff.  There  are  here  large  surfaces  hundreds  of  feet 
square  in  which  a  fluidal  structure  may  be  seen  on  a  large  scale.  Light- 
colored,  pale-pinkish  felsite  and  dark-brownish  felsite  are  twisted  together 
in  various  curling  and  snake-like  forms;  the  mass  as  a  whole  dipping,  as 
usual,  15°  to  the  southeast.  The  contrast  between  the  colors  is  very  strong. 
The  lighter  material  is  the  most  abundant,  and  includes  the  darker.  Fig.  22 
represents  an  area  of  20  by  28  inches.  Some  of  the  dark-colored  bands 
are  a  foot  or  two  in  length  and  an  inch  to  three  inches  in  width,  from  which 


314       GOPPEE-BEAEING  EOGKS  OP  LAKE  SUPEEIOE. 

size  they  run  down  to  mere  threads.  The  brown  bands  themselves  are  often 
streaked  with  lines  of  the  lighter  kinds.  Under  the  microscope  both  light 
and  dark  kinds  are  seen  to  be  felsites  with  a  quai'tz-saturated  base  precisely 
like  that  of  the  rock  of  the  upper  layers  above  described,  and  to  differ  from 
each  other  only  in  the  amount  of  red,  black,  and  brown  ferrite  particles  con- 
tained. The  ferrites  are  not  arranged  in  lines  as  in  the  peculiar  rock  of  the 
upper  part  of  the  cliff. 

Beyond  this  point  the  felsite  continues  to  be  the  coast  rock  through 
section  28,  but  in  the  northeastern  part  of  this  section,  and  the  southeastern 
of  section  21,  it  becomes  involved  with  a  black  diabase,  the  diabase  first  ap- 
pearing to  intersect  it,  and  then  to  become  peculiarly  intermingled  with  it 
in  irregular  areas.  Finally  the  black  prevails,  with  here  and  there  a  vein 
of  the  red,  forming  one  of  the  usual  rudely  columnar  flows  of  moderately 
coarse  black  diabase.  These  confused  rocks,  which  were  not  examined 
thoroughly  enough  to  warrant  further  description  or  conclusions,  terminate 
at  a  shingle  beach  in  the  bay  above  the  Great  Palisades. 

The  rock  exposures  about  the  Palisades  are  of  the  greatest  interest, 
because  of  their  bearing  on  the  question  of  the  relation  of  the  acid  and 
basic  flows  of  the  series.  Since  we  have  here  a  great  flow  of  quartz- 
porphyry  unmistakably  overlying  a  succession  of  plainly-bedded  fine- 
grained diabases  and  amygdaloids,  it  follows  that  all  idea  of  the  greater 
antiquity  of  the  acid  as  compared  with  the  basic  rocks  of  the  Lake  Supe- 
rior region  must  be  abandoned.  This  is  a  conclusion  clearly  indicated  in 
all  parts  of  the  Keweenaw  Series,  but  the  exposures  are  here  so  fine  and-so 
unequivocal  that  I  have  described  them  already  in  some  detail  in  con- 
nection with  the  general  part  of  this  report.  The  description  need  not  be 
repeated  here,  but  a  few  details  may  be  added.  All  along  this  part  of 
the  coast  the  layers  are  trending  more  and  more  away  from  the  coast  line 
towards  the  north,  as  a  result  of  which  the  harder  rocks  form  points  pro- 
jecting towards  the  southwest.  Two  of  these  points,  both  formed  of  quartz- 
porphyry,  are  shown  on  the  accompanying  sketch-map.  They  are  the  Great 
Palisades  and  the  bold  point  just  below  the  mouth  of  Baptism  River.  Each 
has  a  shorter  side  trending  east  and  west,  and  a  longer  side  trending  well 


THE  GEEAT  PALISADES  OF  THE  MINNESOTA  COAST. 


315 


around  to  the  north,  longer  in  the  latter  case  than  the  former  because  of  the 
increased  amount  of  northing  in  the  courses  of  the  strata. 


Fig.  23. — Sketch-map  of  exposures  in  the  vicinity  of  Baptism  River,  Minnesota  coast. 


The  south  side  of  the  Palisade  Point  shows  the  following  section.  A 
of  this  section  is  an  amygdaloid,  with  dark-brown  aphanitic  matrix,  just 
rising  from  the  beach  and  here  and  there  capped  by  a  thin  seam  of  red 
detrital  matter.     B  is  a  flow-bed  made  up  of  50  feet  of  massive,  cross- 


316 


COPPBE-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 


columnar,  fine-grained,    chocolate-brown,    semi-conclioidal   diabase,   with 
sparse  laumontite  and  calcite  amygdules  increasing  in  number  above,  and 


Fig.  24. — Section  on  south  cliff  of  tlie  Great  Palisades,  Minnesota  coast.  A,  amygdaloid;  B,  columnar 
diabase-porphyrite ;  C,  mingled  amygdaloid  and  detrital  matter;  D,  columnar  diabase-por- 
phyrite;  E,  amygdaloid;  F,  quartz-porphyry. 

%  feet  of  true  amygdaloid,  of  which  the  upper  3  feet  is  highly  vesicular. 
€  is  a  thin  seam  (6  inches)  of  mingled  amygdaloid  and  detrital  matter.  D 
is  another  great  flow  of  diabase,  made  vip  of  1  foot  of  basal  amygdaloid  or 
vesicular  portion,  30  feet  of  massive  cross-columnar  diabase,  as  above,  the 
sparse  amygdules  gradually  increasing  in  number  upwards,  and  10  feet  of 
true  amygdaloid  (E)  growing  more  and  more  vesicular  upwards. 

The  compact  diabase  of  these  beds  belongs  with  the  diabase-porphy- 
rites,  the  augite  being  a  quite  subordinate  ingredient,  Avhile  there  is  often 
much  non-polarizing  matter.  A  specimen  from  the  upper  bed  yielded  47.9 
per  cent,  of  silica.  In  the  upper  or  amygdaloidal  portions  of  these  beds 
are  scattered  small  hard  red  patches,  a  few  inches  across.  At  times  these 
patches  are  round,  and  appear  at  the  first  glance  somewhat  like  the  pebbles 
of  a  conglomerate,  bu.t  they  are  more  often  irregular  and  are  mingled  curi- 
ously with  the  surrounding  diabasic  material.  Some  of  the  apparently 
Tounded  particles  are  plainly  seen,  even  by  the  naked  eye,  to  fill  original 
vesicles,  often  of  relatively  large  size,  and  in  these  cases  are  either  on  the 
•outer  wall  of  the  vesicle  with  calcite  within,  or  have  between  them  and  the 
wall  a  lining  of  calcite  and  laumontite.  In  the  thin  section  this  material  is 
easily  seen  to  be  fragmental,  being  composed  of  subangular  quartz  grains, 
"with  some  reddish  interstitial  matter. 

It  is  possibly  this  reddish  matter  that  has  caused  Norwood  and  "Win- 
■chelP  to  speak  of  the  existence  here  of  conglomerate  and  breccia  rocks, 

'  Owen's  Geological  Survey  of  Wisconsin,  Iowa,  and  Minnesota.  Philadelphia,  1852,  pp.  959, 362- 
364.    Seventh  Annual  Report  of  the  Geological  and  Natural  Hist.  Survey  of  Minn.,  p.  10. 


THE  GEE  AT  PALISADES  OF  THE  MINNESOTA  COAST.    317 

which  I  myself  failed  to  find.  All  I  could  find  was  a  series  of  plainly- 
marked  flows,  with  massive  columnar  lower  portions  and  upper  vesicular 
portions  as  strongly  developed  as  anywhere  in  the  typical  region  of  Ke- 
weenaw Point.  The  red  detrital  material  apjDears  to  me  to  always  occupy 
the  open  spaces  of  the  scoriaceous  upper  portions  of  the  lava  flows.  In  the 
thin  section  it  is  invariably  sharply  defined  from  the  matrix,  which  always 
presents  the  usual  appearance  of  the  diabase-amygdaloids.  The  whole 
occurrence  is  closely  like  that  of  the  ashbed  rocks  of  Keweenaw  Point, 
both  as  to  the  curious  intermingling  of  scoriaceous  amygdaloid  and  detrital 
material,  and  as  to  the  peculiar  kind  of  diabase  forming  the  lower  portion 
of  each  flow.-^ 

Above  these  beds  comes  the  mass  of  quartz-porphyry  which  forms 
the  Grreat  Palisades.  The  entire  thickness  of  this  porphyry  is  over  300 
feet,  but  in  the  section  under  description  only  some  50  to  75  feet  are  in 
sight.  The  base  of  this  mass  of  porphyry  presents  a  most  peculiar  appear- 
ance. For  a  thickness  of  some  5  feet  it  is  much  weathered,  and  calcified — 
the  contact  line  of  the  dissimilar  rocks  having  evidently  been  the  course  of 
altering  waters — and  shows  a  strong  appearance  of  contorted  lamination, 
which  is  often  intensified  by  the  calcitic  alteration.  This  peculiar  alteration 
is  the  same  as  that  which  aff'ects,  in  a  less  prominent  degree,  higher  portions 
of  the  mass,  as  is  very  distinctly  seen  in  the  thin  section.  The  feldspar 
crystals  are  found  following  the  curving  lines  and  again  obstructing  them. 
The  quartzes  are  much  smaller  than  the  feldspars,  and  are  in  the  usual 
doubly  terminated  crystals,  with  embay ments  of  the  matrix.  There  are 
also  contained  in  the  quartzes  fine  glass  inclusions,  in  regular  shapes  cor- 
responding to  those  of  the  containing  crystal,  and  afi'ected  by  a  hair-like 
devitrification.  Figs.  11  and  12  of  Plate  XIII  represent  this  laminated  por- 
phyry, and  Figs.  3  and  4  of  Plate  XII,  the  rock  at  the  northeast  end 
of  the  Palisades.  All  of  these  figures  represent  those  portions  of  the 
rock  which  show  the  flowage  structure  most  plainly.  The  columnar  char- 
acter of  this  rock  is  very  noticeable,  and  is  of  especial  interest,  since  a 

^  It  sliould  be  repeated  here  that  both  Norwood  and  Winchell  regard  the  amygdaloids,  the  diahases 
that  go  -with  them,  and  all  of  the  felsitic  perphyries  aa  altered  sediment.  Compare  Eighth  Annual 
Report  of  the  Geological  and  Natiiral  History  Survey  of  Minnesota,  p.  26. 


318  COPPBE-BEARING  EOGKS  OP  LAKE  SUPBRIOE. 

columnar  structure  is  far  less  common  in  rhyolitic  rocks  than  in  basaltic.^ 
The  columns  are  peculiar.  The  intersecting  joints  do  not  lie  at  the  same 
angle  with  the  vertical,  but  incline  slightly  toward  each  other,  so  that  they 
intersect  in  depth.  This  is  especially  well  seen  on  the  south  side  of  the 
Palisades,  and  is  roughly  indicated  on  the  accompanying  figure. 

Of  all  of  the  quartz-porphyries  of  the  Lake  Superior  basin  the  Pali- 
sade rock  presents  the  strongest  appearance  of  sedimentary  origin,  on  ac- 
count of  the  lamination  it  shows,  especially  in  its  lower  portions.  This 
lamination  presents  great  irregularities,  but  in  places,  while  changing  many 
degrees  in  inclination  within  a  few  feet,  it  preserves  for  quite  long  distances 
the  same  general  direction.  Nevertheless,  as  shown  in  a  previous  chapter, 
this  rock,  like  all  the  other  porphyries  of  the  Lake  Superior  basin,  is  of 
eruptive  origin,  and  the  lamination  is  that  which  often  characterizes  felsitic 
porphyries  and  rhyolites  the  world  over. 

A  mile  and  a  half  above  its  mouth,  in  the  S.  E.  4,  Sec.  10,  T.  56,  R.  7 
W.,  Baptism  River  makes  falls  over  a  quartziferous-porphyry  much  like 
that  of  the  Palisades.  From  its  position  it  appears  not  impossible  that  this 
rock  belongs  to  the  same  belt  with  that  of  the  Palisades. 

Below  the  Palisades  for  about  a  mile  the  coast  is  formed  of  a  coarse, 
black,  very  highly  olivinitic  gabbro,  with  strongly  marked  vertically 
columnar  structure.  The  diallages  are  large,  producing  a  nodular  weath- 
ering, and  often  show  a  shining  metallic  cleavage  surface.  The  vertically 
columnar  structure  of  this  rock  renders  it  evident  that  we  have  here  to  do 
with  a  flow  and  not  a  dike,  from  which  it  follows  that  there  must  be  a  fault 
between  this  rock  and  that  of  the  Palisades,  it  being  so  different  from  the 
beds  which  belong  beneath  the  Palisade  porphyry.  Beyond  this  rock  again, 
and  separated  from  it  by  a  short  beach,  comes  in  a  fine-grained,  dark-brown 
ashbed-diabase  with  highly  vesicular  amygdaloids,  the  vesicles  elongated 
in  a  common  direction.  After  a  short  beach  of  only  20  paces,  this  is  re- 
placed on  the  shore  by  a  purple  felsite,  behind  which  it  passes,  forming  the 
bed-rock  of  Baptism  River  a  short  distance  above  its  mouth.  At  the  mouth 
of  this  river  the  purple  felsite  is  the  cliff  rock,  and  at  E  is  faulted  against 

'See  illustrations  to  Clarence  King's  40tli  Parallel  Eeport,  vol.  i,  plate  xxi,  and  vol.  ii,  plate  xxiii. 


BOOKS  NEAE  THE  MOUTH  OF  BAPTISM  EIVBE.  319 

fine-grained  diabase  and  amygdaloid,  which  in  turn  pass,  with  a  steep  dip, 
underneath  the  quartz-porphyry  of  the  bold  point  C. 

The  rock  of  the  latter  point  is  precisely  the  same  as  that  of  the  Pali- 
sades. It  shows  the  same  violet-tinted  base,  the  same  quartzes  with  glass 
inclusions,  the  same  very  abundant  orthoclases,  the  same  strongly  columnar 
structure,  and  the  same  lamination.  Here,  however,  the  lamination  shows 
much  less  tendency  to  confine  itself  to  one  direction,  and  therefore  is  less 
likely  to  be  mistaken  for  sedimentary  lamination.  The  point  as  a  whole 
shows  a  very  distinct  dip  to  the  eastward,  and  yet  on  the  long  face  C  D, 
parallel  to  the  trend,  where  the  laminse  should  look  horizontal,  were  the  rock 
a  sedimentary  one,  they  wander  up  and  down  in  a  wholly  aimless  manner. 
At  the  point  D,  where  this  rock  ends  on  a  small  beach,  its  laminae  dip  60° 
S.  65°  W.,  while  just  beyond  across  the  beach  they  dip  80°  north  of  east. 
Here  the  porphyrjr  is  highly  charged  with  calcite,  which  has  impregnated 
it  in  cross  seams,  and  along  the  lamination,  and  in  places  there  is  a  general 
calcitic  decay,  large  white  pseud-amygdules  of  calcite  dotting  the  rock.  At 
D  F  of  the  map  of  Fig.  23,  this  porphyry  passes  beneath  plainly  bedded 
diabases  and  amygdaloids.  This  horizon  I  have  selected  as  the  base  of  the 
Temperance  River  Group. 

The  close  similarity  of  the  Palisade  porphyry  to  that  of  the  point  just 
below  Baptism  River,  and  of  the  diabases  immediately  underlying  these 
porphyries  to  one  another,  suggest  that  the  two  points  are  but  portions  of 
one  layer  faulted  apart. 

Beyond  the  point  last  described,  in  descending  the  coast,  the  Beaver 
Bay  Group  strikes  back  into  the  country,  having  between  it  and  the  shore 
a  constantly  widening  strip  of  the  beds  of  the  Temperance  River  Group. 
The  exposures,  by  which  has  been  made  out  the  continuance,  in  this  region, 
of  the  Beaver  Bay  Group,  until  it  emerges  again  on  the  shore  at  Grand 
Marais,  do  not  merit  any  particular  description.  They  are  the  usual  steep- 
backed  ridges,  with  flat  lakeward  slope,  composed  of  the  common  black 
gabbro. 

The  exposures  below  Grand  Marais  which  I  have  referred  to  the  Bea- 
ver Bay  Group  may  also  be  more  rapidly  passed  over.  The  dips  here  are 
not  more  than  8°  to  10°  lakeward,  and  the  trend  much  more  to  the  east 


320       COPPEE-BBAEING  EOCKS  OF  LAKE  SUPEEIOE. 

than  that  of  the  shore,  being  at  times  even  due  east.  Since  the  coast  line 
itself  is  trending  here  only  some  20°  to  25°  north  of  east,  it  follows  that  each 
bed  makes  a  very  long  exposure  on  the  coast.  Felsite  and  quartziferous 
porphyry  have  a  great  development  between  Grand  Marais  and  the  Brul^ 
River,  the  ledges  on  the  coast  being  usually  comparatively  low  and  often 
partly  concealed  by  shingle  beaches.  On  the  Devil's  Track  River,  how- 
ever, the  exposures  are  on  a  grand  scale. 

Quartziferous  and  granitic  porphyries  show  again  in  the  vicinity  of  the 
large  bay  in  the  east  part  of  T.  62,  R.  4  E.,  ^and  again  at  Red  Rock  Bay  in 
the  Indian  reservation  (S.  E.  ^,  T.  63,  R.  5  E.).  The  remainder  of  the 
coast  between  here  and  Grand  Marais  is  formed  chiefly  of  coarse  black 
gabbro. 

In  the  angle  of  the  coast  immediately  below  Grand  Marais,  S.  W.  ^, 
Sec.  21,  T.  61,  R.  1  E.,  red  felsite  is  cut  by  dikes  of  dark-colored  rock,  and 
the  same  thing  recurs  for  two  or  three  miles  down  the  coast,  the  dikes  pro- 
ducing projecting  points  and  the  felsite  weathering  down  into  shingle  beaches. 
The  shore-cliff  just  beyond  Grand  Marais  is  red  felsite  for  a  length  of  sev- 
eral hundred  paces  in  a  northeasterly  direction.  At  the  southwest  end  of 
the  cliff  is  a  broad  dike  composed  of  a  medium-grained  olivine-diabase,  of 
which  the  surface  presents  a  mottling  like  that  of  the  luster-mottled  mela- 
phyrs.  The  olivine  is  wholly  altered  to  a  brownish  substance,  but  the  other 
ingredients  are  quite  fresh.  Near  its  junction  with  the  red  felsite  this  rock 
becomes  finer  in  grain,  until  at  the  contact  it  merges  into  an  aphanitic  dia- 
base-porphyrite,  with  much  greenish  alteration  chlorite. 

At  the  northeast  end  of  this  cliff  is  another  broad  dike  of  similar  char- 
acter, with  two  or  three  narrow  ones,  from  a  mere  seam  to  one  or  two  feet 
in  width.  The  rock  of  these  narrow  dikes  is  a  dense  diabase-porphyrite 
with  much  non-polarizing  matter  in  the  base.  The  felsite  cut  by  these  dikes 
presents  a  quite  distinct,  though  irregular,  dip  towards  the  lake,  and  has 
the  usual  wave-like  pseudo-lamination  markings.  It  presents  the  appear- 
ance of  a  hardened  mud  rock  more  here  than  at  most  points  where  the 
felsites  were  observed.  In  the  thin  section  is  shown  a  base  with  much  non- 
polarizing  material,  with  the  usual  ferritic  devitrification-product,  and  much 


BEAVER  BAY  GEOUP  BELOW  GRAND  MAEAIS.       321 

arborescent  and  netted  secondary  quartz.  Calcite  seams  permeate  much 
of  the  rock,  and,  in  many  places,  the  weathering  has  so  affected  it  that  a 
sHghtblow  of  the  hammer  on  the  face  of  the  cliff  will  bring  down  showers 
of  angular  fragments.  This  peculiar  result  of  weathering  is  very  charac- 
teristic of  the  Lake  Superior  red  felsites. 

The  large  exposures  of  red  felsite  on  the  Devil's  Track  River  have 
already  been  alluded  to.  From  the  mouth  of  the  river  nearly  to  the  north 
line  of  Sec.  3,  T.  61,  R.  1  E.,  the  exposures  are  almost  continuous  and  the 
cliffs  occasionally  rise  to  a  height  of  150  feet.  There  is  often  an  appear- 
ance of  lamination,  and  across  these  markings  a  strong  columnar  struc- 
ture is  frequently  seen.  The  thin  section  shows  the  usual  non-polarizing 
base,  with  ferrite  particles  and  net-worked  secondary  quartz. 

Below  the  Devil's  Track  are  long  beaches  of  felsite  shingle.  In  Sec.  9, 
T.  61,  R.  2  E.,  typical  brown  ashbed-diabase  and  diabase-porphyrite  form 
the  shore,  and  at  one  point  show  a  capping  mass,  five  feet  thick,  of  curi- 
ously intermingled  sandstone  and  amygdaloid.  From  this  point  to  the 
mouth  of  the  Bruld  River  red  shingle  beaches  are  interrupted  occasionally 
by  exposures  of  brown  and  red  diabase-porphyrite.  Many  ledges  show  no 
macroscopically  visible  porphyritic  ingredients,  but  are  porphyrites  because 
of  their  content  of  non -polarizing  base;  others  show  numerous  large  por- 
phyritic oligoclases  and  augites.  In  the  groundmass  in  some  slices  more 
or  less  secondary  ramifying  quartz  is  seen,  -when  we  have  a  transition  to 
the  rock  recognized  in  the  pebbles  of  the  South  Shore  conglomerates  as  a 
non-quartziferous  porphyr}^ 

Below  the  Bruld,  coarse  olivine-gabbro  forms  the  coast  for  a  number 
of  miles,  lying  in  flat,  often  cross-columnar  flows.  Just  below  the  mouth 
of  the  Brul^  this  rock  is  peculiar,  on  account  of  its  great  richness  in  brassy- 
lustered  diallage  and  its  very  large  olivines,  which  are  sometimes  one-fourth 
to  one-third  of  an  inch  across,  and  always  altered  to  a  black,  resinous-looking 
substance  with  a  concentric  scaly  structure  (hyalosiderite).  Below  the 
large  bay  in  the  northeast  part  of  T.  62,  R.  4  E.,  these  coarse  rocks  are 
interrupted  by  beaches  with  detached  ledges  of  dense,  brown  diabase-por- 
phyrite.   One  exposure,  near  the  west  line  of  Sec.  12,  T.  62,  R  4  E.,  shows  a 

red  crystalline  rock,  which  macroscopically  presents  a  mass  of  red  feldspar 
21  L  s 


322  C0PPEE-BEAEI2>rG  EOCKS  OF  LAKE  SUPEEIOE. 

crystals,  marked  in  places  by  a  network  of  black  lines.  Under  the  micro- 
scope this  rock  turns  out  to  be  composed  entirely  of  crystalline  matter; 
reddened  oligoclases  and  orthoclases,  completely  saturated  with  secondary 
quartz,  making  up  most  of  the  section.  The  black  markings  mentioned 
resolve  themselves  into  altered  augite  blades,  with  whose  alteration  is  con- 
nected the  production  of  much  magnetite.  Numerous  large  apatite  needles 
are  included. 

Below  the  mouth  of  the  river  forming  the  western  boundary  of  the 
Indian  Reservation,  coarse  olivine-gabbro  forms  the  coast  as  far  as  Red 
Rock  Bay,  a  distance  of  some  five  miles,  lying  in  beds  with  cross-columnar 
structure,  flat  lakeward  dip,  and  easterly  trend.  In  many  places  this  rock 
shows  in  a  marked  manner  the  luster-mottling  due  to  the  presence  of  rela- 
tively large  diallages,  including  many  feldspars. 

At  Red  Rock  Bay,  in  the  Indian  Reservation  (southeast  part  of  T,  63, 
R.  5  E.),  these  black  rocks  give  place  again  to  red  felsite  and  quartziferous 
porphyry.  The  display  of  these  acid  red  rocks  here  is  very  large  and 
among  the  most  interesting  of  this  class  of  rocks  in  the  Lake  Superior  region. 
The  red  rock  at  the  bay  is  commonly  a  true  quartziferous  porphyry,  with  a 
base  in  which  much  non-polarizing  matter  is  mingled  with  the  usual  ferrite 
particles,  and  saturated  with  arborescent  secondary  quartz.  On  the  high 
point  of  red  rock  in  the  bay,  areas  of  a  slightly  differently  colored  rock  are 
included  in  the  general  red  mass.  Under  the  microscope  the  rock  of  these 
areas  shows  a  base  in  which  quartz  and  orthoclase  are  distinctly  individu- 
alized in  good-sized  particles.  The  bold  red  bluff  from  which  the  bay  takes 
its  name  shows  no  trace  of  banding,  but  at  the  point  beyond  the  bay  the 
curving  and  twisting  pseudo-lamination  so  often  seen  in  these  red  porphy- 
ries is  shown  on  a  large  scale.  For  short  distances  the  rock  will  appear  like 
a  much-contorted  schist,  and  then  again  will  pass  into  the  general  structure- 
less mass.     "When  seen,  the  banding  is  as  often  vertical  as  horizontal. 

In  this  vicinity  the  red  rock  is  cut  by  a  heavy  dike,  of  which  part  is 
represented  in  the  accompanying  figure.  This  dike  shows  for  several 
hundred  feet  along  its  length,  and  with  a  width  of  some  75  feet.  The 
junctions  with  the  adjoining  rock  are  sharp,  occasionally  showing  irreg- 
ularities, as  in  the  figure.    In  places  patches  of  red  rock  clinging  to  the  top 


TEMPEEANCE  EIVEE  GEOUP. 


323 


of  the  dike  suggest  that  we  have  here  its  original  top.  The  rock  of  the 
dike  is  a  very  dense,  dark  greenish-gray  diabase-poi-phyrite,  with  a  tendency 
to  become  somewhat  coarser  in  the  middle.  The  silica  content  of  this  dike- 
rock  is  very  low,  being  only  45.88  per  cent.  The  whole  dike  is  intersected 
by  two  sets  of  very  strong  transverse  joints,  which  dip  respectively  S  13J° 
and  E.  69|°.    These  are  cut  by  others  parallel  to  the  walls  of  the  dike  and 


^s  ■:_. 

Fig.  25. — Dike  in  quartz-porphyry,  Eed  Rock  Bay. 

the  whole  mass  looks  much  like  a  pile  of  books.  About  one-fourth  of  a 
mile  farther  down  the  coast  another  dike,  of  more  completely  crystalUne 
diabase,  cuts  the  red  rock,  which  soon  after  ends,  being  replaced  by  the 
rocks  which  I  have  already  described  in  connection  with  the  Duluth  and 
Lester  River  groups. 

Temperance  Biver  Group. — The  rocks  of  this  group  are  displayed  along 
the  coast  from  a  point  a  mile  and  a  half  below  the  mouth  of  Baptism  River 
to  Grand  Marais,  a  total  distance  of  some  50  miles.  The  total  thickness 
appears  to  be  upwards  of  2,000  feet.  The  highest  beds  of  the  group 
form  the  coast  between  Petit  Marais  and  Temperance  River.  As  already 
described,  the  beds  of  this  group,  where  they  first  appear  in  descending  the 
coast,  just  below  Baptism  River,  trend  sharply  to  the  north,  and  even  due 
north  for  a  while,  after  which  they  swing  around  more  to  the  east  of  north, 
the  coast  line  and  strata  trending  together  for  a  long  distance. 

The  easting  in  the  trend  of  the  layers  continuing  to  increase  at  about 
two  miles  below  Temperance  River,  they  finally  begin  to  trend  more  to 
the  east  than  the  coast   line,  so  that  from  this  point  to  Grand  Marais, 


324       COPPEE-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 

where  the  base  of  the  group  comes  to  view,  there  is  a  steady  descent  in 
geological  horizon.  The  iisual  lakeward  dip,  of  course,  holds  throughout 
the  extent  of  the  Temperance  River  Group.  The  angle  ranges  from  6  °  to 
30°,  and  on  the  whole  the  inclination,  especially  in  the  western  portion,  is 
rather  greater  than  usual. 

In  its  kinds  of  rock  the  Temperance  River  Group,  on  the  whole,  con- 
trasts strongly  with  the  preceding  one;  indeed,  with  all  of  the  Minnesota 
shore  groups,  except  the  Agate  Bay  Group,  with  which  it  has  some  charac- 
teristics in  common.  The  rocks  forming  the  greater  part  of  its  thickness 
are  dark-brownish,  fine-grained  diabases  of  the  ordinary  type,  in  thin  layers, 
with  strongly  developed  vesicular  or  amygdaloidal  upper  portions,  and  often 
with  a  more  or  less  plainlj^  marked  columnar  structure  in  the  lower  por- 
tions. Much  less  common,  but  still  occurring  in  a  number  of  layers,  are 
fine-grained,  blackish  olivine-diabases  or  melaphyrs,  with  the  typical  luster- 
mottlings.  They  are  also  furnished  with  amygdaloids.  Layers  of  ashbed- 
diabase  and  diabase-porphyrite  of  conchoidal  fracture,  and  furnished  with 
amygdaloids,  also  occur,  especially  toward  the  base  of  the  group.  Several 
seams  of  reddish  sandstone  and  shale .  are  included,  one  layer  exceeding 
200  feet  in  thickness.  Peculiar  conglomerates  also  occur.  The  whole  suc- 
cession presents  much  the  appearance  of  some  of  the  layers  in  the  middle 
and  upper  portions  of  the  Keweenaw  Point  series. 

The  prevailing  diabase  and  melaphyr  of  this  group  do  not  merit  any 
especial  description  here,  since  they  are  only  repetitions  of  what  have 
already  been  described  in  full  for  other  parts  of  the  extent  of  the  formation. 
They  make  many  very  interesting  exposures,  among  which  may  be  mentioned 
as  especially  fine  those  of  the  shore  of  sections  36  of  T.57,R.  7"W.,  and  sections 
-30  and  31  of  T.  67,  R.  6  W. ;  that  of  the  mouth  of  the  Manitou  River;  and  that 
of  the  point  on  the  east  side  of  Pork  Bay,  where  may  be  seen  a  black  luster- 
mottled  olivine-diabase  or  melaphyr,  which,  under  the  microscope,  shows 
4ill  the  characters  of  this  rock  as  found  in  the  typical  region  of  Keweenaw 
Point.  Another  fine  exposure  is  that  of  the  bay  in  the  S.  W.  ^,  Sec.  21,  T.  58, 
R.  5  W.,  where  quite  a  succession  of  diabases  and  amygdaloids  is  in  sight, 
including  one  or  more  beds  of  luster-mottled  melaphyr.      Those  of  the 


EOCKS  AT  MOUTH  OP  TEMPEEANCE  ElVEE. 


325 


"Two  Islands,"  which  are  columnar  rocks  with  the  outline  of  Fig.  26,  and 
of  the  mouth  of  Temperance  Eiver,  are  also  very  instructive. 


Fig.  26.— Profile  of  island  at  mouth  of  Two  Islands  Eiver. 

The  last-named  place  is  possibly  the  most  interesting  of  all.  A  short 
distance  from  its  mouth  the  river  makes  several  falls,  the  first  one  into,  the 
rest  along  the  course  of  a  narrow  gorge,  which  sometimes  reaches  50  feet 
in  depth,  but  is  so  narrow  that  in  places  one  can  step  from  one  to  the  other 
of  the  overhanging  walls.  The  gorge  is  a  succession  of  well-smoothed  pot- 
holes broken  into  each  other.  There  are  displayed  here  a  number  of  very 
thin  layers,  the  massive  columnar  portions  often  not  exceeding  two  to  four 
feet,  and  the  very  strongly  developed  amygdaloids  running  even  below 
these  figures.  A  number  of  these  beds  have  very  plainly  marked  basal 
amygdaloids,  with  relatively  sparse  spike  am5^gdules.  At  two  or  three  hori- 
zons, streaks  of  red  sandy  shale  were  noticed  between  one  of  these  amyg- 
daloids below  and  a  massive  layer  above.  The  detrital  matter  is  often  in 
mere  films,  and  at  times  is  entirely  absent.  In  places  it  is  found  to  have 
aggregated  in  irregularities  of  the  underlying  amygdaloid,  when  for  a  short 


Fig.  27. — Section  on  Temperance  Eiver. 


distance  it  may  have  something  of  a  thickness.  This  is  finely  displayed 
on  the  northeast  side  of  the  basin  at  the  mouth  of  the  river,  as  shown 
in  the  following  section,  which  represents  a  wall  some  80  feet  long  and  20 


326 


COPPER-BEARING  ROCKS  OF  LAKE  SUPERIOR. 


high.  The  whole  appearance  at  this  place  reminds  one  strongly  of  the 
alternations  at  the  upper  falls  of  the  Montreal,  on  the  South  Shore/  save 
that  the  beds  are  thinner  on  Temperance  River  than  on  the  Montreal. 

The  peculiar  ii'regularities  to  which  these  eruptive  rocks  are  subject 

were  well  seen  in  a  cliff  side 
below  Temperance  River,  where 
-10-  Fig.  28  was  drawn. 

The  ashbed-diabases  of  this 
group  are  only  met  with  at  low 
horizons,  being  found  on  the 
coast  not  far  below  Baptism 
River,  and  again  at  Grand  Ma- 
rais.  Immediately  overlying 
the  quai-tzose-porphyry  of  the 
Beaver  Bay  Group,  C  of  Fig. 
23,  is  found  the  succession 
indicated  in  the  following  dia- 


Fig.  28.— Sectkm  on  Minnesota  Coast,  near  Temperance  gram    (Fig.    29),    in    wHch    the 
EiTsr     The  columnar  portions  are  the  lower  massiyepor-j^^gg^   .^^  j^  ^   1^^,^^^  ^^^_ 

lions  of  the  flows ;  the  dotted  parts  the  amygdaloids  or  -i  '      jr 

vesicuiar  npper  portions  of  the  flows.  nitic    diabase  -  porphyrite,    with 

conchoidal  fracture,  and  containing  52.56  per  cent,  of  silica.  The  next 
is  a  black,  medium-grained  olivine-diabase  or  melaphyr,  containing  50.76 
per  cent,  of  silica,  with  the  olivine  wholly  altered  to  a  green  and  brown 
substance,  and  furnished  above  with  an  amygdaloid,  and  the  uppermost 


Itixi^tine, 


Fig.  29. — Section  on  Minnesota  Coast,  near  Baptism  River. 

layer  is  a  heavy  one  of  brown  diabase-porphyrite,  resembling  that  at  the 
base  of  the  section,  but  not  quite  so  dense  in  grain,  and  carrying  57.87  per 


I  Geol.  of  Wis.,  Vol.  Ill,  p.  191. 


DETRITAL  BOOKS  OF  THE  TEMPEEANCE  EIVEE  GEOUP.       327 

cent,  of  silica.      Just  where  the  sketch  was  taken  the  dip  lakeward  was 
unusually  high.     Further  down  the  coast  it  soon  flattens  again. 

The  rock  constituting  the  point  at  Grand  Marais  is  again  all  ashbed- 
diabase,  though  for  the  most  part  coarser-grained  than  that  described  above. 
It  has  a  brown  color,  is  exceedingly  dense  and  hard,  and  shows  a  well-marked 
cross-columnar  structure  in  places.  In  the  thin  section  it  is  seen  to  contain 
much  augite,  both  in  the  round  particles  characteristic  of  the  ashbed-diabases, 
and  indicative  of  relatively  rapid  solidification,  and  in  particles  whose  con- 
tours are  determined  by  the  pre-existing  plagioclases.  It  is  thus  a  well- 
marked  intermediate  stage  between  the  diabases  of  the  ordinary  type  and 
of  the  ashbed  type.  Besides  the  tabular  plagioclases  of  the  matrix  there  are 
also  present  larger  porphyritic  plagioclases. 

The  strong  resistant  power  of  this  rock  is  shown  in  the  existence  of 
such  an  exposed  ledge  as  that  which  forms  the  protecting  reef  of  Grand 
Marais  Harbor.  The  rock  is  in  part  more  dense  than  that  represented  by 
the  above  description,  and,  to  judge  from  the  numerous  angular  fragments 
on  the  shingle  ridge  forming  the  east  side  of  Grand  Marais,  must  graduate 
downwards  into  an  aphanitic  diabase-porphyrite. 

The  detrital  rocks  of  the  Temperance  Eiver  Group  were  noticed  at 
several  points  along  the  coast.  The  westernmost  of  these  is  in  the  N.  E. 
^,  Sec.  11,  T.  56,  R  7  W.,  where  a  thin  seam  of  red  shale,  with  very  irregu- 
lar thickness,  holds  balls  and  fragments  of  amygdaloid,  and  occasionally  has 
amygdaloidal  material  strangely  mixed  up  with  the  red  sand  of  the  matrix. 
This  seam  overlies  a  porphyritic  amygdaloid,  which  graduates  downward 
into  a  true  ashbed-diabase,  and  is  overlain  by  an  excessively  vesicular  thin 
amygdaloid.  The  whole  occurrence,  then,  is  an  exact  repetition  of  the 
ashbed  of  Keweenaw  Point,  the  same  peculiar  dense  diabase  being  fur- 
nished with  the  same  peculiar  scoriaceous  amygdaloid.  What  appears  to 
be  the  conglomerate  just  mentioned,  but  thickened,  is  seen  in  a  vertical 
wall  36  feet  high  at  the  bottom  of  a  bay  in  the  extreme  northeast  corner  of 
section  11.  The  overlying  and  underlying  rocks  are  not  seen  here,  but  at 
its  eastern  extremity  this  conglomerate  comes  into  abrupt  vertical  contact 
(the  contact  being  seen  on  a  wall  30  feet  high)  with  a  dark  brownish-gray, 
fine-grained  rock.    This  rock  the  thin  section  shows  to  be  a  typical  luster- 


328       COPPER-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 

mottled  olivine-cliabase  or  melaphyr,  the  olivine  in  which  is  wholly  altered 
to  a  reddish  and  greenish  material.     This  rock  is  alluded  to  further  on. 

Just  above  Manitou  River,  Sec.  10,  T.  57,  R.  6  W.,  15  to  20  feet  of  red 
shaly  sandstone  may  be  seen  overlain  by  an  amygdaloid,  and  this  in  turn 
by  a  columnar  diabase.  Numerous  fragments  of  amygdaloid  and  decom- 
posed diabase  are  contained  in  the  sandstone.  The  fragments  are  all  angu- 
lar and  under  an  inch  in  diameter.  The  occurrence  of  seams  of  red  shale 
at  the  mouth  of  Temperance  River  has  already  been  mentioned,  and  the 
peculiar  irregularities  to  which  these  seams  are  subject  shown  in  Fig.  27. 
A  mile  and  a  half  below  the  mouth  of  Poplar  River,  in  the  northwest  corner 
of  Sec.  35,  T.  60,  R.  3  W.,  six  feet  of  a  conglomerate  are  seen  overlain  by 
black  melaphyr  or  fine-grained  olivine  diabase.  The  pebbles  are  larger 
below,  reaching  four  inches  in  diameter,  and  are  principally  of  red  felsite. 
Many  are,  however,  of  various  types  of  amygdaloid  and  diabase-porphyrite. 
Several  thin  sections  were  made  of  these  pebbles,  and  in  all  the  amount  of 
non-polarizing  unindividualized  matrix  seemed  unusually  large,  suggesting 
the  possible  origin  of  these  balls  as  volcanic  scorise. 

At  the  mouth  of  the  creek  in  the  S.  W.  i.  Sec.  19,  T.  60,  R.  2  W.,  the 
following  section  was  noticed;  the  order  is  an  ascending  one: 

1.  Highly  vesicular  amygdaloid 2  feet. 

2.  Fine-grained  diabase,  including  (a)  basal  amygdaloid,  1  foot;  (b)  less 

amygdaloidal,  1  foot;  (c)  massive  portion,  8-12  feet;  [d]  very  irregu- 
lar summit  amygdaloid,  6  inches  to  2  feet;  in  all  about 15  feet. 

3.  Eed  shaly  sandstone,  bunchy,  irregular,  mingled  with  the  amygdaloid, 

1  inch  to 2  feet. 

4.  Amygdaloid 4  inches. 

5.  Sandstone 6  inches. 

6.  Amygdaloid 2  feet. 

7.  Sandstone 3  inches. 

8.  Massive  melaphyr 10  feet. 

The  seams  3  to  7,  inclusive,  are  very  irregular,  and  in  all  the  sand- 
stone probably  belongs  to  one  horizon. 

In  the  bay  immediately  behind  Caribou  Point,  S.  W.  ^,  Sec.  11,  T.  60, 
R.  2  W.,  red  shaly  sandstone  underlies  an  excessively  fine-grained  but 
wholly  typical  melaphyr  or  olivine-diabase.  Some  twelve  feet  of  the  mela- 
phyr and  twenty  of  the  sandstone  are  in  sight.     The  large  bay  here  is 


ISLE  EOYALE  TO  NIPIGON  BAT.  329 

probably  worn  out  in  this  sandstone,  of  which  it  is  supposed  that  only  the 
uppermost  parts  are  in  sight.  Grood  Harbor  Bay,  four  miles  below,  has 
certainly  had  this  origin,  the  sandstone  concerned  being,  however,  a  lower 
layer.  At  the  latter  place  both  underlying  and  overlying  rocks  are  in  sight, 
the  former  in  a  low  ledge  on  the  northeast  shore  of  the  bay,  the  latter  in  a 
vertical  exposure  of  20  feet.  The  sandstone  is  225  feet  thick,  of  which  120 
feet  may  be  measured  in  detail.  The  whole  thickness  dips  9°  east  of  south. 
The  Temperance  River  Group  is  almost  free  from  dikes,  another  feature 
which  it  has  in  common  with  the  bedded  diabases  of  the  upper  part  of  the 
Keweenaw  Point  series.  One  of  aphanitic  black  rock,  six  feet  wide,  and  with 
cross-columnar  structure,  was  noticed  cutting  amygdaloid  near  the  base  of 
the  group  on  the  shore  at  the  east  side  of  Sec.  11,  T.  56,  R.  7  W.  In  the 
same  vicinity.  Sec.  11,  T.  56,  R.  7  W.,  at  two  points,  masses  of  red  augite- 
syenite  were  seen  in  vertical  contact  with  the  diabases  of  the  Temperance 
River  Group.  It  is  possible  that  these  are  faulted  up  from  the  under- 
lying Beaver  Bay  Group,  though  they  look  like  cutting  masses.  The  high 
bluff  known  as  Carlton's  Peak,  Sec.  20,  T.  59,  R.  4  W.,  near  Temperance 
River,  shows  at  its  summit  numerous  large  angular  fragments  of  anorthite- 
rock,  such  as  has  already  been  described  in  connection  with  the  Beaver 
Bay  Group.  None  was  seen  that  could  be  certainly  regarded  as  in  place; 
nevertheless,  the  mountain  is,  without  much  doubt,  composed  of  this  rock, 
and  I  should  regard  the  rock  as  having  antedated  the  Temperance  Group 
flows  rather  than  as  a  cutting  mass. 


Section  II.— ISLE  EOYALE  TO  NIPIGON  BAT. 

All  along  the  eastern  part  of  the  Minnesota  coast,  as  described  in  the 
preceding  section,  the  Keweenawan  beds  strike  away  towards  the  lake 
from  the  coast  line,  so  that  finally,  at  Grand  Portage  Bay,  the  older  slates 
come  out  to  the  shore.  The  Keweenawan  beds  reappear,  however,  in  Isle 
Royale,  having  exchanged  their  easterly  course  for  a  more  northeasterly 
one^  while  concealed  by  the  lake.     This  change  already  begins  to  be  per- 

>  See  the  red  lines  of  Plat©  XX  Vni. 


330       COPPBE-BBAEING  EOCKS  OF  LAKB  SUPEEIOE. 

ceptible  on  the  Minuesota  coast  before  reaching  Grand  Portage,  and  shows 
also  in  the  Lucille  Islands,  off  Pigeon  Point,  the  outer  one  of  these  islands 
being  formed  of  a  typical  Keweenawan  diabase. 

The  following  brief  account,  with  the  accompanying  map  of  Plate 
XXVII,  will  serve  to  present  the  main  features  of  the  geology  of  this 
region.  So  far  as  Isle  Royale  is  concerned,  I  have  had  to  depend  upon  the 
report  of  Messrs.  Foster  and  Whitney,  read  in  the  light  of  a  familiarity  with 
most  of  the  remainder  of  the  extent  of  the  formation,  not  having  visited  the 
island  myself.^  The  detailed  topographical  map  of  the  island,  by  the  United 
States  Lake  Survey,  aids  not  a  little  in  the  understanding  of  the  structure. 
The  region  from  Thunder  Bay  to  Nipigon  Bay  I  have  examined  myself, 
and  am  thus  able  to  draw  information  more  satisfactorily  from  the  descrip- 
tions of  Logan,  Macfarlane,  and  Bell,  as  well  as  to  judge  of  the  correctness 
of  the  views  advanced  by  these  writers. 

ISLE  EOYALE. 

Isle  Royale  is  a  very  long,  narrow  island,  trending  in  a  general  north- 
easterly direction.  From  point  to  point  the  island  is  just  45  miles  in  length; 
but  from  the  Rock  of  Ages,  the  farthest  outlying  reef  to  the  southwest,  to 
the  Gull  Island  rocks  on  the  northeast,  is  57  miles.  The  island  varies  in 
width  from  three  to  eight  miles.  It  does  not  lie  exactly  in  a  straight  line, 
but  curves  from  N.  65°  E.  in  the  southwest  part  to  N.  53°  E.  in  the  north- 
east part.  On  the  southern  side  of  the  southwestern  end  is  quite  an  area  of 
low  land,  underlain  by  sandstone  and  conglomerate,  dipping  some  8°  to  the 
southeast.  This  sandstone  evidently  belongs  to  the  Upper  Division  of  the 
series.  The  remainder  of  the  island  is  made  up  of  very  regularly  bedded 
crystalline  rocks,  with  here  and  there  an  interstratified  conglomerate,  all 
dipping  southward  at  an  angle  which  has  not  been  satisfactorily  determined, 
but  which,  probably,  does  not  often  exceed  25°. 

The  whole  shape  of  the  island,  both  as  to  outline  and  topography, 
expresses  the  geological  structure  in  a  most  striking  manner.  It  is  traversed 
from  end  to  end  by  a  series  of  parallel  ridges,  which  present  always  a  steep, 
often  a  precipitous  side  towards  the  north,  and  a  gradual  slope  towards  the 

'  Since  the  above  was  written,  N.  H.  Wincliell  has  published  some  notes  on  the  geology  of  Isle 
Royale,  especially  on  the  south  side  of  the  island.  Tenth  Annual  Report  of  the  Geological  and  Natural 
History  Survey  of  Minnesota,  pp.  49-54. 


ISLE.EOYALB.  331 

south.  These  ridges  are,  of  course,  made  of  the  heavier,  more  resistant 
layers.  They  seldom  reach  500  feet  above  the  lake  level.  The  inter- 
mediate linear  valleys  are  worn  in  the  softer  amygdaloids  and  other  less 
resistant  rocks.  At  the  ends  of  the  island  these  valleys  are  occupied  by 
long,  narrow  extensions  of  the  lake,  and  between  them  the  ridges  continue, 
constituting  the  so-called  "fingers"  of  Isle  Royale.  Ridges  and  valleys 
both  change  in  trend  as  the  island  is  followed  to  the  northeast,  as  does  the 
island  itself  as  a  whole,  and  this  is  evidently  due  to  a  similar  curving  in  the 
trend  of  the  underlying  rocks.^ 

It  is  evident,  from  the  descriptions  of  Messrs.  Foster  and  Whitney, 
that  we  have  here  merely  a  repetition  of  what  is  seen  everywhere  else  in 
the  course  of  the  Keweenawan  rocks.  They  describe  the  beds  as  exceed- 
ingly well  marked  and  mostly  thin,  and  as  provided  with  strongly-devel- 
oped vesicular  portions,  and  lower  portions  which  are  often  columnar. 
The  more  coarsely  crystalline  kinds  of  which  Foster  and  Whitney  speak — 
such  as  that  of  the  ridge  along  the  northwest  shore  of  the  island,  and  that 
of  Blake's  Point,  at  the  northeast — evidently  belong  with  the  coarse  gabbros 
of  this  memoir;  while  the  porphyritic  kinds  which  they  mention  as  occur- 
ring at  several  points  would  appear  to  belong  with  my  diabase-porphyrites. 

They  say  nothing,  however,  of  the  occurrence  of  red  rocks,  which 
might  be  quartziferous  or  granitic  porphyries,  although  one  would  expect 
such  to  occur,  especially  on  the  north  side  of  the  island.  One  occurrence 
which  they  describe  is  of  interest,  namely,  the  sandstone  veins  running 
down  from  an  overlying  sandstone  into  the  cracks  of  an  amygdaloid  at  the 
mouth  of  Chippewa  Harbor,  on  the  south  side  of  the  island.  The  same 
thing  may  be  seen  at  many  places  on  the  Minnesota  coast,  as  already  indi- 

THUNDER  BAX  TO  NIPIGON  BAT. 

As  shown  in  the  following  chapter,  the  slates  of  the  west  and  north- 
west sides  of  Thunder  Bay,  and  again  those  of  Pie  Island  and  Thunder 
Cape  as  far  around  as  Silver  Islet,  belong  with  the  iron- bearing  rocks  of 
the  South  Shore.  The  east  shore  of  Thunder  Bay,  however,  and  the  whole 
of  the  peninsula  between  Thunder  and  Black  bays,  are  occupied  by  a 

'See,  also,  Chapter  IX. 


332       COPPER-BEAEING  EOCKS  OF  LAKE  SUPERIOE. 

series  of  quartzose  sandstones,  dolomitic  sandstones  and  red  marls,  whicb 
plainly  belong  at  the  base  of  the  Keweenaw  Series.  According  to  Logan,* 
the  whole  thickness  of  these  rocks  is  between  800  and  900  feet;  Bell,  how- 
ever, from  later  study,  making  it  between  1,300  and  1,400  feet.  The  fol- 
lowing is  the  succession,  as  given  by  Bell:^ 

Feet. 

Alternating  red  and  white  dolomitic  sandstone,  With  a  red  conglomerate  layer  at 

the  bottom,  occurring  on  Wood's  location.  Thunder  Cape^*. 40^ 

Light  gray  dolomitic  sandstone,  with  occasional  red  layers  and  spots  and  patches 
of  the  same  color.  These  sandstones  occur  along  the  southwest  side  of 
Thunder  Bay  and  on  Wood's  location^ 200' 

Eed  sandstones  and  shales,  interstratifled  with  white  or  light-gray  sandstone 
beds,  frequently  exhibiting  ripple-marked  surfaces,  and  also  with  conglom- 
erate layers  composed  of  pebbles  and  bowlders  of  coarse  red  jasper  in  a 
matrix  of  white,  red,  or  greenish  sand 50& 

Compact  light-reddish  limestones  (some  of  them  fit  for  burning  into  quicklime), 

interstratified  with  shales  and  sandstones  of  the  same  color 8ft 

Indurated  red  and  yellowish-gray  marl,  usually  containing  a  large  proportion  of 
the  carbonates  of  lime  and  magnesia.^  This  division  runs  through  the  center 
of  the  peninsula  between  Thunder  Bay  and  Black  Bay,  and  may,  in  this  re- 
gion, have  a  thickness  of  350  feet  or  more 350' 

Eed  and  white  sandstones,  with  conglomerate  layers,  the  red  sandstones  being 
often  very  argillaceous  and  variegated  with  green  spots  and  streaks,  and  hav- 
ing many  of  their  surfaces  ripple-marked.  These  rocks  are  found  all  along 
the  northwest  side  of  Black  Bay  as  far  up  as  the  township  of  McTavish 200 

This  succession  is  of  interest  as  presenting  us  with  the  only  instance 
of  a  considerable  accumulation  of  detrital  matter  at  so  low  a  horizon  in  the 
entire  extent  of  the  Keweenaw  Series;  and  the  only  instance,  also,  so  far  as. 
the  immediate  basin  of  Lake  Superior  is  concerned— not  taking  local  infil- 
trations of  calcite  into  account — of  the  occurrence  of  calcareous  and  dolo- 
mitic matter  in  the  sandstones  of  this  series.  The  reference  of  these  rocks 
to  the  Keweenaw  Series  has,  in  fact,  been  questioned  by  Macfarlaiie^  and 
Hunt;''  the  former  regarding  them,  along  with  the  underlying  slates,  as  the 

1  Geology  of  Canada,  1863,  p.  70.  =  Qp.  cit.,  p.  319. 

^Macfarlane  finds  the  red  sandstone  to  contain  12^  per  cent,  of  carbonate  of  lime  and  11  per  cent, 
of  carbonate  of  magnesia. 

■•Macfarlane  found  them  to  contain  13  per  cent,  of  carbonate  of  lime  and  12  per  cent,  of  carbonate 
of  magnesia. 

f^The  amount  varying,  in  the  specimens  analyzed  by  Macfarlane,  from  21  to  34^  per  cent,  of  the 
carbonate  of  lime,  and  from  7|  to  13J  of  the  carbonate  of  magnesia. 

"^Canadian  Naturalist;  New  Series,  III,  p.  252;  IV,  p.  38. 

'Second  Geological  Survey  of  Pennsylvania;  Azoic  Kocks;  Part  I,  p.  241. 


SANDSTONES  OF  THUNDER  BAY.  333 

•equivalents  of  the  horizontal  sandstones  of  the  South  Shore  and  therefore  as 
newer  than  the  Keweenawan,  while  the  latter  separates  them  from  the  under- 
lying slates,  but  considers  both  groups  as  newer  than  the  Keweenawan. 

The  underlying  slates,  however,  I  refer,  as  indicated  in  the  next 
•chapter,  to  the  Huronian,  while  I  can  have  little  hesitation  in  following 
Bell  and  Logan  as  to  the  inferior  position  of  these  sandstones  to  the  great 
thickness  of  unmistakably  Keweenawan  rocks  which  constitute  Isle  Royale, 
the  peninsula  between  Black  and  Nipigon  bays,  and  the  line  of  islands  in 
front  of  Nipigon  Bay.  This  relation  is  indicated  by  the  existence  of  a 
southeasterly  dip  of  from  3°  to  10°  throughout  the  peninsula  between 
Thunder  and  Black  bays — the  higher  angle  being  reached  on  the  Black 
Bay  shore — and  of  the  same  southeasterly  dip  along  the  southeast  shore  of 
Black  Bay,  where  reddish  sandstone  and  conglomerate  may  be  seen  pass- 
ing under  typically  Keweenawan  diabases  and  amygdaloids.  This  sand- 
stone and  conglomerate  seem  to  be  the  upward  continuation  of  those  on 
the  west  side  of  Black  Bay.  Further  evidence  is  found  in  the  occurrence 
of  heavy  calcite  seams  and  veins  in  the  sandstone,  and  of  dikes  intersecting 
it  towards  the  southeast  end  of  the  peninsula  west  of  Black  Bay,  both 
things  unknown  in  the  horizontal  sandstone  of  the  South  Shore.  Yet  more 
-conclusive  than  any  of  these  points  is  the  fact,  that  along  the  Black  Stur- 
geon River  and  thence  westward  to  the  northeast  corner  of  Nipigon  Bay, 
red  sandstones  and  marls,  which  are  beyond  question  the  continuation  of 
those  of  the  west  side  of  Black  Bay,  are  found  to  be  overlain  by  heavy 
"beds  of  olivine-gabbro. 

According  to  Bell,  the  belt  of  level  sandy  country  which  runs  from 
the  northwest  corner  of  Nipigon  Bay  westward  to  the  Black  Sturgeon  River 
IS  bounded  both  north  and  south  by  "hills  of  columnar  trap"  resting  upon 
the  "indurated  red  marls  and  associated  rocks." ^  This  superposition  of 
■coarse  gabbro  to  red  marl  may  be  beautifully  seen  on  the  northwest  shore 
■of  Nipigon  Bay,  and  on  both  sides  of  Nipigon  Harbor  near  the  Red  Rock 
Post  of  the  Hudson's  Bay  Company.  The  overlying  rock  is  medium- 
grained  to  coarse-grained,  white-  and  black-mottled  olivine-gabbro.  A  sec- 
tion of  a  specimen  from  the  cliff  on  the  northwest  shore  of  Nipigon  Bay, 
just  outside  of  the  mouth  of  Nipigon  Harbor,  shows  under  the  microscope 

^Op.  cit,p.  338. 


334       COPPER-BEABING  EOCKS  OF  LAKE  SUPERIOE. 

very  abundant  and  extraordinarily  fresh  olivine,  anorthite,  predominant 
diallagic  augite,  and  a  little  titaniferous  magnetite ;  the  whole  rock  being  in 
an  unusually  fresh  condition.  Similar  occurrences  obtain  in  several  of  the 
islands  which  lie  within  Nipigon  Bay. 

On  the  south  side  of  the  sandy  level  belt  above  mentioned  the  sand- 
stones and  red  marls  pass  beneath  the  great  series  of  diabases  and  amyg- 
daloids  which  form  the  southern  half  of  the  peninsula  lying  to  the  southeast 
of  Black  Bay.  The  latter  rocks  make  up  a  great  belt  which,  beginning  in 
the  islands  about  Point  Magnet,  takes  at  first  a  course  north  of  northeast ; 
but  at  Nipigon  Straits  this  has  veered  around  more  to  the  northeast.  Beyond 
the  straits  the  same  belt  is  continued  in  the  line  of  islands  which  lie  to  the 
south  of  Nipigon  Bay,  changing  its  course  to  an  easterly  direction  in  the 
Saint  Ignace  Island,  and  to  south  of  east  toward  the  eastern  end  of  the 
Battle  Island  Group. 

Throughout  this  belt,  which  is  bold  in  character  and  often  makes  ele- 
vations of  a  thousand  feet  or  more  above  the  lake,  there  is  presented  a  con- 
stant lake  ward  dip  of  some  8°,  the  direction  at  first  being  southeast  and 
then  due  south,  as  the  middle  of  Saint  Ignace  Island  is  reached.  According 
to  Logan  the  rocks  of  which  this  belt  is  made  up  reach  a  total  thickness  of 
some  6,000  to  ]  0,000  feet,  consisting  of  amygdaloidal  and  non-amygdaloidal 
beds 

with  intrusive  masses  of  a  more  solid  and  a  more  highly  crystalline  character.  These 
appear  in  general  to  consist  of  greenstone,  sometimes  passing  into  well-marked  colum- 
nar basalt,  and  they  are  associated  with  other  masses  of  a  vitreous  aspect,  exhibiting 
the  forms  of  pitchstone  and  pitchstone  porphyry.' 

The  amygdaloidal  layers  are  described  by  Logan  as  plainly  stratified, 
as  thinner  than  the  associated  crystalline  beds,  and  as  having  the  general 
characters  of  the  amygdaloids  of  the  South  Shore,  the  usual  vesicular  fillings 
— calcite,  quartz,  agate,  prehnite,  epidote,  copper,  and  various  zeolites — 
occurring  here  also  Wrinkles  indicative  of  a  viscous  flow  are  described  as 
characterizing  some  beds.  Dikes  are  said  to  be  numerous,  for  the  most  paat 
of  some  kind  of  fine-grained,  dark-colored  greenstone,  but  also  in  part  of  a 
porphyry  which  "  contains  large  crystals  of  feldspar  disseminated  through 

1  Geology  of  Canada,  p.  71. 


EOCKS  OF  BLACK  AND  NIPIGON  BATS.  B35 

a  base  of  greenstone."     Still  other  porpliyriti(;  kinds  are  described  as  par- 
taking "  of  the  character  of  a  syenite." 

In  this  a  dark-gray  mixture  of  hornblende  and  feldspar,  with  magnetic  oxide  of  iron 
and  iron  pyrites,  similar  to  the  greenstone  already  mentioned,  incloses  a  multitude  of 
irregular  patches  composed  of  red  feldspar  and  of  quartz,  generally  hyaline,  and  rarely 
of  an  opaque  white  resembling  chalcedony.  The  quartz  is  also  occasionally  dissemi- 
nated throughout  the  matrix  without  the  red  feldspar.  More  rarely  red  feldspar  occurs 
without  the  quartz,  and  still  more  rarely  small  quantities  of  calcareous  spar  are  met 
with.  The  whole  mass  of  the  dyke,  however,  sometimes  passes  into  a  uniform  small- 
grained  mixture  of  red  feldspar  and  green  hornblende  with  very  little  quartz,  and 
ceases  to  have  either  a  porphyritic  or  syenitic  aspect.' 

Still  a  third  kind  of  porphyritic  dike-rock  is  described  as  consisting 
of  a  very  fine-grained  mixture  of  red  feldspar  and  quartz,  holding  distinct  and  not  very 
large  crystals  of  the  same  minerals ;  the  quartz  crystals  being  colorless  transparent 
hexagonal  prisms,  terminated  by  a  pyramid  at  each  extremity,  and  rather  uniformly 
disseminated  through  the  mass.    *    *     * 

The  greenstone  dykes,  whether  porphyritic  or  not,  possess,  without  a  single  ob- 
served exception,  a  well-marked  transverse  columnar  structure,  which  is  in  general  so 
truly  at  right  angles  to  the  plane  of  the  dykes  that  their  underlie  can  be  correctly  deter- 
mined by  it.  This  structure  belongs  equally  to  them,  whether  their  dimensions  are 
great  or  small ;  but  the  size  of  the  columns  increases  with  the  breadth  of  the  dyke, 
which  sometimes  attains  200  feet.  The  number  of  these  dykes  is  very  great:  thirteen 
of  them,  of  good  size,  have  been  counted  in  the  width  of  two  miles,  and  their  parallel- 
ism for  great  distances  is  as  remarkable  as  their  number. 

The  directions  of  the  greenstone  dykes,  as  well  as  those  of  the  other  descriptions 
which  have  been  mentioned,  are  in  general  two,  one  with  the  stratification  and  the 
other  transverse,  changing  with  any  important  change  in  the  general  strike ;  and  they 
appear  to  maintain  what  might  be  considered  a  continuation  of  these  courses  into  the 
older  sedimentary  rocks,  with  a  less  precise  relation  to  their  strike  where  stratified. 
The  point  of  intersection  of  the  two  sets  of  dykes  has  been  seldom  seen.  In  one  in- 
stance, however,  on  the  island  of  Saint  Ignace,  a  dyke  of  eighteen  inches,  coincident 
with  the  stratification,  cuts  another  of  nearly  the  same  breadth  running  transversely. 
Both  of  these  possess  a  columnar  structure,  which  has  not  been  observed  in  the  dykes 

of  syenitic  trap.^ 

The  dykes  in  general  appear  to  be  more  durable  than  the  rocks  cut  by  them,  from 
which  results  a  peculiarity  in  the  geographical  features  of  the  country.  The  destruc- 
tive action  of  the  water  upon  the  coast  is  partially  arrested  In  its  progress  upon 
meeting  with  the  dykes,  and  those  which  run  with  the  strike  are  in  consequence  often 
found  to  shield  the  shore  for  considerable  distances.  They  sometimes  run  out  into 
long  prongs  or  promontories,  with  deep  recesses  behind  them,  or  present  a  succession 
of  long  narrow  islands,  which  act  as  breakwaters  in  defending  the  neighboring  main- 
and ;  and  it  frequently  happens  that  a  narrow  breach  having  been  effected  in  a  dike, 
it  will  be  found  to  be  the  entrance  to  a  spacious  cove  worn  out  on  each  side  in  the 

softer  rock  behind  it. 

'  Geology  of  Canada,  p.  72.  ^  Geology  of  Canada,  p.  73. 


336       COPPER  BE AEING  EOCKS  OF  LAKE  SUPEEIOE. 

According  to  tlie  same  author  this  series  includes  also  detrital  beds : — 
On  Edward  Island  and  other  islands  northward,  grits  and  conglomerates  are 
found  interstratifled  with  trap  layers.  The  same  interstratification  is  met  with  in  the 
rocks  bordering  the  southeast  side  of  Black  bay,  while  those  fronting  the  lake  on  the 
southeast  side  of  the  peninsula  are  composed  almost  entirely  of  various  descriptions  of 
conformably  overlying  trap.  This  arrangement  of  the  stratification,  occupying  a  belt 
of  from  seven  to  ten  miles  in  breadth  (which  on  the  lake  front  is  carved  out  into  a  multi- 
tudeof  deep  coves,  and  includes  a  great  collection  of  small  rocky  islands,)  runs  in  a  north- 
easterly direction  across  ISTeepigon  Strait,  from  the  mainland  to  Saint  Ignace  Island. 
Gradually  changing  its  direction  about  the  middle  of  this  island  to  due  east,  it  con- 
tinues on  through  Simpson's  island,  and  farther  to  the  eastern  extremity  of  the  Battle 
Islands. 

A  high  precipitous  escarpment  of  red  sandstone,  with  white  bands  and  conglom- 
erate layers  all  interstratifled  with  occasional  beds  of  variegated  red  shales,  and  having 
a  pretty  constant  dip  of  8°  or  9°  to  the  southward,  keeps  its  place  on  the  north  side 
of  each  succeeding  island  standing  in  the  line,  which  curves  a  little  to  the  south 
of  eastward  towards  the  eastern  extremity.  A  section  from  the  gneiss  through  the 
large  center  island  of  the  Battle  group  would  shew  in  place  both  the  blue  shales  and 
the  succeeding  sandstones,  apparently  diminished  in -their  proportions.  In  the  cliffs  on 
the  north  side  of  the  last  island  of  the  group  the  limestones  are  displayed,  associated 
with  white  sandstones  and  a  conglomerate  layer  beneath,  resting  on  a  trap  of  a 
porphyritic  character,  and  overlaid  by  more  porous  volcanic  products.'    *    *    * 

The  last-named  sandstones  w^ould  appear  to  be  in  all  probability  the 
uppermost  layers  of  the  thick  stratum  vrhich  underlies  most  of  the  peninsula 
west  of  Black  Bay,  the  sandy  isthmus  between  Black  and  Nipigon  bays, 
and  much  of  Nipigon  Bay  itself. 

These  quotations  from  Logan  show  conclusively  enough  that  he  was 
correct  in  placing  these  rocks  with  those  of  Isle  Royale  and  Keweenaw 
Point.  My  own  examination  of  this  region  has  served  chiefly  to  convince 
me  of  the  accuracy  of  Logan's  general  statements,  but  also  enables  me  to 
add  that  the  kinds  of  rocks  here  developed  are  precisely  those  which  char- 
acterize the  Keweenaw  Series  elsewhere,  and  no  others;  that  Logan  was  prob- 
ably incorrect  in  supposing  any  of  them  to  be  hornblendic,  and  that  while 
his  statements  as  to  dikes  are  generally  correct,  one  might  draw  from  his 
descriptions  a  mistaken  inference  in  supposing  that  all  of  the  prominent 
points  and  fringing  islands  of  this  part  of  the  Lake  Superior  coast  are  due 
to  the  resistant  power  of  dikes.  Many  of  these  points  and  islands  are  plainly 
fragments  of  hard  and  resistant  layers,  and  often  have  the  usual  lakeward 

'  Geology  of  Canada,  p.  78. 


EOOKS  OF  NIPIGON  AND  BLACK  BAYS.  337 

dip  very  plainly  brought  out  in  a  long  front  slope  and  precipitous  back 
slope,  and  at  the  same  time  a  most  beautifully  developed  columnar  structure. 
Some  of  these  islands  must  be  remnants  of  very  heavy  layers,  the  columnar 
back  cliffs  sometimes  considerably  exceeding  a  hundred  feet  in  height. 

I  may  add  the  results  of  a  microscopic  study  of  a  few  of  the  specimens 
collected  by  me  from  this  region. 

A  rock  from  the  cliff  on  the  southeast  shore  of  Black  Bay  is  very  fine- 
grained and  black,  with  sparsely  scattered  and  ijpinute  true  vesicle-fillings 
of  calcite,  quartz  and  chlorite.  The  thin  section  shows  a  groundmass  con- 
sisting of  a  dirty  brownish- white,  impellucid  isotrope  material  (altered  glass), 
with  tabular  feldspar  microliths.  In  this  are  included  as  porphyritic  ingre- 
dients abundant  areas  of  augite,  each  made  up  of  a  number  of  detached 
grains.  Small  plagioclases  (oligoclase)  also  occur  porphyritically.  The 
vesicles  filled  with  the  minerals  named  above  are  seen  to  have  sharply 
defined  outlines,  and  to  have  the  material  immediately  about  them  more 
dense  than  the  rest  of  the  rock.  Chlorite  also  occurs  in  pseud-amygdaloidal 
areas,  when  it  is  an  alteration-product  of  the  augite.  The  rock  is  somewhat 
peculiar  from  its  great  abundance  of  augite. 

Another  rock  from  the  same  cliff  farther  to  the  southwest  is  finely 
crystalline,  and  black,  with  a  very  rough,  lumpy  fracture.  Under  the 
microscope  it  proves  to  be  a  very  highly  augitic  diabase  of  the  ashbed  type, 
and  is  plainly  a  non-amygdaloidal  phase  of  the  last  described  rock. 

About  a  mile  still  farther  southwest  the  east  shore  of  Black  Bay  shows 
fifteen  feet  of  red-  and  white-mottled  and  striped,  cross-laminated  sandstone, 
underlying  black  diabase  The  thin  section  of  this  sandstone  shows  that 
it  is  chiefly  made  up  of  angular  and  sub-angular  quartz  grains,  which  occur 
of  two  sizes,  the  prevailing  small  ones  constituting  a  sort  of  matrix  in  which 
the  large  ones  float.  Large-sized  particles  of  orthoclase,  microcline  and 
oligoclase  are  also  noticeable  in  a  tolerably  fresh  condition ;  and  a  fine 
cloudy  material  in  the  base  is  probably  comminuted  and  decomposed  feldspar. 
The  red  blotching  is  due  to  the  iron  staining.  The  sandstone  is  plainly 
made  up  of  granitic  debris  and  not  of  the  usual  porphyritic  detritus,  a  fact 
which  is  easily  explained  by  the  proximity  of  this  sandstone  to  the  granites 
of  the  west  and  northwest  sides  of  Black  Bay. 
22  L  s 


338       COPPEE-BBAEINa  EOCKS  OF  LAKE  SUPEEIOE. 

A  fine-grained,  close-textured,  brownisli-black  rock,  with  semi-con- 
choidal  fracture,  from  a  large  island  near  Cabmous-Neiding  Point,  thirteen 
miles  southwest  of  Lamb  Island  Light,  at  the  mouth  of  Nipigon  Straits, 
proved  to  be  a  typical  diabase  or  diabase-porphyrite,  having  as  its  con- 
stituents tabular  plagioclase  (anorthite),  abundant  augite  in  irregularly 
rounded  granules,  titaniferous  magnetite,  and  much  of  an  interstitial  sub- 
stance now  largely  stained  by  red  and  brown  iron  oxide. 

A  similar  but  more  dense  and  less  highly  augitic  rock  forms  the  small 
islets  at  the  west  side  of  the  mouth  of  the  large  bay  of  which  the  Eoche  de 
Bout  is  the  eastern  cape. 

A  coarse-grained  black-  and-white-mottled  rock,  which  forms  the  north 
point  of  the  large  island  lying  directly  in  the  mouth  of  Nipigon  Straits, 
turned  out  to  be  a  typical  orthoclase-gabbro,  with  plagioclase,  orthoclase, 
twinned  diallage,  large  augite  granules,  titaniferous  magnetite,  and  second- 
ary quartz  as  the  constituents.  A  similar  rock  from  the  mainland  immedi- 
ately to  the  northwest  of  the  last  rock  yielded  the  same  results  in  the  thin 
section. 

Farther  northward  along  the  west  side  of  Nipigon  Straits  the  following 
rocks  were  encountered,  among  others:  (1)  A  medium-grained  to  fine- 
grained, black  rock,  carrying  numerous  large  brownish  translucent  por- 
phyritic  plagioclases,  which,  in  the  thin  section,  shows  a  wholly  crystalline 
groundmass,  consisting  of  plagioclase,  orthoclase,  augite  partly  in  grains, 
and  partly  filling  the  interstices  between  the  feldspars,  titaniferous  magnetite, 
secondary  quartz,  and  epidote.  The  porphyritic  feldspars  yield  the  polari- 
zation angles  of  labradorite.  The  rock  lies  between  the  ashbed-diabases 
and  the  orthoclase-gabbros.  (2)  A  fine-grained,  brownish-black  rock  con- 
sisting of  fresh  tabular  labradorites,  arranged  so  as  to  indicate  flowage, 
abundant  and. partly  fresh  augites  and  magnetite.  The  rock  is  a  diabase  of 
the  ordinary  type.  (3)  A  black,  conchoidally  fracturing,  aphanitic  ashbed- 
diabase  consisting  of  predominating  tabular  labradorites,  augite  in  grains, 
and  a  little  magnetite. 

NIPIGON  LAKE  BASIN. 

The  red  sandstones,  shales,  and  marls  which  in  the  region  of  Thunder, 
Black,  and  Nipigon  bays  He  at  the  base  of  the  Keweenaw  Series  extend,  as 


NIPIGON  LAKE  EOCKS.  339 

Bell  has  shown,  northward  from  the  last  two  of  these  bays,  in  a  broad  belt, 
spreading  from  the  Nipigon  River  to  some  twenty  miles  west  of  the  Black 
Sturgeon  River.  According  to  BelP  the  rocks  of  this  belt  are  chiefly  the 
red  sandstones  and  marls  above  referred  to,  with  an  overlying  mass  of  "black 
trap,"  similar  f  o  the  olivine-gabbro  already  described  as  overlying  red  marl 
at  the  mouth  of  Nipigon  River.  Still  farther  north  the  western  half  of  the 
basin  of  Lake  Nipigon  is  described  by  the  same  geologist  as  occupied  by 
rocks  which  he  assigns  to  the  Keweenaw  Series,  and  which  he  regards  as  a 
direct  continuation  northward  of  those  of  the  valley  of  the  Black  Sturgeon 
River.  The  rocks  thus  assigned  by  Bell  are  stated  to  be  chiefly  "black  trap,"^ 
at  times  coarse,  and  again  fine ;  but  also  to  include  "  brick-red  porphyry," 
sandstone,  "  argillites,"  "felsite,"  and  green  and  gray  limestones. 

The  "  black  trap  "  is  nearly  everywhere  the  only  rock  seen.  Much  of 
it  is  evidently  coarse  olivine-gabbro.  It  is  described  as  only  occasionally 
showing  distinct  bedding,  but  when  it  does  so  as  lying  sometimes  at  high 
angles  to  the  east  or  west,  though  often  as  more  nearly  horizontal.  It  was 
seen  both  plainly  interbedded  with  sandstone,  and  then  generally  standing 
at  a  high  angle,  and  also  apparently  overlying  all  the  rocks  of  the  region. 
The  red  porphyry  is  desci'ibed  as  presenting  itself  in  one  principal  exposure 
on  the  east  side  of  Lake  Nipigon,  where 

the  lake  shore  and  the  islands  from  the  Hudson  Bay  Company's  farm  at  Nipigon  House 
to  English  Bay,  a  distance  of  three  miles,  are  occupied  by  a  brick-red  porphyry,  com- 
posed of  crystalline  red  orthoclase  feldspar,  with  grains  of  translucent  quartz,  inclos- 
ing finer  stratified  patches  of  the  same  color,  and  others  of  white  quartz.  It  also  holds 
spots  of  a  soft  green  earthy  mineral,  and  small  cavities  lined  with  crystals  of  feldspar.* 

This  porphyry  evidently  belongs  to  my  group  of  augite-syenites  and 
granitic  porphyries. 

The  sandstone  is  met  with  at  several  points  on  the  east  side  of  Lake 
Nipigon,  where  it  is  "  rather  fine-grained,  hard,  and  quartzose,"  and  flanks 
the  granite,  and  strikes  northward  with  the  shore,  dipping  eastward  at  an 
angle  of  15°  at  one  place,  and  at  another  westward  into  the  lake  at  an 
angle  of  80°.  Trap,  either  in  the  form  of  beds  or  great  dykes,  is  asso- 
ciated with  it. 

>  Report  of  the  Geological  Survey  of  Canada,  for  1867-'69,  p.  338. 
»0p.  cit.,  p.  348. 


340       COPPEE-BEAEING  EOCKS  OF  LAKE  SUPEETOB. 

On  the  west  shore  of  the  lake  sandstone  is  said  to  occur  interstratified 

with  beds  of  "  argillite,"  felsite,"  and  "  trap," 

all  ou  edge,  and  runuing  in  a  northerly  direction.  *  *  *  Some  of  the  felsite  beds  are 
soft  greenish  and  earthy;  others  harder  and  schistose.  The  argillite  is  hard  dark  col- 
ored and  compact,  with  a  conchoidal  fracture ;  while  the  sandstones  are  light-colored 
and  soft.  One  bed  of  the  latter,  of  a  very  light  greenish-gray  color,  is  composed  of  fine 
silicions  and  argillaceous  particles,  with  scattered  grains  of  translucent  quartz.  [At 
another  point  farther  up  the  west  shore  there  shows  under  high  cliffs  of  trap]  a  band 
of  light-gray  tender  harsh-grained  sandstones,  about  100  feet  thick,  dipping  S.  80°  W. 
(mag.)  <  50°,  which  appears  to  come  between  great  masses  of  coarse  crystalline  trap. 
Two  miles  farther  south,  or  about  a  quarter  of  a  mile  north  of  the  extremity  of  Black 
Sturgeon  Lake,  beds  of  a  coarse  light-gray  sandstone,  holding  occasional  pebbles,  mostly 
of  white  quartz,  are  found  lying  against  the  side  of  a  hill  of  gray  splintery  schistose 
felsite.  The  sandstone  dips  southwestward  at  an  angle  of  about  40°,  while  the  felsite 
■dips  in  the  opposite  direction,  with  an  inclination  of  about  60°.' 

The  limestones  were  seen  in  two  places,  one  on  the  south  side  of  Lake 
Nipigon  and  one  on  the  west.     In  the  former  case  the  limestone  is — 

thinly  bedded,  and  consists  of  alternating  whitish  and  olive-green  layers.  The  rock, 
which  has  a  fine  homogeneous  texture  and  conchoidal  fracture,  is  magnesian  and  ar- 
gillaceous, and  when  burnt  would  i^robably  form  a  good  cement.  Some  indistinct 
forms,  resembling  fossils,  occur  in  it,  but  nothing  definitely  organic  was  observed.  The 
limestone  band  is  generally  horizontal,  but  in  some  places  it  is  thrown  into  a  series  of 
small  anticliuals,  having  their  axes  north  and  south.  It  is  overlaid  by  the  trap,  which 
rises  to  a  height  of  about  100  feet  immediately  above  it.^ 

The  limestone  of  the  west  shore   shows  on  the  north  side  of  the 

Narrows  at  the  mouth  of  Chief's  Bay.^     Here — 

trap  is  overlaid  by  compact  argillaceous  magnesian  limestone,  with  a  conchoidal  frac- 
ture, dipping  S.  25°  W.  (mag.)  <5o.  The  beds  are  from  three  inches  to  two  feet  and  a 
half  in  thickness,  and  present  different  shades  of  a  grayish  and  olive-green  color. 
Although  the  section  exposed  does  not  appear  to  exceed  ten  feet  in  thickness,  so  regu- 
lar and  slight  is  the  dij)  that  these  rocks  extend  for  a  quarter  of  a  mile  along  the  shore, 
and  are  seen  along  a  brook  to  the  northwestward  and  in  the  bottom  of  the  lake  in  front. 
Small  pear-shaped  bodies,  about  the  size  of  peas,  weather  out  on  the  surfaces  of  some 
of  the  beds,  but  they  show  no  organic  structure,  either  outwardly  or  in  sections  exam- 
ined under  the  microscoi)e.  The  same  olive-green  limestone  occurs  again  on  the  north- 
east shore  of  Chief's  Bay,  about  two  miles  from  the  Narrows.  The  beds  are  from  six 
inches  to  two  feet  thick,  and  dip  S.  40°  W.  (mag.)  <8o.  A  section  of  six  or  eight  feet 
as  exposed,  and  the  strata  are  underlaid  conformably  by  beds  of  fine-grained  compact 
black  trap,  showing  crack-marks  on  the  surface.^ 

1  Op.  cit,  p.  345.  2  Op.  cit.,  p.  342. 

=  See  map  accompanying  Bell's  report  in  Report  of  Progress  of  the  Geological  Survey  of  Canada 

for  1867-'69. 

< Op.  cit.,  p.346. 


NIPIGON  LAKE  ROCKS— MICHIPICOTEN.  341 

These  limestones  are  of  interest,  since  no  such  rocks  are  anywhere  else 
known  throughout  the  entire  extent  of  the  Keweenaw  Series. 

Bell's  statements,  thus  quoted,  certainly  seem  to  show  that  the  Nipigon 
Lake  rocks  are  Keweenawan,  and  that,  to  judge  from  lithological  char- 
acters, they  belong  always  low  down  in  the  series.  I  have  already  stated 
my  disbelief  in  the  existence  of  any  one  "  crowning  overflow"  closing  the 
entire  series  of  Keweenawan  eruptions  in  the  Thunder  Bay  region.  Still 
more  doubtful  to  me  seems  the  reference  to  this  "crowning  overflow"  of 
much  of  the  trap  of  the  Nipigon  region.  The  structural  relations  of  the 
Nipigon  Basin  to  that  of  Lake  Superior  would  be  an  interesting  subject 
for  disciission,  but  while  the  structure  in  the  Nipigon  Basin  itself  is  so  little 
known,  speculation  on  the  relations  would  hardly  be  profitable. 


Section  III.— MICHIPICOTEIsr  ISLAND  AND  THE  EAST  COAST  OF  LAKE 

SUPERIOR. 

Beyond  the  easternmost  of  the  Battle  Islands  the  north  and  east  coasts 
of  Lake  Superior,  for  nearly  200  miles,  are  composed  wholly  of  rocks  more 
ancient  than  the  KeVeenaw  Series.  As  shown  subsequently,  there  are 
reasons  for  believing  the  Keweenawan  rocks  continuous  in  this  distance 
underneath  the  waters  of  Lake  Superior.  They  first  reappear  to  view, 
however,  in  Michipicoten  Island, -which  lies  about  100  miles  southeast  of 
the  last  of  the  Battle  Islands.  The  following  is  Logan's  account  of  Michipi- 
coten Island: 

The  strata  of  which  it  is  composed  have  a  general  dip  to  the  east  of  south, 
and  the  inclination  appears  seldom  to  fall  short  of  thirty  degrees.  The  lower  strata, 
towards  the  north  side  of  the  island,  particularly  as  indicated  at  the  upper  end,  appear 
to  be  composed  chiefly  of  amygdaloidal  trap,  with  occasional  beds  of  trap  conglomerates, 
red  sandstones,  and  shales  j  while  towards  the  south  these  are  overlaid  by  a  consider- 
able amount  of  compact  earthy  or  sub-resinous  red  trap,  assuming  sometimes  an 
obscure  and  sometimes  a  distinct  porphyritic  character,  by  the  display  of  ill-defined 
crystals  of  red  feldspar  or  well-marked  crystals  of  transparent  colorless  quartz. 

Along  nearly  the  whole  of  the  south  side  of  the  island  the  trap  assumes  a  more 
resinous  aspect,  and,  its  color  becoming  black,  it  presents  the  characters  of  pitchstone 
and  pitchstone  porphyry.  Some  of  the  beds  associated  with  these  are  of  an  amygda- 
loidal character,  and  exhibit  large  agate  veins,  which  run  chiefly  in  the  direction  of  the 
strike,  but  frequently  also  transverse  to  it. 


342       COPPER-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 

About  three-fourths  of  a  mile  out  in  front  of  the  harbor,  which  is  half-way  down 
the  south  side,  a  few  narrow  islands  occur,  presenting  beds  of  peculiar  character, 
amounting  to  between  sixty  and  seventy  feet,  dipping  southward  at  an  angle  of  twenty 
degrees.  They  are  of  a  general  red  color,  spotted  and  patched  with  yellowish-white, 
and  wherever  a  crack  exists  the  rock  is  blanched  to  a  small  distance  on  each  side  of 
it.  The  surfaces  are  uneven,  and  peculiarly  marked  with  festooned  and  finely  wrinkled 
forms,  composed  of  very  thin  close-fitting  laminae,  with  a  ligneous  aspect,  having  a 
thickness  sometimes  exceeding  one  or  two  inches.  The  rock  scarcely  resembles  a  trap, 
nor  does  it  bear  the  character  of  indurated  shale;  but  it  may  perhaps  be  an  indurated 
mixture  of  volcanic  mud  and  ashes,  in  which  the  wrinkles  result  from  a  partial  flow. 
The  total  volume  of  the  formation  developed  in  Michipicoten  Island  at  the  most  mod- 
erate dip  observed  would  not  fall  short  of  12,000  feet. 

Subsequently  to  the  publication  of  this  description  of  the  Michipicoten 
rocks  Macfarlane  made  a  further  study  of  them  for  the  Canada  survey. 
His  more  detailed  examinations  established  a  total  thickness  of  over  18,500 
feet  of  plainly  bedded  eruptive  flows,  with  interstratified  conglomerates 
and  sandstones.  From  the  numerous  dip  and  strike  observations  which  he 
records,  it  is  evident  that  the  beds  throughout  the  island  have,  as  Logan  says, 
a  southerly  dip,  but  also  that  they  take  a  sort  of  curving  course  as  they  are 
followed  on  the  length  of  the  island,  the  strike  directions  becoming  more 
and  more  north  of  east  as  the  eastern  end  of  the  island  is  reached.  The  dip 
also  flattens  to  the  southward,  the  beds  on  the  north  side  of  the  island 
dipping  south  as  much  as  30°  to  36°,  while  on  the  south  shore  the  angle 
appears  to  be  often  less  than  20°.  Macfarlane  also  describes  the  existence 
on  the  east  and  northeast  shores  of  the  island  of  peculiar  masses  of  red 
quartziferous  porphyry,  which  occur  in  confused  relations  to  the  associated 
basic  rocks,  the  very  regular  succession  of  beds  seen  on  the  east  and  south 
sides  of  the  island  failing  to  repeat  itself  here. 

By  the  kindness  of  Mr.  A.  R.  C.  Selwyn,  Director  of  the  Geological 
Survey  of  Canada,  I  am  in  possession  of  a  suite  of  nineteen  type  specimens 
of  the  Michipicoten  rocks,  collected  and  determined  according  to  the  older 
lithological  methods  by  Mr.  Macfarlane.  These  I  have  carefully  studied 
under  the  microscope,  and  am  thus  the  better  able  to  institute  a  satisfactory 
comparison  between  the  Michipicoten  rocks  and  those  of  the  typical  Ke- 
weenawan  localities  of  the  South  Shore. 

The  peculiar  red-  and  white-blotched,  wrinkled  rock  described  by 
Logan  as  forming  the  small  islands  which  lie  about  three-fourths  of  a  mile 


EOCKS  OF  MICHIPICOTEN  ISLAND.  343 

south  of  the  harbor  on  the  south  side  of  the  island,  I  find  to  be  a  highly 
siliceous  felsite,  closely  resembling  and  plainly  belonging  with  the  red  rock 
of  which  Mount  Houghton,  on  Keweenaw  Point,  is  formed,  which  makes  up 
much  of  the  central  mass  of  the  Porcupine  Mountains,  and  which  forms  so 
many  of  the  red  chffs  of  the  Minnesota  coast  of  Lake  Superior.  The  resem- 
blance is  both  macroscopic  and  microscopic;  while  the  pecuhar  "festooned 
and  wrinkled"  markings,  "composed  of  very  thin  close-fitting  laminse,  with 
a  ligneous  aspect,"  noticed  by  Logan,  are  precisely  what  I  have  repeatedly 
described  in  the  foregoing  pages  as  characterizing  similar  rocks  in  so  many 
places  in  the  western  half  of  the  Lake  Superior  Basin.  These  markings  are 
due  doubtless  to  a  viscous  flow,  and  are  much  the  same  as  are  found  to 
characterize  the  modern  rhyolites.  The  high  stratigraphical  position  of  this 
felsite,  which  is  described  by  Logan  as  plainly  dipping  with  the  rest  of  the 
Michipicoten  series,  is  particularly  worthy  of  notice,  since  it  places  these 
acid  rocks  at  a  higher  horizon  than  elsewhere  in  the  Lake  Superior  Basin. 

Of  the  pecuhar  resinous-looking  rocks,  which,  under  the  name  of 
pitchstone  and  pitchstone-porphyry,  Logan  describes  as  showing  all  along 
the  south  shore  of  the  island,  I  find  several  specimens  in  Macfarlane's  col- 
lection. One  of  these  specimens,^  labeled  by  him  "  compact  melaphyr," 
presents  a  nearly  aphanitic,  dark-gray  rock,  with  a  conchoidal  fracture,  and 
without  porphyritic  ingredients.  It  bears  a  strong  resemblance  to  the  ash- 
bed  traps  of  Keweenaw  Point.  In  the  thin  section  this  resemblance  is  borne 
out  completely,  the  rock  proving  to  consist  of  predominant  tabular  oligo- 
clases,  with  augite  in  the  characteristic  irregular  grains  whose  contours  are 
not  determined  by  the  feldspars.  Magnetite  and  some  non-polarizing  ma- 
terial, which  is  taken  to  represent  residuary  magma,  are  also  present.  The 
rock  is  thus,  according  to  the  Rosenbusch  nomenclature,  a  diabase-porphy- 

rite. 

Another  specimen,^  also  called  melaphyr  by  Macfarlane,  is  aphanitic, 
of  a  dark  chocolate-brown  color,  has  a  conchoidal  fracture,  and  shows  no 
porphyritic  ingredients.  The  thin  section  of  this  rock  proves  it  also  to  be 
a  diabase-porphyrite,  the  ingredients  being  the  same  as  in  the  last,  the  only 
perceptible  differences  being  that  in  this  rock  the  tabular  feldspars  are  two 

» Macfarlane's  No.  2.  *  Macfarlane's  No.  5. 


344       COPPEE-BEARING  ROCKS  OF  LAKE  SUPERIOR. 

or  three  times  as  large  and  have  their  longer  axes  arranged  with  a  tendency 
to  a  common  direction,  while  the  augite  is  sparser  and  in  more  minute  par- 
ticles. The  brown  color  appears  to  be  connected  with  a  ferritic  alteration 
of  the  residuary  magma. 

Still  another  specimen/  called  porphyrite  by  Macfarlane,  and  coming 
from  the  southeast  corner  of  the  island,  presents  an  aphanitic,  very  com- 
pact matrix,  of  a  greenish-gray  color,  in  which  are  included  very  abundant 
porphyritic  white  feldspars,  one- sixteenth  to  one-eighth  inch  in  length, 
and  also  much  rarer  and  more  minute  porphyritic  black  particles.  In  the 
thin  section  the  base  is  seen  to  contain  much  of  a  light-brown  and  dark- 
brown  stained  non-polarizing  material,  which  is  thickl}'-  strewn  with  minute 
tabular  plagioclases,  and  contains  also  rarer  and  more  minute  particles  of 
black  magnetite.  The  porphyritic  feldspars  turn  out  to  be  labradorite,  the 
crystals  of  which  mineral  are  often  grouped  in  peculiar  clusters,  and  are 
always  very  fresh.  The  porphyritic  black  particles,  seen  macroscopically, 
turn  out  to  be  augite  largely  altered  to  a  greenish  material,  with  which  there 
is  associated  much  black  magnetite,  also  as  an  alteration-product.  This 
rock,  then,  is  another  phase  of  diabase-porphyrite,  with  a  larger  proportion 
of  uncrystalline  matter.  The  thin  section  of  this  rock  is  figured  on  Plate 
IX  at  Figs.  1  and  2. 

A  fourth  specimen,^  called  by  Macfarlane  basaltic  melaphyr,  is  aphan- 
itic, nearly  black,  has  a  highly  conchoidal  fracture,  with  an  almost  vitre- 
ous aspect,  and  shows  no  porphyritic  ingredients.  It  is  evidently  one  of 
the  rocks  especially  referred  to  by  Logan  under  the  name  of  pitchstone. 
The  thin  section  shows  an  excessively  dense  rock,  in  which,  with  a  high 
power,  and  with  the  polarized  light,  are  recognizable  very  numerous  minute 
augite  particles,  embedded  in  a  non-polarizing  matrix,  with  very  much  rarer 
minute  plagioclases  and  magnetite  particles.  This  rock  is  again  a  diabase- 
porphyrite,  but  is  nearer  to  the  glassy  condition  than  any  of  those  previ- 
ously described.  It  is  also  peculiar  for  its  large  content  of  augite.  Still 
nearer  to  the  glassy  state  is  a  specimen  from  Sir  William  Logan's  collection, 
labeled  "  pitchstone."  It  is  completely  aphanitic,  of  a  jet-black  color,  and 
greasy  semi- vitreous  luster,  and  has  a  glass-like  fracture.     In  the  thin  sec- 

'  Macfarlane's  No.  16.  ^  Macfarlane's  No.  18. 


EOCKS  OF  MICHIPICOTEN  ISLAKD.  345 

tion  the  brownish-stained  matrix  is  seen  to  be  composed  in  a  considerable 
measure  of  nnihdividualized  material  (large  areas  remaining  dark  between 
the  crossed  nicols)  exceedingly  minute  tabular  plagioclases,  occasional  minute 
brightly  polarizing  augite  points,  and  magnetite  particles  in  the  groundmass. 

The  still  lower  strata  which  form  the  bulk  of  the  thickness  present  in 
the  island  are  not  so  well  represented  in  the  collection.  One  specimen^  from 
the  copper  mines  on  the  north  side  of  the  island,  called  by  Macfarlane  mela- 
phyr,  is  fine-grained,  plainly  crystalline,  and  has  a  rough  fracture,  and  a  dark 
purpHsh-gray  color,  mottled  with  still  darker  shades.  Its  aspect  is  identical 
with  that  of  many  of  the  finer-grained,  luster-mottled,  olivinitic  rocks  to  which 
Pumpelly  has  given  the  name  of  melaphyr.  This  resemblance  is  fully  borne 
out  by  the  appearance  of  the  thin  section,  in  which  the  characteristic  rela- 
tively large  augites  include  numbers  of  tabular  plagioclases,  while  the  abun- 
dant olivines,  wholly  altered  to  green  and  brown  substances,  are  crowded 
with  the  magnetite  into  the  interspaces  of  the  augites.  A  number  of  large 
porphyritic  plagioclases  occur  in  the  section,  an  unusual  thing  for  this  class 
of  rocks  elsewhere  in  the  Keweenaw  Series. 

The  copper-bearing  "vein"  at  this  place  is  evidently  merely  one  of  the 
usual  altered  cupriferous  amygdaloids,  and  is,  according  to  Macfarlane, 
almost  identical  with  the  cupriferous  amygdaloid  of  the  Pewabic  and  Quincy 
mines.  Portage  Lake.  It  has  been  traced  for  a  considerable  distance. 
Overlying  this  cupriferous  bed  is  a  rock  which  is  represented  in  the 
Macfarlane  collection.  This  rock^  is  completely  aphanitic,  of  a  dark-gray 
color  and  highly  conchoidal  fracture,  and  shows  as  porphyritic  ingredients 
only  rare  black  crystals  of  augite.  The  thin  section  proves  it  to  be  a  typical 
diabase-porphyrite,  with  a  predominating  isotropic,  pinkish-tinted,  and  cloudy 
base,  in  which  may  be  recognized,  with  a  high  power,  excessively  minute 
tabular  plagioclases,  larger  but  still  minute  magnetite  particles,  and  rare 
and  very  minute  brightly  polarizing  particles,  probably  belonging  to  augite. 
The  porphyritic  augites  are  largely  replaced  by  a  greenish  alteration- 
product.  Aphanitic  black  rocks  of  similar  appearance  to  that  last  described 
form,  according  to  Macfarlane,  a  large  proportion  of  the  lower  half  of  the 
Michipicoten  section.  Rocks  of  this  character  show  all  along  the  west  shore 
of  the  island  to  its  western  point. 

iMaofarlane'sNo.  1.  'Macfarlane's  No.  9. 


346  COPPEE-BBAEING  EOCKS  OF  LAKE  SUPEEIOE. 

One  specimen  ^  in  the  collection,  from  a  bed  near  the  middle  of  the 
series,  is  called  by  Macfarlane  porphyritic  melaphyr.  It  turns  out,  how- 
ever, to  be  a  quartzless  porphyry,  and  is  of  interest  as  presenting  a 
gradation  phase  between  the  wholly  crystalline  augite-syenites  and  the 
typical  quai'tzless  porphyries.  Macroscopically  it  presents  an  aphanitic 
light-brownish  base,  with  very  abundant  minute  pink  porphyritic  feldspars. 
In  the  thin  section  the  base  proves  to  be  chiefly  made  up  of  isotropic  material 
and  small,  but  not  excessively  minute,  feldspars,  which  are  in  large  measure 
crthoclase.  Minute  quartz  particles  and  clusters,  some  plainly  secondary, 
dot  this  background,  which  is  also  affected  by  a  general  red  stain.  The 
quartz  also  occurs  in  excessively  fine  radiating  and  parallel  lines.  This 
radial  arrangement  is  also  brought  out  by  lines  of  brown  ferrite.  Small 
augite  points  are  here  and  there  recognizable.  The  porphyritic  feldspars 
are  orthoclase  and  oligoclase.  A  single  rather  large-sized  apatite-crystal 
is  contained  in  the  section.  There  are  splotches  of  green  chloritic  substance, 
which  shows  also  macroscopically,  but  it  is  not  evident  which  mineral  has, 
by  its  alteration,  given  rise  to  them. 

The  red  porphyry  which  Macfarlane  describes  as  making  so  confused 
an  appearance  on  the  east  shore  of  the  island  is  represented  in  the  col- 
lection. It  is  a  quartziferous  porphyry,^  with  a  dark  purplish-red,  aphanitic 
matrix,  in  which  the  porphyritic  quartzes  and  feldspars  are  extraordinarily 
abundant  and  large.  It  is  near  to  the  rock  seen  on  the  Torch  Lake  Railroad, 
south  of  the  Calumet  mine,  on  Keweenaw  Point,  to  that  of  which  many 
of  the  pebbles  of  the  Keweenaw  Point  conglomerates  are  composed,  and  to 
that  which  makes  large  exposures  on  Bead  Island,  at  the  mouth  of  Nipigon 
Straits.  In  the  thin  section  the  ground-mass  of  this  rock  appears  faintly 
pinkish-tinted  and  cloudy,  and  contains  abundant  and  very  minute  ferrite 
particles,  which  in  the  vicinity  of  the  porphyritic  ingredients  show  crowding 
and  a  tendency  to  linear  directions.  In  the  polarized  light  the  matrix  shows 
a  dark  background,  strewn  with  particles  and  flocks  of  particles,  of  feebly 
double-refracting  substances,  but  only  rarely  showing  any  networked 
secondary  quartz.  The  porphyritic  quartzes  are  in  the  usual  dihexahedral 
forms,  which  are  generally  much  eaten  into  by  the  matrix.  The  feldspars 
are  both  orthoclase  and  oligoclase,  and  are  much  altered. 

iMacfarlane's  No.  15.  'Maofarlane's  No.  11. 


THE  EAST  COAST  OP  LAKE  SUPEEIOE.  347 

The  facts  given  above,  quoted  from  Macfarlane  and  Logan,  together 
with  the  microscopic  observations  that  I  have  added,  establish,  then,  a 
complete  identity  between  the  Michipicoten  Island  rocks  and  those  of  the 
typical  cupriferous  districts  of  the  south  shore  of  Lake  Superior.  To  judge 
from  these  data,  the  only  peculiarity  about  the  Michipicoten  section  appears 
to  be  the  relatively  great  abundance  of  diabase-porphyrites. 

EAST   COAST   OP   LAKE    SUPEEIOR. 

According  to  Logan  and  Macfarlane,  several  of  the  prominent  points 
along  the  east  coast  of  Lake  Superior  are  formed  of  rocks  of  the  copper- 
bearing  series,  the  intervening  shores  being  occupied  by  older  formations. 
The  appearance  is  as  if  the  Michipicoten  rocks,  leaving  that  island  with  a 
north-of-east  trend,  run  up  well  into  the  bight  into  which  the  Michipicoten 
River  empties,  and  then,  turning  abruptly  at  more  than  right  angles,  skirt 
the  east  coast  as  far  as  the  Sault,  being,  however,  in  most  of  this  distance 
concealed  by  the  waters  of  the  lake.  The  northernmost  point  at  which 
Keweenawan  rocks  have  been  recognized  on  the  east  coast  is  at  or  near 
Cape  Choyye,  18 J  miles  south  of  the  mouth  of  Michipicoten  River.  With 
regard  to  this  point  and  others  farther  south  I  quote  from  Logan : 

About  two  miles  north  of  Cape  Choyye  a  coarse-grained  bed,  supporting  some 
thickness  of  sandstone  colored  red,  with  white  bands,  and  dipping  a  little  to  the  south 
of  west  at  an  angle  of  about  ten  degrees,  abuts  against  a  precipitous  cliff  of  the  older 
rocks,  as  if  let  down  by  a  north-east  and  south-west  fault. 

About  nine  miles  to  the  south  of  this,  the  peninsula  of  Cape  Gargantua,  and  some 
of  the  small  islands  immediately  near  display  amygdaloidal  trap  disposed  in  beds  dip- 
ping to  the  south  of  west  at  an  angle  of  about  forty  degrees,  and  resting  unconformably 
on  the  gneiss.    *    *    * 

To  the  south  of  Montreal  Island,  sandstones  and  amygdaloidal  trap  occupy  the 
lower  side  of  the  cove  above  Pointe  aux  Mines.  The  sandstones,  where  first  seen,  are 
nearly  in  contact  with  the  gneiss,  against  which  they  appear  to  abut,  as  if  brought  in 
by  a  dislocation.  Their  dip,  at  an  angle  varying  from  ten  to  twenty  degrees,  gradually 
changes  from  a  direction  F.  45°  W.,  to  N.  15°  W.  The  trap,  coming  apparently  from 
below,  after  an  interval  of  about  one  hundred  yards,  in  which  it  is  difficult  to  ascertain 
its  true  attitude  from  its  being  worn  down  level  with  the  surface  of  the  water,  exhibits 
a  decided  dip  S.  80°  W.  <  30° — 40°,  maintained  for  such  a  distance  across  the  measures 
as  to  yield  a  thickness  of  3,000  feet.  This  trap  is  interrupted  at  Pointe  aux  Mines  by  a 
south-easterly  dislocation,  which  brings  up  the  Laurentian  gneiss,  of  which  the  extremity 
of  the  point  is  composed.  Prom  this  point  the  line  of  demarkation  between  the  gneiss 
and  the  overlying  unconformable  rocks,  as  has  already  been  indicated,  appears  to  run 


348       COPPEE-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 

across  in  a  southeasterly  direction  to  Batchehwahnung  Bay,  leaving  tlie  promontory  of 
Mamainse  between  it  and  the  lake. 

This  promontory  is  composed  of  amy gdaloidal  trap  and  coarse  interstratified 
conglomerates,  whose  pebbles  and  bowlders  consist  chiefly  of  the  ruins  of  the  sub- 
jacent slate,  gneiss,  and  associated  rocks.  The  general  dip  of  the  strata  which  occupy 
this  area  is  maintained  with  considerable  constancy  in  a  direction  rather  south  of 
west,  at  an  angle  of  twenty  or  twenty-five  degrees,  and  the  breadth  across  the  meas- 
ures is  sufficient  to  give  a  thickness  probably  of  not  far  from  10,000  feet,  of  which 
about  1,500  feet  consist  of  conglomerate  layers,  one  of  them  being  400  feet. 

More  recently  Macfarlane  has  published  a  detailed  section  of  the  rocks 
forming  Mamainse  Point,  in  which  he  makes  the  total  thickness  of  the 
Keweenawan  beds  here  in  sight  16,208  feet,  the  interstratified  conglomerates 
aggi'egating  2,138  feet.  The  conglomerates  are  often  bowlder-conglom- 
erates leather  than  pebbly,  and  are  peculiarly  among  Keweenawan  conglom- 
erates for  carrying  bowlders  and  pebbles  of  granitic  and  gneissic  rocks,  a 
fact  evidently  to  be  connected  with  the  near  neighborhood  of  such  rocks  in 
place.  The  rocks  which  are  described  as  melaphyrs  by  Macfarlane,  and 
which  with  their  amygdaloids  make  up  the  greater  part  of  the  Mamainse 
section,  are  precisely  the  same  types  of  diabase,  etc.,  which  characterize  all 
the  other  Keweenawan  regions  about  Lake  Superior.  This  is  in  effect 
stated  by  Macfarlane  and  Logan,  and  their  statements  are  confirmed  by  a 
collection  made  here  for  me  by  Mr.  A.  C.  Campbell.  Diabase-porphyrites 
and  ashbed-diabases  are  very  plenty  among  the  specimens,  but  typical 
luster-mottled  melaphyr  and  felsite  are  also  represented.  The  usual 
amygdaloids  are  developed  here,  and  native  copper  has  been  observed  in 
several  places. 

Macfarlane  describes  the  south  side  of  Mamainse,  on  Batchewanung 
Bay,  as  presenting  a  confused  appearance: 

Although  the  sandstones  occasionally  protrude,  they  become  much  less  frequent, 
while  the  overlying  traps  become  much  more  regular,  and  gradually  assume  the  same 
strike  and  dii>  as  the  strata  on  the  west  coast.  The  hills  to  the  north  of  Anse  aux 
Crepes  consist  of  the  same  beds  of  melaphyr  and  conglomerate  as  were  observed  on 
the  west  coast,  with  similar  strike  and  dip. 

According  to  Logan,  amygdaloid  shows  again  in  the  easternmost  part 
of  Batchewanung  Bay,  "where  it  reposes  on  the  gneiss,  with  a  dip  S.  80° 
W.  •<  42°."  Trap  and  amygdaloid  show  again  on  the  south  side  of  the  bay. 
"The  worn  condition  of  the  rock  renders  the  dip  obscure,  but  it  appears  to 


THE  EAST  COAST  OF  LAKE  SUPERIOE.  349 

be  N.  60°  W.<22°."  Similar  rocks  appear  again  "at  the  extremit)-  of  Gros 
Cap,  where  there  is  but  a  small  quantity  of  the  amygdaloid,  and  where 
trap  of  a  porphyritic  character  appears  to  be  associated  with  it.  The  dip 
is  W.  <  45°." 

To  these  quotations  I  have  only  to  add  that  the  specimens  brought  to 
me  from  the  above-described  places,  south  of  Mamainse  Point,  show  only  the 
typically  Keweenawan  kinds,  including  not  only  the  basic  kinds  but  also 
red  quartziferous  porphyry  and  felsite,  the  latter  from  Grros  Cap.  There  is 
still  much  obscurity  hanging  about  the  structural  relations  of  the  rocks  of  the 
east  shore  from  Pointe  aux  Mines  to  Grros  Cap,  and  especially  is  this  true  of 
the  rocks  in  and  about  Batchewanung  Bay.  To  add  to  the  difficulties,  the 
horizontal  Eastern  Sandstone  seems  to  be  present  here,  and  has  been  con- 
founded with  true  Keweenawan  sandstones.  The  peculiar  way  in  which  the 
traps  and  amygdaloids  of  the  latter  formation  skirt  the  shores  of  Batchewa- 
nung Bay,  appearing  in  a  small  patch  even  at  the  deepest  point  of  the  bay, 
suggests  very  strongly  the  thought  that  the  existence  of  this  bay  is  deter- 
mined by  a  peculiar  loop-like  bend  in  the  general  course  of  the  Keweenawan 
belt  of  the  East  Shore.  Possibly  the  confused  appearances  noted  on  the 
south  side  of  Mamainse  by  Macfarlane  may  have  something  to  do  with  this 
convolution. 


CHAPTER   VIII. 

RELATIONS    OF   THE    KEWEENAW    SERIES    TO   THE 
ASSOCIATED   FORMATIONS. 

Section  I.  To  the  newer  formations. 

Thk  Eastekn  Sandstone. — Position  and  extent  of  the  Eastern  Sandstone. — Cambrian  age  of  this  sand- 
stone.— Relations  between  it  and  the  Keweenaw  Series  on  the  south  side  of  the  Keweenaw  Kange; 
along  the  "  South  Range." — Different  views  as  to  the  relations  of  the  Keweenaw  Series  and  the 
Eastern  Sandstone. — The  south  face  of  the  Keweenaw  Range  is  both  a  fault  line  and  a  line  of 
unconformable  contact. 

The  Western  Sandstone. — Position  and  extent. — Contact  with  the  Keweenawan  rocks  in  Douglas 
County,  Wisconsin ;  this,  also,  is  both  a  fault  line  and  a  line  of  unconformable  contact. — Its  equiva- 
lence with  the  Eastern  Sandstone. 

Thb  Mississoti  Valley  Cambelan  Sandstone. — Relations  of  this  sandstone  to  the  Keweenawan  diabases 
in  the  Saint  Croix  Valley. — It  overlies  them  unconformably. 
Section  II.  To  the  older  formations. 

The  Animikie  Group. — At  Grand  Portage  Bay,  Minnesota ;  on  Wauswaugoning  Bay ;  on  the  Lucille 
Islands ;  on  Pigeon  Point ;  on  Pigeon  River ;  on  the  west  shore  of  Thunder  Bay ;  in  the  interior 
between  Thunder  Bay  and  Pigeon  River ;  on  the  north  shore  of  Thunder  Bay ;  on  the  east  shore 
of  Thunder  Bay ;  at  Thunder  Cape  and  to  the  eastward  from  there. — Relations  of  the  Animikie  and 
Keweenawan  in  this  region. — Summary  statement  as  to  the  Animikie  rooks  of  the  Thunder  Bay- 
Pigeon  River  region. — Views  of  the  Canadian  geologists  as  to  the  Animikie  Group. — The 
Animikie  rocks  in  the  Mesabi  range  of  Minnesota;  at  Pokegoma  Falls,  on  the  Mississippi 
River ;  in  the  Saint  Louis  River  region  of  Minnesota. — Relations  of  the  Animikie  and  Kewee- 
nawan rocks  in  general ;  the  former  are  not  only  a  downward  continuation  of  the  latter. — The 
Animikie  rocks  are  Huronian. 

Thb  original  Huronian. — Descriptions  quoted  from  Logan. — Nature  of  the  eruptive  rocks  of  the  orig- 
inal Huronian. — Resemblances  between  the  Animikie  rocks  and  the  original  Huronian. 

Thb  Penokee  Huronian. — Descriptive  section  of  the  Penokee  rocks ;  Huronian  in  Barron  County,  Wis- 
consin.—Resemblances  between  the  Animikie  and  Penokee  rocks ;  they  are  the  same  formation. 

Thb  Marquette  and  Menominee  Huronian. — Relations  to  the  Penokee  Huronian ;  they  are  the  same  for- 
mation.— Rooks  peculiar  to  the  Marquette  and  Menominee  Huronian,  and  not  found  in  that  of 
the  Penokee  region. — The  hornblendic  rocks  of  the  Huronian  of  the  Marquette  and  Menominee 
regions  are  suspected  to  be  merely  uralitic  or  altered  augitic  rocks. — The  Animikie  Huronian 
and  that  of  the  Marquette  and  Menominee  regions  are  the  same  formation ;  the  former  being  un- 
folded, the  latter  folded. 

Cbibtalline  schists  of  doubtful  relations. — Insufficient  knowledge  of  these  ancient  rocks. — Confusion 
with  regard  to  them  in  the  reports  of  various  geologists. — Folded  crystalline  schists  north  of 
Lake  Superior,  from  Nipigon  Lake  to  Vermillion  Lake,  Minnesota. — The  iron-bearing  rocks  of 
Vermillion  Lake ;  their  relations  to  the  Animikie  rocks  of  the  Mesabi  Range. — Doubtfully  related, 
folded  crystalline  schists  of  the  south  side  of  Lake  Superior ;  of  the  east  side  of  Lake  Superior. 

Belations  of  the  Keweenaw  series  and  the  Huronian  in  general. — Similarity  between  the  basic  erup- 
tives  of  the  Huronian  and  Keweenawan. — Absence  of  amygdaloids  in  the  Huronian. — Contrast 
between  the  sedimentary  members  of  the  two  groups. — Structural  relations  of  the  two  series  of 
rocks. — Close  approach  to  conformity,  with  an  intervening  erosion,  between  the  unfolded  Huro- 
nian and  the  Keweenaw  Series. — The  relations  of  the  Keweenaw  Series  to  the  folded  Huronian 
schists  are  not  so  plain ;  the  folding  may  have  taken  place  before  or  during  the  Keweenawan 
period. 
350 


•  THE  EASTERN  SANDSTONE.  351 

Section  I.— TO  THE  NEWER  FORMATIONS. 
THE  EASTERN  SAKDSTONE. 

By  this  term  is  meant  that  sandstone  which,  as  already  indicated,  fills 
the  valley  between  the  Keweenaw,  or  Main  Trap  Range  of  Michigan,  and 
the  so-called  South  Range.  The  eastern  end  of  this  depression  is  occupied 
by  the  waters  of  Keweenaw  Bay.  The  whole  area  has  a  characteristic  flat 
appearance  and  sandy  soil,  standing  thus  in  strong  contrast  with  the  high- 
lands of  crystalline  rocks  on  both  sides.  On  the  northern  edge  of  this 
depression  the  sandstone  may  frequently  be  seen  exposed,  from  the  head  of 
B^te  Grise  Bay,  on  Keweenaw  Point,  westward  to  beyond  Lake  Agogebic; 
and  on  the  southern  edge  may  be  traced  east  and  northeast  from  the  vicinity 
of  Lake  Agogebic  to  the  head  of  Keweenaw  Bay.  On  the  west  side  of  this 
bay  the  sandstone  is  constantly  exposed  in  cliff;  but  on  the  east  side  the 
older  crystalline  rocks  come  out  to  the  water's  edge.  Gneiss  and  schists 
form  the  mass  of  the  peninsula  between  Keweenaw  and  Huron  bays,  but 
skirting  the  immediate  shore  of  the  lake  is  a  band  of  sandstone  varying 
in  breadth  from  a  few  rods  to  one  or  two  miles,  the  older  rocks  only  now 
and  then  reaching  to  the  lake.  -  Similar  conditions  obtain  from  here  to 
Marquette,  beyond  which  point,  to  the  eastward,  sandstone  forms  all  of  the 
shore  cliffs  as  far  as  the  Sault. 

The  sandstones  of  Keweenaw  Bay  and  its  vicinity,  and  eastward 
thence  to  White  Fish  River,  are  reddish  and  often  highly  argillaceous.  At 
White  Fish  River  the  red  sandstone  is  overlain  by  a  light-colored  sand- 
stone, which  is  in  turn  succeeded  by  a  magnesian  limestone,  in  which  are 
casts  of  Pleurotomaria}  This  limestone  is  the  Lower  Magnesian  of  the 
Wisconsin  reports,  and  the  Calciferous  Sandrock  of  the  eastern  states. 
That  it  is  succeeded  in  regular  order  by  the  fossiliferous  limestones  of  the 
Trenton,  Cincinnati,  and  Niagara  groups  was  long  since  shown,  and  has 
been  demonstrated  anew  of  late  years  by  the  labors  of  the  geological  sur- 
veyors of  Wisconsin  and  Michigan.  There  thus  seems  little  room  for 
doubt  as  to  the  correctness  of  the  view  held  for  years  by  a  succession 
of  geological  workers  in  the  Lake  Superior  region,  from  Owen  to  Romin- 
ger,  viz.,  that  in  the  Eastern  Sandstone  we  have  to  do  with  the  same 

>  Geological  Survey  of  Mich.,  Vol.  I,  Part  III,  pp.  89,  90. 


352       COPPEE-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 

formation,  or  with  its  downward  continuation,  as  the  fossiliferous  Cam- 
brian sandstone  which,  in  the  Mississippi  Valley,  forms  the  base  of  the 
Paleozoic  column.  There  appears  to  be  but  one  waj^  in  which  this 
conclusion  can  be  avoided,  and  that  is  by  supposing  that  where,  east 
of  Marquette,  the  red  sandstone  is  overlain  by  the  lighter-colored,  there 
is  a  discordance  of  greater  or  less  extent,  the  red  sandstone  having  been 
thus  separated  by  a  relatively  large  time-gap  from  that  which  overlies 
it.  This  view  was,  indeed,  held  as  long  ago  as  1841,  by  Houghton,^ 
who,  however,  so  completely  altered  his  opinion  in  the  next  few  years  as  to 
consider  the  red  sandstone  of  Keweenaw  Bay  and  Keweenaw  Point  the 
newer  of  the  two,  and  as  of  Triassic  age;  while  the  light  sandstones  east 
of  Grand  Island  he  believed  to  antedate  the  Trenton  limestone.^ 

It  is  needless  to  discuss  the  idea  of  a  Triassic  age  for  any  of  the  Lake 
Superior  rocks,  since  its  incorrectness  has  been  so  abundantly  and  repeat- 
edly proved,  from  the  tinie  of  Foster  and  "Whitney  down  to  the  publication 
of  the  third  volume  of  the  Wisconsin  reports,  in  which  the  demonstration 
is  clinched  by  the  descriptions  of  the  perfectly  plain  relations  of  the  fossil- 
iferous sandstone  of  the  Mississippi  Valley  and  the  western  extension  of  the 
Keweenaw  Point  rocks,  in  the  region  of  the  Saint  Croix  River.  To  judge 
from  Rominger's  account  of  the  exposures  in  the  vicinity  of  White  Fish 
River,  the  falsity  of  the  idea  of  an  unconformable  superposition  of  the 
lighter-colored  upon  the  red  sandstone  is  equally  well  proven.^  In  the 
same  connection  allusion  should  be  made  to  the  long-known  occurrence 
of  an  isolated  patch  of  fossiliferous  Trenton  limestone  within  the  area  of 
the  Eastern  Sandstone.  This  limestone  forms  a  line  of  bluffs  extending 
through  sections  13,  14,  23,  and  24,  of  T.  51,  R.  35  W.,  Michigan,  fourteen 
miles  west  of  the  head  of  Keweenaw  Bay.  The  rock  contains  a  number  of 
well-known  Trenton  fossils.*  In  this  occurrence  we  have  demonstration  of 
the  former  extension  of  the  Lower  Silurian  limestones  far  to  the  westward 
of  their  present  limit. 

In  presenting  the  facts  upon  which  are  based  my  conclusions  as  to  the 

'  Fourth  Annual  Report  on  the  Geological  Survey  of  Michigan,  1841. 

2  American  Journal  of  Science,  1843,  XLV,  p.  160. 

3  Geological  Survey  of  Michigan,  Vol.  I,  Part  III,  pp.  62-63. 
<  Geological  Survey  of  Michigan,  Vol.  I,  Part  III,  p.  69. 


THE  EASTEEN  SANDSTONE  ON  B^JTE  GRISE  BAY. 


353 


relations  subsisting  between  the  Keweenawan  rocks  and  the  Eastern  Sand- 
stone, I  follow  the  northern  junction  westward  from  Bete  Grise  Bay  to  Lake 
Agogebic,  and  then  the  southern  from  that  lake  eastward. 

The  north  shore  of  Bete  Grise  Bay,  as  shown  on  a  previous  page,  is  made 
of  low  cliifs  of  Keweenawan  diabase  and  melaphyr,  with  some  quartzifer- 
ous  porphyry,  all  dipping  northward  at  a  high  angle ;  while  the  west  shore  of 
the  bay  lies  in  the  lowland  underlain  by  the  Eastern  Sandstone.  In  the  angle 
of  the  bay  the  tw^o  formations  come  together,  and  their  contact  may  be  fol- 
lowed for  a  long  distance.  The  sandstone,  of  which  a  considerable  thickness 
may  be  seen  in  continuous  exposure,  dips  southward  at  angles  varying  from 
55°^  at  the  contact  to  30'^  and  less  at  the  -point  farthest  removed  from  the 
contact.  It  is  made  up  of  alternating  whitish,  quartzose,  fine-grained  lay- 
ers, and  thinner  ones  of  red  shale;  the  latter  running  from  a  few  inches  to 
several  feet  in  thickness.  Some  of  the  red  layers  are  strongly  conglom- 
eratic, the  pebbles  being  generally  of  small  size  and  often  angular,  and 
composed  in  the  main  of  red  felsite,  but  also  in  some  measure  of  the  ordinary 
Keweenawan  diabase  and  melaphyr.     The  accompanying  section,  repre- 


£eie.  Gi-tse 


Fig.  30. — Showing  relation  of  the  Eastern  Sandstone   and  Keweenawan  melaphyr,  Bete  Grise 
Bay.     Length  of  section  about  150  feet. 

senting  a  length  of  about  160  feet,  is  designed  to  illustrate  the  nature  of 
this  contact.  The  junction  line  between  the  sandstone  and  the  older  rocks 
is  quite  irregular,  and  as  the  shore  of  the  bay  is  followed  eastward,  patches 
of  the  sandstone  are  seen  remaining  in  embayments  of  the  older  rocks  on 
the  cliff-side.  Underneath  the  clear  waters  of  the  lake  the  beveled  edges 
of  the  alternating  bands  of  red  and  white  sandstone  may  be  traced  for 
hundreds  of  feet  in  great  sweeping  curves.  On  the  south  point  of  Bete 
Grise  Bay,  below  the  ship  canal,  the  sandstone  lies  horizontally. 


23  L  s 


'  Not  78°,  as  reported  by  Foster  and  Whitney,  o^).  cit.,  p.  112. 


354       COPPER-BE AEING  BOOKS  OF  LAKE  SUPEEIOE. 

As  already  shown,  the  contact  line  of  the  sandstone  and  northward-dip- 
ping Keweenawan  rocks  west  of  B^te  Grise  Bay  is  plainly  marked  by  a 
sharp  break  in  the  topography.  At  a  niunber  of  points  along  this  line, 
phenomena  similar  to  those  observed  at  B6te  Grrise  have  been  noted — i.  e., 
the  sandstone  inclining  southward  at  an  angle  which  lessens  in  amount  very 
rapidly  as  one  passes  away  from  the  contact.  This  may  be  seen,  for  in- 
stance, in  the  vicinity  of  Lac  La  Belle  and  Grratiot  Lake,  and  on  some  of  the 
head  streams  of  Tobacco  River.  As  long  since  shown  by  Foster  and  Whit- 
ney, the  amount  of  southward  dip  in  the  sandstone,  even  near  the  contact, 
lessens  westward,  so  that,  in  the  vicinity  of  Torch  and  Portage  lakes,  it  lies 
horizontally  at  the  contact,  or,  at  most,  inclines  but  a  very  few  degrees 
southeastward. 

The  contact  line  is  crossed  and  exposed  by  several  of  the  small  streams 
entering  Torch  Lake  on  its  west  side.  These  streams  run  in  quite  deep 
gorges,  Avhich  are  carved  in  the  Eastern  Sandstone,  and  end  abruptly,  often 
with  a  vertical  wall,  where  they  reach  the  more  enduring  rocks  of  the  Trap 
Range.  The  gorges  of  two  of  these  streams,  the  Hungarian  and  Douglas 
Houghton  rivers,  were  examined  with  some  care.  As  the  Hungarian  River 
is  ascended,  the  sandstone  is  first  met  with  on  the  sides  of  the  ravine,  and 
then  in  its  bed  also,  where  it  forms  several  falls.  For  the  most  part  the  sand- 
stone is  light-colored  and  quartzose,  but  conglomerate  bands  are  included 
in  which  the  pebbles  are  in  the  main  of  some  of  the  red  acid  eruptives 
of  the  Keweenawan.  Often  the  sandstone  lies  horizontally;  at  times 
it  appears  to  have  a  slight  northwesterly  dip,  and  as  often  a  slight  south- 
easterly one.  These  deviations  from  horizontality  are  often  plainly  the 
result  of  the  undermining  on  the  side  of  the  ravine.  At  the  uppermost  fall 
the  contact  with  the  older  rocks  is  seen.  The  occurrences  here,  and  for 
some  distance  below,  are  as  shown  in  the  accompanjnng  sketch  made  on 
the  ground  by  Mr.  W.  M.  Chauvenet,  in  which  B  is  the  bank  of  the  gorge 
without  exposures ;  A,  sandstone  layers  projecting  from  the  sides  of  the  bank; 
D,  amygdaloid  and  pseud-amygdaloid  dipping  northwesterly;  E,  the  con- 
tinuation of  the  amygdaloid  in  a  crumbling  condition;  C,  porphyry-con- 
glomerate; and  F,  an  overlying  diabase.  At  G,  at  the  very  foot  of  the  fall, 
is  a  smoothed  sui-face  of  sandstone  jointed  in  two  directions,-  the  two  joint 


THE  EASTERN  SANDSTONE  ON  THE  HUNGAEIAN  EIVEE.      355 


Fig.  31. — Section  ou  the.  Hungarian  River,  Keweenaw  Point. 

surfaces  dii)ping  N.  W.  25°  and  S.  20°  E.;  and  a  few  steps  farther  down  the 
sandstone  is  seen  lying  perfectly  flat  In  the  same  vicinity  true  bedding,  as 
shown  by  the  differences  in  the  coarseness  and  coloring  of  the  sandstone, 
gave  dips  of  N.  W.  10°,  S.  E.  20°,  N.  E  20°.  The  irregularities  seem  to  be 
due,  in  a  measure,  to  undermining  on  the  sides  of  the  ravine,  but  are  also 
apparently  somewhat  analogous  to  those  described  and  figured  on  a  pre- 
vious page  as  occurring  on  the  gorge  of  Black  River,  in  Douglas  County, 
Wisconsin — i.  e.,  are  the  product  of  faulting  motion. 

In  his  account  of  the  occurrence  on  the  Hungarian  River,^  Mr.  M.  E. 
WadsAvorth  has  represented  the  Eastern  Sandstone  as  pi-esenting  an  gradu- 
ally increasing  northwesterly  dip,  as  it  is  followed  up  the  stream,  until  it 
is  plainly  seen  plunging  beneath  the  Keweenawan  diabase  and  interbedded 
conglomerate.  But  neither  the  increasing  northwesterly  dip  nor  the  subor- 
dinate position  of  the  sandstone  to  the  diabase  could  be  detected  by  Mr. 
Chauvenet.  Northwesterly  dips  are  found  in  the  sandstone  for  some  dis- 
tance below  the  contact,  but  southeasterly  ones  just  as  often,  or  oftener,  and 
both  seem  distinctly  subordinate  to  a  general  horizontality.  Again,  sand- 
stone lies  vertically  beneath  an  amygdaloid,  but  the  mass  of  sandstone  ap- 
pears to  be  a  fallen  one,  and  if  it  is  riot,  the  crumbling  amygdaloid  above 
certainly  is. 

The  occurrences  on  the  Douglas  Houghton  River  are  much  like  those 
seen  on  the  Hungarian,  with  the  exception  that  the  true  Keweenawan 
beds  extend  down  stream  for  some  300  paces  from  the  head  of  the  ravine, 
for  the  reason  that  they  include  just  here  a  considerable  thickness  of  soft 
conglomerate.  Below  the  last  of  these  beds  is  a  gap  of  some  200  paces, 
when  the  horizontal  layers  of  the  Eastern  Sandstone  come  in,  here  and 


'Notes  on  the  Geology  of  the  Iron  and  Copper  District  of  Lake  Superior,  p.  113. 
Museum  of  Comparative  Zoology;  Whole  Series,  Vol.  VII,  Geological  Series,  Vol.  I. 


Bulletin  of  the 


356       COPPER-BE AEING  EOCKS  OF  LAKE  SUPEEIOE. 

there  with  a  slight  northwesterly  dip  (2°-5°),  but  more  often  with  a  south- 
easterly one.  These  conditions  obtain  for  a  mile  or  more  down  the  stream. 
Mr.  Wadsworth  has  also  described  the  exposures  on  the  Douglas  Hough- 
ton River,  and  correctly,  so  far  as  showing — which  he  was  the  first  to  do — 
that  the  conglomerate  for  some  distance  below  the  falls  does  not  belong 
with  the  Eastern  Sandstone,  but  is  really  interbedded  between  diabases  of 
the  Keweenaw  Series.  When  he  represents,  however,  the  sandstone  still 
farther  down  stream  as  passing  beneath  the  last  Keweenawan- diabase,  he 
bridges  in  his  imagination  a  covered  gap  of  several  hundred  paces ;  beyond 
which,  to  the  eastward,  the  sandstoile  lies  flat,  or  inclines  varyingly  and  in- 
differently slightly  to  the  northwest,  southwest,  or  southeast,  not  showing 
any  sign  of  a  persistent  and  gradually  decreasing  northwestern  dip.  Were 
this  ravine  the  only  place  where  the  Eastern  Sandstone  could  be  seen  in 
proximity  to  the  north-dipping  Keweenawan  beds,  and  were  there  not 
other  considerations  rendering  such  a  conclusion  untenable,  the  idea  that 
Mr.  Wadsworth  has  advanced  might  perhaps  suggest  itself  as  a  possibility, 
although  so  far  as  the  exposures  here  are  concerned  it  could  be  nothing 
more.  There  would  remain  even  then  as  looking  the  other  way  a  marked 
lithological  difference  between  the  intercalated  sandstone  and  that  farther 
down  the  stream;  the  latter  being  a  much  more  purely  quartzose  rock, 
while  at  the  same  time  containing  pebbles  of  the  porphyry  whose  detritus 
composes  the  usual  interbedded  sandstones  of  the  trappean  series. 

About  a  mile  south  from  the  head  of  the  Douglas  Houghton  ravine, 
on  the  line  of  the  Torch  Lake  Railroad,  is  a  large  quarry  in  the  Eastern 
Sandstone.  The  sandstone  is  disposed  horizontally  in  heavy  massive 
layers.  It  is  nearly  white  and  almost  wholly  composed  of  rolled  quartz 
grains.  It  also  contains  here  and  there  grains  of  feldspar,  somewhat 
altered,  but  on  the  whole  singularly  fresh  for  such  a  rock,  some  particles 
:  showing  the  twin  lamellation  very  beautifully.  A  very  minute  quantity  of 
a  brownish  cement  is  present,  and  in  each  thin  section  may  be  seen  two 
or  three  grains  worn  from  some  of  the  fine-grained  diabases  of  the  Kewee- 
naw Series.  Not  a  trace  is  to  be  seen  of  anything  like  the  fragments  of 
porphyry  matrix,  so  abundant  in  the  Keweenawan  sandstones;  nor  was  I 
able  to  discover  any  satisfactory  indications  that  the  quartz-grains  are  the 


EASTEEN  SANDSTONE  AT  TOECH  LAKE.  357 

quartzes  of  the  quartziferous  porphyries,  ahhough  one  might  expect  to  do 
so.  In  his  description  of  this  quarry  quoted  below,  Mr.  Wadsworth  speaks 
of  the  grains  of  the  sandstone  as  furnished  with  crystalhne  outhnes,  and 
regards  these  outhnes  as  showing  the  derivation  of  the  quartzes  from  a  quartz- 
porphyry.  My  sections  fail  to  show  any  such  outlines,  but  if  they  occur, 
they  are  probably  rather  in  the  nature  of  those  of  the  crystal  grains  so 
frequently  met  with  in  the  Potsdam  sandstone  of  the  Mississippi  Valley, 
in  which  case  the  crystalline  outlines  are  the  result  of -a  secondary  deposi- 
tion of  quartz  upon  the  surfaces  of  the  originally  rolled  grains.  Eare  peb- 
bles of  quartz  of  some  size  are  contained  in  this  sandstone,  and  patches  and 
lines  of  red  clayey  substance,  which  do  not  show  any  persistent  inclination 
in  any  one  direction.  The  clayey  material  often  expands  into  large  bunches 
of  red  clay,  forming  the  usual  claj^holes,  so  characteristic  everywhere  of  the 
Eastern  and  Western  horizontal  sandstones. 

My  description  of  the  rock  of  this  quarry  differs  from  one  published 
by  Mr.  Wadsworth,  in  which  he  says — 

In  the  sandstone  quarry  at  the  head  of  the  incline  on  the  Hecla  and  Torch  Lake 
Eailroad,  the  sandstone  layers  have  been  regarded  as  being  nearly  horizontal.  The 
joint  planes  that  form  the  floors  of  the  quarry  are  nearly  so,  having  only  a  slight  dip 
to  the  northwest;  but  these  joint  planes  cannot  be  the  bedding  planes,  for  we  find 
on  close  examination  that  numerous  layers  of  coarser  material,  ipebbles,  clay  masses, 
etc.  occur  in  the  rock.  These  layers  extend  for  long  distances  through  the  sandstone, 
and  are  always  parallel,  having  the  same  dip,  which  is  N.  45°  W.  15°.  These  of 
course,  from  their  character  and  regularity,  must  mark  the  old  planes  of  bedding, 
while  the  generally  supposed  bedding  planes  are  secondary  joint  planes  *  cutting  the 
bedding  i)laues  at  a  small  angle.  This  sandstone  has  been  leached  and  acted  upon 
by  water  the  same  as  that  below  the  Douglas  Houghton  Falls,  and  its  feklspathic 
material  converted  into  clay  or  entirely  removed.  Part  of  the  materials  composing 
the  sandstone,  especially  in  the  coarser  portions,  are  similar  to  those  in  the 
sandstone  at  Marquette.  The  quartz  grains  are  partly  water-worn,  but  a  large 
proportion  are  seen  to  be  short  crystals  formed  of  the  bexagoual  prism,  terminated 
on  both  euds  by  the  pyramid,  or  the  usual  form  found  in  the  acidic  i^orphyritic 
rocks.  It  appears,  then,  as  the  facets  of  these  crystals  are  comparatively  unworn, 
that  they  were  derived  from  the  destruction  or  decomposition  of  trachytic  and  rhy- 
olitic  rocks  (granitic  and  quartz-porphyries),  the  feldspathic  material  having  been 
removed  since  by  water,  leaving  a  quartzose  sandstone.  It  is  a  question  worthy  of 
examination  whether  any  other  sandstones  have  been  formed  from  acidic  volcanic 
material,  irom  Avhich  nearly  all  the  other  parts  of  the  rock  have  b^en  removed  by  perco- 
lating waters;  especially  as  other  sandstones  have  been  said  to  be  comiiosed  of  quartz 
crystals. 


358       COPPEE-BEAEIKG  EOCKS  OF  LAKE  SUPEEIOE. 

In  my  examination  I  failed  to  find  any  evidence  of  the  northwesterly 
dip  described  by  Wadsworth,  and  a  subsequent  examination  by  Mr.  W.  M. 
Chaiivenet  with  Wadsworth's  description  in  hand  was  eqvially  futile.  The 
reddish  bands,  as  stated  above,  showed,  so  far  as  I  observed,  no  one  direc- 
tion of  inclination  any  more  decided  than  the  others,  and  even  if  they  did, 
it  would  be  necessary  for  any  one  trying  to  establish  their  direction  as  that  of 
the  general  bedding  of  the  rock,  to  prove  that  they  should  not  rather  be  taken 
as  instances  of  the  cross-bedding  so  commonly  aflPecting  the  similar  sand- 
stone of  the  Mississippi  Valley,  while  both  they  and  the  larger  clay  bunches 
are  precisely  what  may  be  seen  in  the  plainly  horizontal  sandstones  of  the 
Apostle  Islands.  It  would  seem  that  Mr.  Wadsworth,  having  previously 
formed  a  theory  as  to  the  relation  of  the  Eastern  Sandstone  to  the  Kewee- 
awan  beds,  has  felt  it  necessary  to  explain  away  the  plain  horizontality 
of  the  rock  in  this  quarry. 

A  similar  process  has  led  him  to  the  view  that  the  feldspathic  ingredient 
has  been  leached  out  of  the  Eastern  Sandstone,  in  order  that  he  may  explain 
the  quartzose  character  of  this  sandstone  and  of  that  of  the  Douglas  Hough- 
ton and  Hungarian  rivers — a  character  which  is  in  fact  a  common  one  of  the 
Eastern  Sandstone,  wherever  met  with  on  the  line  between  B^te  Grise  Bay 
a,nd  Lake  Agogebic,  and  again  along  the  north  face  of  the  South  Eange  east 
of  Lake  Agogebic.  This  leaching  process  would  have  but  a  slender  theo- 
retical basis  at  the  best,  and  in  the  present  case  seems  to  be  distinctly  dis- 
proved by  the  appearance  of  the  thin  section,  nearly  the  whole  of  which  is 
formed  of  rounded  quartz  grains  without  any  space  for  the  feldspathic  material 
to  have  been  leached  from;  while  the  few  feldspar  grains  present  are  singu- 
larly fresh  for  the  grains  of  a  fragmental  rock.  Moreover,  the  quartz  particles 
cannot  represent  a  secondary  substitute  for  feldspar,  such  as  so  often  occurs 
in  the  granitic  porphyries  of  the  Keweenaw  Series.  I  cannot  conceive  of  a 
leaching  process  which  leaves. neither  space  nor  substitute  for  the  original 
material.  Possibly  it  is  meant  that  the  leaching  has  affected  the  rock  as  a 
mass,  and  that  the  remaining  material  has  collapsed.  But  this  could  not 
happen  so  as  to  leave  the  rock  so  distinctly  marked  by  the  original  bedding 
structure.  The  thin  section  shows,  moreover,  the  quartz  grains  frequently 
in  the  often  observed  relation  which  indicates  that  they  lie  where  rolled 


EASTERN  SANDSTONE  ON  THE  ONTONAGON  EIVEE. 


359 


together  by  shifting  waters — i.  e.,  one  grain  enters  a  depression  in  the  side 
of  another.  Again,  it  is  difficult  to  see  why  the  supposed  hot  waters 
should  have  selected  this  one  sandstone  for  leaching,  removed  as  it  is,  now 
at  least,  from  the  heating  lava-flows;  while  the  beds  directly  intercalated 
with  these  flows  should  in  no  instance  show  any  signs  of  such  a  leaching. 
Although  Mr.  Wadsworth  seems  to  have  felt  it  necessary  thus  to  explain 
away  the  peculiarly  quartzose  character  of  the  Eastern  Sandstone  as  com- 
pared with  the  sandstones  of  the  Keweenawan,  this  lithological  dissimilarity 
seems  to  me  really  rather  more  in  favor  of  his  peculiar  view,  as  to  the 
structural  relations  of  the  Eastern  Sandstone,  than  against  it. 

Southwestward  from  the  vicinity  of  Torch  Lake,  the  Eastern  Sand- 
stone may  be  seen  in  close  proximity  to  the  Keweenawan  diabases,  at  a 
number  of  places,  and  always  with  the  same  relatively  quartzose  character, 
and  horizontal  position  or  southeasterly  dip.  Pumpelly  figures  such  an 
occurrence,  for  instance,  in  the  N.  W.  i  Sec.  6,  T.  54,  R.  33  W.,  on  the 
south  side  of  Portage  Lake.^  Again,  the  streams  in  sections  22  and  23  of  T. 
54,  R.  34  W.,  run  over  horizontal  quartzose  sandstone,  and  the  same  is  true 
of  the  streams  in  the  central  and  southeastern  parts  of  T.  52,  R.  35  "W., 
where  the  exposures  are  quite  large.  But  still  farther  west,  on  and  near 
the  Ontonagon  River,  much  larger  and  more  instructive  exposures  of  the 
Eastern  Sandstone  are  to  be  found.  The  occurrences  here  are  like  those  on 
B^te  Grise  Bay — i.  e.,  the  sandstone  dips  away  southward  from  the  north- 
dipping  Keweenawan  diabases,  at  quite  a  high  angle  near  the  contact  and 
rapidly  grows  flatter  as  the  contact  is  receded  from. 


^^euiecnoauan,    2Habaees.  Eascern    acavOntane. 

Fig.  32.— Section  showing  relation  of  Eastern  Sandstone  to  Keweenawan  diabase,  T.  50,  R.  39  W., 
Michigan.     Length  of  section,  one-half  mile. 

The   above  section  was  taken  by  Mr.  W.  M.  Chauvenet  along  the 
course  of  the  small  stream  in  sections  23  and  24,  T.  50,  R.  39  W.    The  sand- 

'  Atlas  GeoL  Survey  Mich.,  Plates  XIV  and  XV. 


360       COPPEE-BEARIKG  EOCKS  OF  LAKE  SUPEEIOR. 

stone  seen  here  is  the  usual  saccharoidal  quartzose  kind,  often  perfectly- 
white,  and  at  times  mingled  and  streaked  with  more  or  less  brownish  material. 
It  carries  frequent  pebbles  of  white  quartz,  but  none  of  the  Keweenawan 
diabase  against  which  it  rests.  One  mile  south,  in  section  27,  the  sandstone 
was  observed  in  a  horizontal  attitude,  and  in  the  S.  W.  ^,  Sec.  28  was  seen 
in  large  exposures  at  the  falls  of  the  Ontonagon  River.  Here  it  dips  south- 
ward at  an  angle  of  15'^,  but  as  it  is  followed  northward  some  200  yards, 
this  dip  changes  to  18°  and  20°.  A  short  distance  farther  north  is  a  bold 
south-facing  bluff  of  Keweenawan  diabase.  It  should  be  said  that  these 
south  dips  are  not  wavering  and  uncertain  like  those  observed  on  the  Douglas 
Houghton  River,  but  are  persistent  and  pronounced,  affecting  many  hun- 
dred feet  in  thickness,  while  the  exposures  are  to  be  likened  in  extent  and 
inclination  to  those  seen  on  B6te  Grise  Bay.    - 

Farther  west  again,  as  far  as  Lake  Agogebic,  the  west  branch  of  the 
Ontonagon  River  has  its  course  just  under  an  overhanging  bluff  of  diabase, 
following  closely  the  junction  line  of  the  two  formations.  Here  and  there 
it  exposes  the  sandstone  under  conditions  like  those  just  described.  Ex- 
posures of  horizontal  sandstone  are  also  often  met  with  in  the  country 
south  of  this  line. 

Along  the  north  face  of  the  South  Range  eastward  from  Lake  Ago- 
gebic, the  sandstone  is  not  unfrequently  met  with  in  exposures.  The 
principal  point  of  interest  in  this  connection  is  the  way  in  which  it  com- 
jDletely  overlaps  the  Keweenawan  rocks,  which,  as  previously  shown,  con- 
stitute this  range.  This  overlapping  is  not  merely  an  inference  from  the 
supposed  continuation  of  the  South  Range  Keweenawan  beds  beneath  the 
sandstone  —  as  for  instance  in  the  fifty  miles  southwest  from  the  head  of 
Keweenaw  Bay — but  may  be  directly  demonstrated  by  closely  approxi- 
mated exposures  of  the  formations  concerned.  This  was  first  shown  by 
Pumpelly'^  from  exposures  examined  by  him  on  the  west  branch  of  the 
Ontonagon  in  the  northeast  part  of  T.  46,  R.  41  W.  This  place  was  sub- 
sequently visited  under  my  direction  by  Mr.  Robert  McKinlay,  who  found 
the  occurrences  as  shown  in  Fig.  3.     The  sandstone  is  horizontally  bed- 

'  Geological  Survey  of  Mich.,  Vol.  II,  Part  II,  p.  4.  It  will  he  seen  that  Mr.  McKinlay  found 
large  exposures  of  the  Keweenawan  rocks  much  nearer  the  sandstone  than  indicated  by  Professor 
PumpeUy 


rOSTEE  AND  WHITNEY  ON  THE  EASTERN  SANDSTONE.        361 

ded,  showing  in  a  south-facing  cliff  60  feet  high  and  350  feet  long.  It  is 
reddish,  very  coarse,  and  composed  almost  entirely  of  rounded  grains  of 
quartz.  One  hundred  paces  from  the  foot  of  this  cliff  are  reddish  decom- 
posed schists  trending  N.  E.  and  dipping  45°  to  60°  S.  E.  Seven  hundred 
paces  northeast,  near  the  southeast  corner  of  section  11,  is  a  small  ledge  of  a 
dark-brown,  weathered,  medium-grained  diabase,  and  in  the  northeast  part 
of  the  same  section,  and  running  thence  through  sections  10  and  9,  and 
terminating  in  the  S.  E.  ^  of  Sec.  5,  is  a  series  of  exposures  of  diabase 
pseud-amygdaloid  and  amygdaloid.  Further  west  and  again  east  of  the 
sandstone  are  other  exposures  of  amygdaloid,  so  that  there  can  be  no  ques- 
tion whatever  that  the  Eastern  Sandstone  lies  directly  across  the  course 
of  the  Keweenawan  belt. 

Four  different  views  have  been  held,  since  the  publication  of  the  well- 
known  report  of  Foster  and  Whitney,  as  to  the  relations  of  the  Eastern 
Sandstone  to  the  northward-dipping  rocks  against  which  it  abuts. 

Foster  and  Whitney's  idea^  evidently  was  that  the  Eastern  Sandstone 
and  that  which,  with  a  very  great  thickness,  forms  the  west  side  of  Ke- 
weenaw Point,  were  originally  the  same,  but  are  separated  by  a  longitu- 
dinal fault  extending  from  Bete  Grise  to  Black  River.  In  the  region  of 
Bete  Grise  this  favilt  was  supposed  to  be  accompanied  by  the  protrusion  of 
the  mass  of  the  Bohemian  Range,  to  whose  elevation  was  attributed  the 
northward  inclination  of  the  whole  succession  of  "bedded  traps,"  with  the 
overlying  conglomerates  and  sandstones,  which  constitute  the  greater  part 
of  Keweenaw  Point,  and  the  inclination  southward  of  the  Eastern  Sand- 
stone in  the  Bete  Grise  region,  the  Bohemian  Range  being  taken  as  the 
center  of  an  anticlinal.  Farther  west  this  fissure  was  supposed  to  have 
been  unaccompanied  by  any  outflow,  and  the  Eastern  Sandstone  to  have 
been  left  horizontal. 

I  have  shown  on  a  previous  page  that  the  rocks  of  the  Bohemian  Range 
are  simply  a  downward  continuation  of  the  Keweenaw  Point  Series,  being 
made  up  of  the  usual  flows,  and  that  there  is  no  evidence  of  antichnal  struc- 
ture. Otherwise  the  theory  of  Foster  and  Whitney  has  some  plausibihty  in 
it.  I  have  myself  already  argued  in  favor  of  the  view  that  the  southern 
escarpment  of  the  Keweenaw  Range  is  a  fault  line,  though  with  different 

> Report  on  tlie  Lake  Superior  Land  District,  Part  I,  p.  66. 


362       COPPEE-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 

reasons  from  those  appealed  to  by  Foster  and  Whitnej^.  In  its  quartzose 
character  the  Eastern  Sandstone  has,  too,  something  in  common  with  the 
uppermost  layers  of  the  sandstone  of  the  western  side  of  Keweenaw  Point, 
where  a  distinct  tendency  to  become  more  quartzose  is  to  be  seen,  although 
there  is  always  a  considerable  difference  in  this  respect  between  the  two 
sandstones.  It  would  also  be  easy  to  understand  how  to  the  eastward  this 
uppermost  sandstone  might,  by  overlapping,  pass  on  to  the  older  rocks  with 
a  small  thickness,  while  constituting  to  the  west  only  the  uppermost  layer 
of  a  great  series. 

There  are  some  difficulties,  however,  in  the  way  of  an  acceptance  of 
this  view.  The  throw  of  the  fault  would  have  to  be  enormously  great — 
at  least  35,000  feet — and  much  greater  than  is  needed  for  the  fault  which  I 
have  supposed  to  exist  along  this  contact  line.  Subsequently  to  the  fault- 
ing, or  during  it,  an  amount  of  hard  resistant  material  35,000  feet  in  height, 
several  miles  in  width,  and  over  one  hundred  in  length,  must  have  been 
denuded  on  one  side  of  this  fault,  while  on  the  other  an  insignificant 
amount  of  a  fragile  sandstone  was  left  standing.  A  yet  more  serious 
difficulty  is  found  in  the  way,  already  described,  in  which  the  Eastern 
Sandstone  crosses  the  course  of  the  beds  of  the  South  Range  east  of  Lake 
Agogebic.  Were  it  merely  an  upper  member  of  a  series  of  which  they  form 
the  bottom  portions,  the  two  formations  could  not  possibly  sustain  any  such 
relations  as  they  do.  They  were  very  plainly  shown  by  Pumpelly,  in  his 
description  of  the  place  above  alluded  to,  to  be  in  true  unconformity  to 
one  another,  and  the  additional  facts  obtained  by  Mr.  McKinlay  amply 
sustain  Pumpelly's  descriptions. 

Pumpelly's  conclusion,  after  making  these  observations,  was,  that  the 
junction  line  between  the  Eastern  Sandstone  and  the  inclined  beds  of  the 
Keweenaw  Range  was  an  old  shore-cliff,  instead  of  a  fault  line,  against  which 
the  sandstones  were  deposited.  This  conclusion  was  supposed  to  be  cor- 
roborated by  the  finding  of  abundant  pebbles  of  the  Keweenawan  diabase 
in  the  Eastern  Sandstone  near  the  contact,  on  the  Douglas  Houghton  River. 
It  is  not  impossible,  however,  that  both  he  and  Agassiz  before  him^  did,  as 
"Wadsworth  says,  mistake  a  bed  intercalated  with  the  Keweenaw  Series  as 

'ProcBost.  Soc.  Nat.  Hist.,  1867,  XI,  p.  244. 


PUMPELLY  ON  THE  EASTERN  SANDSTONE.  363 

part  of  the  Eastern  Sandstone.  However,  on  Bete  Grise  Bay,  where  there 
can  be  no  possibility  of  doubt,  the  Eastern  Sandstone  at  the  contact  con- 
tains layers  in  which  diabase  and  amygdaloid  pebbles  are  abundant,  along 
with  others  of  red  felsite  and  quartziferous  and  granitic  porphyries.  In 
advancing  this  view  Pumpelly  was  simply  attempting  to  carry  to  demon- 
stration what  had  before  been  suspected  by  Logan  and  other  earlier  geolo- 
gists. In  abandoning  the  idea  of  a  fault  along  the  south  side  of  Keweenaw 
Point  he  saw  that  it  would  be  necessary  to  account  for  the  disappearance 
of  the  seven  miles  in  thickness  of  rocks  constituting  Keweenaw  Point — of 
which  fully  two  miles  are  red  sandstone  and  conglomerate  of  unquestioned 
sedimentary  origin — in  the  few  miles  intervening  between  the  point  and  the 
southern  end  of  Keweenaw  Bay,  where  the  Eastern  Sandstone  lies  directly 
upon  the  Huronian  slates.  This  he  did  by  supposing  an  enormous  Pre- 
Cambrian  erosion,  thus  making  the  break  between  the  Keweenaw  and 
Eastern  Sandstone  an  immensely  great  one. 

Recently  Mr.  M.  E.  Wadsworth  has  maintained  a  view,  previously  sug- 
gested by  Credner,^  namely,  that  the  Eastern  Sandstone  passes  underneath 
the  entire  copper  series,  forming  its  lowermost  member,  or  lowermost  mem- 
ber in  sight.  This  conclusion  he  rests  on  observations  made  on  the  Douglas 
Houghton  and  Hungarian  rivers.  I  have  already  shown  that  the  exposures 
on  the  Hungarian  River  will  not  admit  of  any  such  explanation,  while  those 
on  the  Douglas  Houghton,  taken  alone,  could  only  be  thus  explained  by 
imagining  an  appropriate  structure  within  an  interval  where  there  are  no 
exposures.  But  there  is  no  necessity  of  going  to  these  streams  to  prove  the 
untenableness  of  Wadsworth's  peculiar  position,  although  he  considers  that 
it  "settles  the  long-disputed  question  of  the  relative  age  of  the  traps  and 
Eastern  Sandstone  of  Lake  Superior."  The  large  exposures  of  south-dipping 
sandstone  on  Bete  Grise  Bay,  at  the  contact  with  the  Keweenawan  melaphyr, 
and  the  similar  exposures  on  the  south  side  of  the  Trap  Range  in  the  vicinity 
of  the  Ontonagon  River,  are  enough  to  disprove  absolutely  any  such  struc- 
tural theory.  In  Mr.  Wadsworth's  view  the  Eastern  Sandstone  antedates 
all  the  Keweenawan  eruptions,  and  yet  it  holds  frequent  pebbles  of  both 
acid  and  basic  Keweenawan  eruptives,  whose  characters  are  so  pronounced 

'Elemente  der  Geologie,  4tli  edition,  1878,  p.  416. 


364       COPPEE-BEARING  EOCKS  OF  LAKE  SUPEEIOE. 

that  there  can  be  no  doubt  as  to  the  source  of  the  pebbles.  The  more  dis- 
tinctly quartzose  character  of  the  Eastern  Sandstone,  as  compared  with  the 
detrital  beds  of  the  Keweenaw  Series,  finds  its  explanation,  I  think,  in  the 
derivation  of  the  larger  part  of  its  material  from  the  granites  and  schists  of 
the  region  south  and  east  from  Keweenaw  Bay.  Although  Mr.  Wadsworth 
feels  obliged  to  explain  away  its  silicious  character  by  a  supposed  process 
of  leaching  by  hot  water,  the  lithological  character  of  the  Eastern  Sand- 
stone, but  for  its  carrying  pebbles  of  Keweenawan  eruptives,  would  be 
more  in  favor  of  his  view  than  against  it.  If  the  Eastern  Sandstone  ante- 
dates all  of  the  Keweenaw  Series  it  should  present  a  strong  lithological 
contrast  with  the  detrital  rocks  of  that  series,  having  been  derived  from  a 
wholly  different  source. 

Moreover,  there  are  general  considerations  which  would  make  this  view 
impossible  of  acceptance,  even  were  the  occurrences  at  the  contact  not  so 
conclusively  against  it  as  they  are.  If  the  whole  mass  of  the  Keweenaw 
Point  succession  overlies  the  Eastern  Sandstone,  what  has  become  of  this 
seven  miles  of  rock  thickness  to  the  eastward  1  It  will  not  do  to  say  that  we 
are  dealing  here  with  eruptive  rocks  which  thicken  and  thin  suddenly,  and 
cannot  therefore  be  reasoned  about  in  the  same  manner  in  which  we  would 
deal  with  beds  of  sediment.  No  eruptive  agencies  ever  pile  up  seven  miles 
of  rock  with  a  vertical  wall  of  that  height  extending  over  a  hundred  miles 
in  length.  Besides,  in  this  case  the  eruptive  beds  or  flows  are  structurally 
just  like  beds  of  sediment,  which  thicken  and  thin  also.  Yet  more,  fully 
two  miles  of  the  thickness  is  of  sediment.  Nothing  but  an  immense  erosion 
on  Mr.  Wadsworth's  view  of  the  inferior  position  of  the  Eastern  Sandstone 
can  explain  the  disappearance  of  so  great  a  thickness  of  strata.  But  an  ero- 
sion which  has  stopped  suddenly  on  so  sharp  a  line,  parallel  to  the  general 
trend  of  the  layers,  and  yet  has  left  nowhere  behind  this  line  a  trace  of  the 
former  extent  to  the  south  and  east  of  this  immense  thickness  of  resistant 
beds,  while  leaving  undenuded  over  a  wide  area  an  inconsiderable  thickness 
of  an  underlying  fragile  sandstone,  is  incredible.  Again,  east  of  Marquette 
the  Eastern  Sandstone  appears  to  pass  upwards  insensibly  into  the  beds  of 
the  Calciferous.    Yet  on  Keweenaw  Point,  only  a  few  miles  away,  this  the- 


THE  WESTERN  SANDSTONE.  365 

ory  supposes  that  between  these  horizons  there  exists  a  thickness  of  seven 
miles  of  rock,  of  which  two  are  of  purely  sedimentary  material. 

It  seems  to  me  that  the  south  face  of  the  Keweenaw  Range  is  both 
a  fault  cliff  and  a  shore  cliflf,  against  which  the  newer  Eastern  Sandstone  was 
laid  down,  but  not  until  after  a  large  erosion ;  and  that  faulting  took  place 
again  after  or  else  continued  until  after  the  deposition  of  the  sandstone.  The 
original  faulting  seems  to  be  demanded  on  this  line  by  the  general  structural 
relations  of  the  Keweenaw  and  South  ranges,  as  shown  on  a  previous 
page,  and  by  the  absence  of  outliers  of  the  immense  thickness  of  rocks  of 
the  Keweenaw  Range  to  the  southward.  That  the  Eastern  Sandstone  was 
deposited  subsequently  to  this  first  faulting  is  evidenced  by  its  containing 
conglomerate  layers  in  which  the  pebbles  are  frequently  of  Keweenawan 
eruptives,  basic  as  well  as  acid  (Bete  Grise  Bay),  and  by  the  way  in  which 
it  cuts  across  the  course  of  the  South  Range  beds.  That  faulting  motion 
took  place  along  the  fault  line  after  or  during  the  deposition  of  the  Eastern 
Sandstone,  is  indicated  by  the  way  in  which  the  sandstone  dips  southward 
along  the  junction  at  the  south  side  of  the  Keweenaw  Range. 

THE    WBSTEElSr    SANDSTONE. 

The  Apostle  Islands  and  the  adjoining  coast  of  Bayfield  County,  Wis- 
consin, are  composed  of  a  horizontally  placed  sandstone,  closely  resembling 
in  character  the  Eastern  Sandstone  of  Keweenaw  Point.  I  have  described  this 
sandstone  somewhat  fully  in  another  place.^  From  the  head  of  Chaquamegon 
Bay  eastward  there  are  no  rock  exposures  on  the  coast  until  Clinton  Point 
is  reached,  four  miles  above  the  mouth  of  Montreal  River.  Here  are  flat 
ledges  of  sandstone  of  some  size  at  the  water  level.  I  take  them  to  mark 
the  easternmost  point  of  the  Western  Sandstone,  though  this  cannot,  from 
their  position  only,  be  regarded  as  certain.  At  the  mouth  of  the  Montreal 
occurs  the  vertically  placed  sandstone  of  the  Upper  Division  of  the  Ke- 
weenaw Series,  with  an  immense  thickness,  as  already  described.  West- 
ward from  the  Apostle  Islands  this  sandstone  has  been  traced  to  the 
head  of  the  lake,  and  in  Douglas  County,  Wisconsin,  may  be  seen  at  a 
number  of  points  in  direct  contact  with  the  south-dipping  Keweenawan 
diabases.     In  all  of  this  region  this  sandstone  preserves  its  predominatingly 

'Geology  of  Wisconsin,  Vol.  Ill,  p.  207. 


366       COPPEE-BBAEIKG  EOCKS  OF  LAKE  SUPEEIOE. 

qiiartzose  character,  being  at  times  just  like  the  darker  colored  portions  of 
the  quartzose  sandstone  of  the  central  part  of  Wisconsin. 

The  phenomena  of  the  contact  in  the  Douglas  County  Copper  Range 
have  been  described  in  some  detail  on  a  previous  page,  where  I  have  also 
shown  that  the  Western  Sandstone  in  all  probability  sustains  the  same  rela- 
tions to  the  Keweenawan  diabases  against  which  it  rests  as  does  the  Eastern 
Sandstone  of  Keweenaw  Point  to  the  north-dipping  beds  of  that  typical 
region,  this  similarity  of  relation  being  carried  out  even  to  the  faulting  that 
I  have  shown  to  obtain  in  the  latter  district. 

This  similarity  of  structural  relations,  taken  together  with  similarity  in 
lithological  character,  renders  it  very  probable  that  the  Eastern  and  West- 
em  Sandstones  are  geologically  equivalent.  But  they  are  nowhere  con- 
nected, and  the  Western  Sandstone  has  not  been  traced  to  any  point  where 
its  relation  to  any  of  the  Mississippi  Valley  fossiliferous  formations  can  with 
certainty  be  made  out,  although  the  appearances  in  northwestern  Wisconsin 
and  northeastern  Minnesota  are  decidedly  in  favor  of  its  being  the  down- 
ward continuation  of  the  Mississippi  Valley  Cambrian  Sandstone. 

THE    MISSISSIPPI  VALIilfiT   CAMBRIAX   OB   POTSDAM   SANDSTONE. 

I  have  already  shown  that  the  Keweenawan  diabases  and  interbedded 
conglomerates  are  traceable,  mile  by  mile,  from  the  typical  region  of  Ke- 
weenaw Point  to  the  Saint  Croix  Eiver  on  the  west  side  of  Wisconsin ;  and 
that  here  they  underlie  the  fossiliferous  Cambrian  Sandstone  of  the  Missis- 
sippi Valley,  in  such  a  manner  as  to  render  certain  the  tilting  and  great 
erosion  of  the  Keweenawan  beds  before  the  deposition  of  the  sandstone; 
the  latter  for  fifty  miles  in  an  N.  E.— S.  W.  direction,  with  interruptions  due 
to  denudation,  lying  athwart  the  course  of  the  tilted  Keweenawn  beds, 
which  are  here  disposed  in  synclinal  form.  Whatever  difficulties  may  hang 
about  the  structural  relation  of  the  Eastern  and  Western  Sandstone  of  Lake 
Superior,  there  are  here  none ;  so  unmistakable  are  their  structural  relations 
in  this  region,  that  any  geologist  still  doubting  the  separation  of  the  Ke- 
weenawan rocks  from  the  overlying  Cambrian  Sandstones  by  an  interven- 
ing disturbance  and  erosion  should  feel  himself  debarred  from  denial  until 
he  has  thoroughly  examined  the  facts  in  the  field. 


THE  ANIMIKIE  GEOUP,  367 

Section  II.— THE  OLDER  FORMATIONS. 
THE    ANIMIKIE    GROUP. 

At  Grand  Portage  Bay,  on  the  east  end  of  the  Minnesota  coast,  there 
rise  from  beneath  the  typical  Keweenawan  diabases,  beds  of  slate  and 
quartzite.  These  beds  are  finely  shown  immediately  behind  the  Indian 
village  at  Grand  Portage.  Here  may  be  seen  a  large  thickness  of  a  thin- 
laminated,  black  to  dark-gray  slate,  which  is  now  aphanitic  and  clay-slate- 
like, and  now  more  distinctly  arenaceous.  Some  of  the  layers  carry  nu- 
merous shiny  mica  scales  along  the  lamination  planes,  and  the  whole 
exposure  is  in  striking  contrast  to  anything  in  the  Keweenaw  Series  above. 
The  whole  thickness  trends  N.  70°  W.  and  dips  lO'^  to  the  S.  W.  The 
cleavage  planes  are  lamination  planes  and  not  due  to  slaty  cleavage.  A 
great  dike,  standing  vertically,  and  trending  east  and  west,  with  a  width  of 
50  to  75  feet,  cuts  the  slate,  which  for  a  long  distance  is  weathered  away, 
leaving  the  dike  standing  as  a  bold  wall,  in  places  over  a  hundred  feet  in 
height.  The  dike  rock  is  a  fine-grained,  black  diabase,  which  is  peculiar  for 
having  the  black  iron  oxide  constituent  in  long  rods,  which  often  lie  parallel 
for  considerable  distances,  two  parallel  systems  at  times  crossing  each  other. 
The  augite  individuals  have  their  contours  only  in  part  determined  by  the 
feldspars. 

At  the  northeast  end  of  the  large  island  at  the  mouth  of  Grand  Port- 
age Bay,  these  slates  may  be  seen  directly  overlain  by  the  Keweenawan 
diabases,  as  described  on  a  previous  page.  The  slates  do  not  rise  here  very 
high  above  the  water,  most  of  the  island  being  composed  of  the  overlying 
diabase.  Much  of  the  slaty  rock  here  is  a  very  highly  but  finely  arenace- 
ous, nearly  white  quartzite,  carrying  pebbles  of  white  quartz,  and  consisting, 
as  seen  under  the  microscope,  of  wholly  fragmental  material,  in  the  shape  of 
subangular  to  angular  quartz  grains,  mingled  with  a  few  of  decomposed 
feldspars,  all  imbedded  in  a  finer  material,  which  appears  to  be  partly 
clayey  and  partly  arenaceous.  The  dip  of  these  slates,  10°  S.  E.,  is  in 
entire  conformity  with  that  of  the  overlying  diabases.  The  slates  of  Port- 
age Bay  Island  belong  above  those  of  the  cliff  behind  the  village,  having 
their  low  position  by  virtue  of  the  southeast  dip. 


368      COPPEE-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 

On  Hat  Point  again,  at  the  east  side  of  Portage  Bay,  the  slates  are 
finely  exposed  on  both  sides  of  the  point,  where  they  are  gray  to  black 
argillaceous  quartz-slates,  marked  by  thin  lines  of  lamination,  and  strongly 
jointed  by  cross-joints.  Cutting  the  slate  on  the  east  side  of  the  point, 
where  the  dip  is  nearer  15°  than  10°  S.  E.,  are  several  east  and  west  dikes. 
One  of  these,  twenty  feet  wide,  is  of  a  black  rock,  which  under  the  micro- 
scope resembles  the  rock  of  the  dike  back  of  Grand  Portage  village,  but  is 
coarser  in  grain.  It  is  distinctly  an  orthoclase-gabbro.  The  body  of  Hat 
Point,  however,  is  formed  by  what  may  be  a  great  dike,  though  it  seems  to 
be  an  immense  overlying  mass,  upwards  of  300  feet  thick,  of  a  medium- 
grained  to  coarse-grained,  very  highly  crystalline,  light-gray  olivine-gabbro. 
Thin  sections  of  this  rock  show  very  abundant  olivines  of  large  size — up 
to  two-tenths  of  an  inch  across — and  extraordinarily  fresh,  they  being  only 
here  and  there  crossed  by  brown  ocherous  bands.  The  other  ingredients 
are  greatly  predominant  and  very  fresh  anorthite,  the  usual  titaniferous 
magnetite,  and  sparse  diallage.  In  the  gabbro  are  included  irregular 
blotches  of  darker- colored  rock,  which  in  the  thin  section  are  seen  to  be 
almost  or  quite  without  the  olivine,  and  to  have  the  diallage  relatively  very 
abundant.  The  Hat  Point  gabbro  is  not  to  be  distinguished  in  the  thin 
section  from  the  similar  olivinitic  rocks  of  the  Bad  River  region  of  Wis- 
consin and  of  the  Cloquet  River  of  Minnesota.  I  have  already  suggested 
the  possibility  of  the  two  belonging  to  a  corresponding  horizon,  or  rather 
to  the  same  general  time  of  outflow.  At  the  end  of  Hat  Point  the  over- 
lying gabbro  mass  sinks  to  the  water's  edge,  and  the  whole  appearance  is 
that  of  slight  discordance  to  the  underlying  slate. 

Hat  Point  forms  the  west  side  of  Wauswaugoning  Bay.  All  around 
the  head  of  the  bay  are  bold  cliffs  of  slate,  and  of  the  gabbro  of  a  great 
dike.  At  the  east  side  of  Wauswaugoning  Bay  are  immense  exposures  of 
slate  and  cutting  dike  masses.  Here  the  slates  are  often  hard,  dense,  sharp- 
edged,  ringing  quartzites,  in  layers  one  inch  to  six  and  even  eight  inches  in 
thickness.  These  hard  kinds  are  interbedded  with  more  slaty  kinds,  some 
of  which  are  lustrous,  dark-gray  to.  black  clay-slates,  and  others  quartz- 
schists,  in  which  the  quartzose  and  argillaceous  portions  are  mingled  in 
various  proportions.     Some  of  these  slaty  layers  are  indistinguishable  from 


AEIMIKIE  SLATES  ON  THE  LUCILLE  ISLANDS.  369 

the  quartz-schists  of  the  lower  part  of  the  Huronian  Series  at  Penokee 
Gap;  and  the  whole  aspect  is  precisely  that  of  the  South  Shore  Huronian 
quartzites  and  quartz-schists  of  different  horizons.  A  thin  section  of  one  of 
the  more  quartzose  kinds  showed  it  to  be  made  up  of  angular  quartz  frag- 
ments, with  a  finer  matrix,  and  some  areas  of  quartz  not  so  distinctly  frag- 
mental.  The  strike  and  dip,  obtained  at  a  number  of  places  in  this  vicinity 
from  large  dip-surfaces,  range  between  N.  55°  and  65°  E.  for  the  strike,  and 
10°  S.  E.  and  15°  S.  E.  for  the  dip,  the  higher  angles  being  the  more  common. 
The  dikes  on  the  east  side  of  Wauswaugoning  Bay  run  east  and  west,  and 
are  closely  like  those  of  Grand  Portage  Bay  in  character.  The  thin  section 
of  the  rock  of  one  of  them  showed  the  same  peculiar  rod-like  magnetites 
seen  in  the  Grand  Portage  dike-rock,  but  the  rock  is  intensely  altered, 
having  all  the  augite  turned  into  greenish  material. 

The  inner  ones  of  the  Lucille  group  of  islands  are  again  composed  of 
the  slates,  dipping  in  the  same  way;  the  outer  ones,  as  already  said,  being 
formed  of  the  overlying  Keweenawan  diabases.  On  one  island,  however 
— the  one  called  Brick  Island  on  the  United  States  Lake  Survey  chart — 
the  rock  is  very  peculiar.  It  is  pink  to  bright  brick-red  in  color,  thinly 
and  very  distinctly  stratified,  dipping  S.  E.  8J°,  and  is  plainly  part  of  the 
slate  series.  In  the  field  this  rock  was  taken  to  be  simply  a  red  variety  of 
the  usual  quartzite  seen  all  about  on  the  adjoining  coast  and  islands.  But 
an  inspection  of  the  hand  specimen  shows  that  it  is  finely  crystalline,  while 
the  thin  section  reveals  a  rock  very  close  to  those  red  rocks  of  the  Kewee- 
naw Series  which  I  have  described  under  the  names  of  augite-syenite  and 
granitic  porphyry;  that  is  to  say,  it  is  a  mass  of  feldspar  crystals,  satu- 
rated with  secondary  quartz  arranged  in  the  usual  graphic  form,  while 
other  larger  quartz  areas  seem  also  to  belong  with  the  secondary  quartz. 
Here  and  there  is  an  augite  crystal  to  complete  the  resemblance,  and  there 
is  no  trace  of  fragmental  texture. 

Pigeon  Point,  the  extreme  eastern  end  of  the  Minnesota  coast,  shows 
the  slates  in  fine  exposure  along  its  southern  side,  and  with  much  the 
same  characters  as  above  described;  i.  e.,  dark-gray  to  black,  more  or  less 
highly  argillaceous  or  clay-slate-like  layers,  alternating  with  others  that  are 
more  quartzitic.  Places,  too,  were  noted  on  the  point  where  a  red  rock,  ap^ 
24  L  s 


370       COPPEE-BEAEmG  EOCKS  OP  LAKE  SUPEEIOE. 

parently  bedded  just  like  the  slates,  and  dipping  like  them,  was  exposed 
on  a  large  scale.  The  thin  section  shows  that  this  rock  is  the  same  as  that 
from  Brick  Island,  above  described.  Its  relation  to  the  surrounding  slates 
is  worthy  of  very  careful  study.  Unfortunately  at  the  time  of  my  visit  its 
nature  was  not  realized,  and  being  taken  merely  as  a  phase  of  the  slaty 
quartzites  around,  it  was  not  given  any  especial  attention.  It  is  this  red  rock 
on  Pigeon  Point  that  has  caused  Foster  and  Whitney,  on  Mather's  author- 
ity,^ to  mark  this  point  as  granitic  on  their  large  map  of  the  Lake  Superior 
region.  The  Pigeon  Point  slates  maintain  a  constant  lakeward  dip  at  an 
angle  of  15°  to  20°,  and  even  25°,  the  usual  trend  being  more  to  the  north 
than  that  of  the  point  itself  The  high  dip  along  most  of  the  point  is  note- 
worthy. 

Cutting  the  slates  along  the  south  side  of  Pigeon  Point  are  a  number 
of  dikes  trending  often  nearly  east  and  west,  and  in  other  cases  in  a  north- 
erly direction.  Some  of  these  dikes  are  composed  of  the  same  orthoclase- 
gabbro  as  that  noted  in  the  dikes  on  Grand  Portage  and  Wauswaugoning 
bays.  One  dike  was  observed,  however,  in  which  the  rock  was  a  fine- 
grained olivine-gabbro,  or  luster-mottled  rock,  close  to  the  melaphyrs  of 
Pumpelly.  It  is,  in  fact,  save  as  to  greater  fineness  of  grain,  identical  with 
the  type  rock  of  the  Greenstone  of  Keweenaw  Point.  On  the  north  side  of 
Pigeon  Point  are  cliffs  of  a  coarse,  light-gray,  fresh  olivine-gabbro,  which 
appear  to  form  a  dike  of  large  size  through  much  of  the  length  of  the  point. 

Following  Pigeon  River  upwards  from  its  mouth,  slates  and  slaty 
quartzites  are  found  all  the  way  to  the  head  of  the  stream,  and  beyond  to 
Gunflint  Lake,  on  the  national  boundary  line.  The  general  dip  of  these 
rocks,  which  have  never  been  examined  in  detail,  is  southeast,  at  an  angle 
•of  some  10°,  which  would  give  a  thickness  of  some  10,000  feet  for  the  slate 
series  as  seen  along  the  national  boundary  line.  Peculiar  cherty  layers  are 
met  with  at  low  horizons  in  the  slates,  and  also  banded  lean  magnetic  iron 
ores  closely  similar  to  the  lean  iron  ores  of  the  Penokee  region  of  Wisconsin. 

By  all  the  geologists^  who  have  traversed  this  region  these  slates 
are   described  as  cut   by  immense  numbers  of  large  dikes,  which  often 

*  Seo  large  map  of  Lake  Superior,  accompanying  Foster  and  Whitney's  Report. 
sRicliard  Owen,  BeU,  N.  H.  Winchell. 


ANIMIKIE  EOCKS  OK  THUNDER  BAY.  371 

form  bold  ridges  several  hundred  feet  in  height,  crossing  the  country  in 
straight  lines.  Dikes  like  the  smaller  ones  of  Pigeon  Point  also  occur,  and 
also  great  overflows  and  interbedded  masses  of  basaltic  rocks.  At  the  first 
great  fall,  two  miles  above  the  mouth  of  the  river,  two  of  these  great  dikes 
are  to  be  seen.  One  of  these,  over  which  the  river  falls,  is,  according  to 
Richard  Owen,^  212  feet  wide,  and  its  course  northeast,  which  corresponds 
with  the  course  of  similar  great  masses  noted  on  the  west  side  of  Thunder 
Bay.  The  rock  of  this  great  dike  is  a  medium-grained  to  coarse-grained, 
light-gray,  rough-textured,  highly  crystalline  olivine-gabbro.  The  thin 
section  shows  exceedingly  fresh  and  abundant  olivine,  along  with  tabular 
anorthite,  very  fresh  diallage,  and  a  little  magnetite. 

Following  the  west  shore  of  Thunder  Bay  from  Pigeon  Point,  the 
slates  with  interbedded  and  overlying  masses  of  fine-grained  diabase  and 
coarse  gabbro,  and  dikes  of  the  same  rocks,  are  displayed  on  a  grand  scale. 
The  numerous  islands  in  the  mouth  of  the  bay  are  also  composed  of  the 
same  rocks,  many  of  the  smaller  islets  showing  only  the  dike  rock,  a 
whole  line  of  islands  marking  often  the  course  of  a  single  dike.  The  shore 
of  the  bays  south  of  the  valley  of  the  Kaministiquia  is  often  overlooked 
by  bold  clifis  of  slate  and  gabbro,  500  to  800  feet  in  height.  The  slates  of 
these  exposures  vary  from  soft,  thin-laminated,  black  or  dark-gray  clay- 
slates  to  hard,  ringing,  light-gray  quartzites  occurring  in  layers  several  inches 
in  thickness.  All  show  a  distinctly  fragmental  character  beneath  the  mi- 
croscope, but  many  of  the  quartzites  show  also  quartz  areas  like  those  of 
an  ordinary  gneiss.  The  dip  of  the  slates  continues  to  the  southeast  in  this 
distance,  the  trend  growing,  however,  more  and  more  northerly  as  the  valley 
of  the  Kaministiquia  is  approached,  while  the  dip  is  usually  flatter  than  10°, 
being  often  not  more  than  2°  or  3°. 

Of  the  dikes  in  this  distance  one  class,  including  the  broader  ones,  in 
which  the  rock  is  relatively  coarse-grained,  are  commonly  of  a  very  fresh 
olivine-gabbro  like  that  of  the  great  dike  at  the  falls  of  Pigeon  River,  which 
belongs  to  this  class.  These  larger  dikes  can  be  finely  seen,  for  instance,  on 
the  north  side  of  the  south  point  of  Big  Trout  Bay,  where  they  project  into 
ihe  water  in  great  buttress-like  forms,  100  to  200  feet  in  height  and  trend- 

'  Owen's  Geological  Survey  of  Wisconsin,  Iowa,  and  Minnesota,  p.  405. 


372       COPPER-BBAEING  EOCKS  OF  LAKE  SUPEEIOE. 

ing  N.  60°  E.     The  following  are  the  results  of  the  examination  of  some' 
thin  sections  of  the  rocks  from  some  of  these  larger  dikes. 

The  coarse-grained,  dark-gray,  rough-textured  rock  from  the  island  in 
the  mouth  of  the  south  arm  of  Pigeon  Bay  shows  fresh  diallage  predomi- 
nating, labradorite,  a  little  titanic  magnetite,  and  a  few  grains  of  olivine 
altered  to  a  brownish  substance.  The  rock  from  the  island  in  the  mouth 
of  Big  Trout  Bay  is  dark-gray,  medium-grained  to  coarse-grained,  and 
marked  by  very  large-sized  and  noticeable  luster-mottlings,  due  to  the^ 
large  augites.  The  thin  section  shows  exceedingly  fresh  diallage,  in  rela- 
tively great  areas;  very  fresh  anorthite,  numbers  of  crystals  of  which  min- 
eral are  often  inclosed  within  the  diallages;  olivine,  often  quite  fresh,  and. 
not  abundant  magnetite.  The  rock  is  an  exceedingly  fresh  one,  and  is  very 
close  to  that  of  the  Greenstone  of  Keweenaw  Point. 

Other  dikes,  nearly  as  large  as  these,  are  of  an  orthoclase-bearing  gab- 
bro,  allied  to  that  occurring  in  the  Keweenaw  Series  as  flows,  as,  for  instance, 
the  rock  of  a  broad  dike  on  the  north  side  of  Big  Trout  Baj-,  in  which  the 
constituents  are  orthoclase  and  oligoclase  much  reddened  and  clouded, 
and  now  and  then  replaced  by  secondary  quartz;  diallage  partly  fresh  and 
partly  altered  to  a  greenish  substance,  and  titanic  magnetite.  Apatite  is 
also  present,  and  the  whole  aspect  is  much  like  that  of  the  finer-grained 
Keweenawan  orthoclase-gabbro,  as,  for  instance,  that  of  the  ledges  just 
west  of  Lester  River,  and  especiall}^  that  of  the  Bohemian  Mountain,  just 
north  of  Lac  la  Belle,  Keweenaw  Point. 

The  coarse-grained,  light-gray  rock  from  just  north  of  the  mouth  of 
Pigeon  River  is  another  orthoclase-bearing-  gabbro,  in  a  much  fresher  con- 
dition and  with  relatively  less  orthoclase.  Abundant  and  rather  fresh  dial- 
lage, labradorite,  orthoclase,  magnetite,  a  little  secondary  quartz,  and  apa- 
tite are  the  constituents.  This  rock  is  much  like  the  Duluth  orthoclase- 
gabbro,  except  that  it  is  finer  in  grain.  It  is  a  kind  nearer  than  usual  to 
the  non-orthoclastic  gabbros.  A  pink-  and  black-mottled  rock  from  Victoria 
Island,  which  is  part  of  a  dike  continuing  to  the  northeast  through  Spar 
and  Thompson  islands,  is  at  the  other  extreme.  The  diallage  is  relatively 
sparse  and  wholly  altered  to  uralite.  The  feldspars  are  very  profoundly 
altered,  and  largely  replaced  by  secondary  quartz,  which  also  is  present  in 


ANIMIKIE  BOOKS  ON  THUNDER  BAY. 


373 


■quite  large  areas,  filling  corners  between  the  feldspars.  This  rock  is  as  near 
to  the  augite-syenites  as  to  the  orthoclase-gabbros.^  The  smaller  dikes, 
usually  under  twenty  feet  in  width,  are  of  a  denser  i-ock,  the  only  section  of 
which  examined  showed  a  very  highly  augitic  diabase  allied  to  the  ashbed 
kinds.  All  of  these  dikes  have  a  general  northeasterly  trend,  but  the 
amount  of  easting  lessens  as  they  are  followed  northeastward.  This  may 
be  seen  in  a  single  dike  in  the  case  of  the  Victoria  Island  group  of  islandt,, 
which  to  the  southwest  trends  N.  60°  E.  and  to  the  northeast  N.  52°  E. 

Of  the  crystalline  rocks  interbedded  with  the  slates,  the  greatest  masses, 
often  over  a  hundred  feet  in  thickness,  appear  to  be  olivine-gabbros  and 
orthoclase-gabbros,  identical  with  the  two  kinds  of  rocks  of  the  broader 
dikes.  These  masses  beyond  question  are  interbedded  with  the  slates,  and 
though  no  direct  connection  was  seen  between  them  and  the  dikes,  from 
the  nature  of  the  rocks  such  a  connection  is  probable  enough.  The  rock 
■of  the  great  mass  capping  the  bluff  just  north  of  Sucker  Brook,  for  instance, 
is  an  orthoclase-bearing  gabbro  very  close  to  the  dike  rock  of  the  mouth  of 
Pigeon  River,  and  on  the  same  bluff,  at  a  lower  level,  precisely  the  same 
orthoclase-bearing  rock  is  seen  cutting  the  slate,  the  probability  being  very 
strong  that  the  two  are  connected.  In  thin  section  this  rock  shows  labra- 
dorite,  some  orthoclase,  abundant  augite  or  diallage,  partly  much  altered 
and  partly  in  quite  fresh,  long,  twinned  blades,  and  very  abundant  magnetite 
in  long,  rod-like  forms. 


Fig.  33. — Bed  of  intrusive  diabase  in  Animikie  slates,  Pigeon  Bay,  Canada. 

Besides  these  greater  imbedded  masses,  thinner,  strongly  cross-colum- 
nar beds  of  a  dark  crystalline  rock  are  often  seen.  These  are  at  times 
-only  four  to  six  feet  thick.  They  generally  run  in  strict  parallelism  to  the 
included  slates,  but  in  one  or  two  places  were  noticed  distinctly  traversing 

■  To  judge  from  other  hand  specimens  brought  away,  the  one  from  which  ihls  slice  was  made 
does  not  represent  the  dike  of  Victoria,  Thompson,  and  Spar  islands  for  any  great  distance.  The  other 
specimens,  not  sectioned,  look  more  like  some  of  the  olivine-bearing  kinds. 


374       COPPEE-BEAEING  EOCKS  OP  LAKE  SUPEEIOE. 

the  slate  for  a  short  distance,  as  indicated  in  the  preceding  sketch,  which 
was  taken  on  the  north  side  of  the  south  arm  of  Pigeon  Bay.  The  slate- 
laminje  run  against  the  crystalline  rock  without  alteration  or  disturbance, 
and  the  whole  structure  is  an  unmistakable  one.  The  columnar  I'ock  is  black, 
excessively  fine-grained,  and  breaks  with  a  conchoidal  fracture.  Under  the 
microscope  it  shows  a  groundmass  in  which  are  recognizable  tabular  oligo- 
clases,  rounded  augite  particles,  magnetite,  and  some  non-polarizing  sub- 
stance. A  few  porphyritic  oligoclases  are  seen,  and  the  section  is  close  to 
that  of  the  so-called  ashbed-diabases.  It  might  very  well  be  a  very  fine- 
grained phase  of  the  coarser  orthoclase-gabbro  that  forms  the  neighboring 
large  dikes  and  overflows. 

The  researches  of  Bell  ^  and  Logan  ^  have  shown  that  an  area  to  the- 
west  of  Thunder  Bay,  bounded  on  the  north  by  a  line  from  Gunflint  Lake, 
on  the  national  boundary  line,  to  the  Grand  or  Kakabika  Falls,  on  the 
Kaministiquia,  is  very  often  underlain  by  the  same  series  of  slates  that  form 
the  west  side  of  Thunder  Bay,  intersected  here  by  the  same  grand  system  of 
dikes,  which  are  often  met  with  crossing  the  country  in  great  walls  or  form- 
ing bold  linear  ridges.  The  same  overflows  and  interbedded  masses  also 
occur,  and  often  form  the  tops  of  table-like  elevations.  Among  the  boldest 
of  these  is  the  line  of  blufi's  forming  the  south  side  of  the  lower  stretch  of 
the  valley  of  the  Kaministiquia,  McKay's  Mountain,  a  portion  of  this 
elevation,  rising  to  a  height  of  1,000  feet.  Throughout  all  of  this  area  the- 
flat  southeasterly  dip  prevails,  though  at  times  there  is  a  little  variation  from 
horizontality,  while  occasionally  there  is  a  slight  slant  northward  for  short 
distances. 

It  is  of  the  greatest  interest  in  this  connection  that  Bell  reports  the 
existence  as  part  of  the  slate  series  in  the  valley  of  the  Kaministiquia — as, 
for  instance,  at  "a  place  called  the  Algoma  mine" — of  "thinly -bedded,, 
flaggy,  hard,  dark-gray  sandstone,  largely  composed  of  particles  of  mag- 
netic iron,  and  weathering  to  a  rusty  color."  There  are  exposed  about 
twenty  feet  of  the  beds.  An  analysis  showed  37.73  per  cent,  of  metallic 
iron.     "  The  same  highly  ferruginous  sandstone,  dipping  very  slightly  east- 

'  Geology  of  Canadar,  1863,  pp.  C7-70. 

=  Geol.  Survey  of  Canada,  Report  for  1866-'69,  p.  321,  et  scq. 


ANIMIKIE  EOCKS  OF  THUNDER  BAY.  375 

northeast,  is  again  exposed  on  tlie  banks  of  a  brook  rather  more  than  a 
mile  north  of  the  Algoma  mine."  The  same  interest  attaches  to  the  occur- 
rences at  numbers  of  places  of  layers  of  concretionary  chert  and  "  bands  of 
a  reddish  jasper,"  and  dolomite. 

Bounding  this  slate  region  on  the  north  is  a  district  or  belt  of  granite 
and  contorted  gneiss.  From  Kakabika  Falls  the  southern  boundary  of  the 
gneiss  is  "  roughly  indicated  by  a  line  drawn  from  the  Great  Falls  to  a 
point  on  the  north  shore  of  Thunder  Bay,  about  six  miles  east  of  the  mouth 
of  Current  River."  All  along  this  shore  of  Thunder  Bay  I  found  the  con- 
ditions observed  on  the  west  shore  repeated,  save  that  the  great  bluffs  of 
slate  and  gabbro  are  wanting.  Numerous  interbedded  layers  of  black 
crystalline  rocks  were  observed,  however,  dipping  southeast  5°  with  the 
accompanying  slates,  and  affected  by  a  strong  cross-columnar  structure.  At 
Bare  Point,  for  instance,  three  miles  below  Prince  Arthur's  Landing,  and  a 
mile  below  the  mouth  of  Current  River,  there  is  a  thin  interstratified  bed, 
dipping  6°  to  8°  southeast,^  of  a  very  fine-grained,  dark-gray  diabase-por- 
phyrite,  with  a  groundmass  composed  of  tabular  plagioclases,  augite  gran- 
ules, and  non-polarizing  substance,  and  abundant  porphyritic  crystals  of 
labradorite,  and  a  few  of  altered  augite.  The  rock  is  closely  allied  to  the 
ashbed  traps  of  the  Keweenawan. 

East  from  Goose  Point,  nearly  to  the  head  of  Thunder  Bay,  the  older 
gneiss  is  generally  seen  at  the  bottoms  of  the  coves,  while  the  points  and 
islands  are  formed  of  the  slates  with  interstratified  crystalline  rocks.  A 
ten-foot  bed  of  cross-columnar,  light-gray,  coarse-grained  rock  from  one 
of  the  islands  nearest  below  Goose  Point  proves  under  the  microscope 
to  be,  as  was  expected,  a  very  fresh  olivine-gabbro,  with  abundant  oli- 
vine, diallage,  augite,  anorthite,  and  titanic  magnetite  as  the  constituents. 
A  twenty-foot  columnar  layer,  seen  on  some  of  the  islands  just  opposite 
Caribou  Island,  and  on  an  adjoining  point  of  the  mainland,  is  also  olivinitic 
diabase  or  gabbro,  but  finer-grained  than  the  last,  and  with  the  olivine 
much  altered.  Near  the  head  of  Thunder  Bay  some  of  the  cherty  layers^ 
of  this  series  are  to  be  seen.     Some  of  these  layers  show  chert  interlami- 

'Not  20°  to  25°,  as  Bell  has  it.     Op.  cit,  p.  325. 


376  OOPPEE-BBARIlSrG  EOOKS  OF  LAKE  SUPERIOR. 

nated  with  the  usual  slaty  material  of  the  series.     Others  show  a  peculiar 
concretionary  arrangement. 

Still  nearer  the  head  of  the  bay  come  in  the  dolomitic  sandstones  of 
the  overlying  Keweenawan.  Where  first  seen  these  sandstones  are  often 
conglomeratic,  holding  pebbles  of  the  older  slates  and  especially  of  the  chert 
layers  upon  which  they  directly  rest.  Along  the  southeast  shore  of  Thun- 
der Bay  the  slates  are  here  and  there  seen  at  the  water's  edge,  but  above 
them  the  cliffs  of  newer  sandstone,  with  one  or  more  beds  of  chert-conglom- 
erate, rise  to  a  height  of  200  feet  above  the  lake.  These  conditions  continue 
for  a  number  of  miles.  Before  reaching  the  deep  bay  on  the  north  side  of 
Thunder  Cape  a  number  of  narrow  dikes  were  passed  which  intersect  these 
slates.  Fifteen  of  these  dikes  were  counted  in  a  distance  of  about  ten  miles. 
They  are  usually  under  15  feet  across,  the  widest  not  exceeding  20  feet, 
while  some  are  only  4  feet  in  width.  Most  of  them  stand  vertically,  trend- 
ing from  N.  75°  E.  to  east  and  west.  One  was  noticed  dipping  60°  south, 
with  a  strong  columnar  cross-jointing  at  right  angles  to  this  direction.  It 
could  not  be  made  out  that  these  dikes  cut  the  overlying  sandstones.  The 
one  thin  section  of  the  dike-rock  examined  showed  an  olivine-free  diabase 
or  gabbro  of  m.xlerately  coarse  grain,  containing  predominating  augite, 
partly  fresh  and  partly  altered  to  greenish  material,  labradorite,  a  little 
orthoclase,  and  magnetite.  The  rock  lies  between  the  ohvinitic  gabbros 
and  the  orthoclase-gabbros  proper. 

At  the  deep  bay  just  above  Thunder  Cape  the  white  sandstones,  which 
have  heretofore  formed  the  cliffs  of  the  entire  southeast  side  of  the  bay, 
swing  away  from  the  coast  to  the  southeastward,  reappearing  on  the  lake 
front  near  Silver  Islet  Landing,  and  leaving  between  them  and  the  extremity 
of  the  cape  a  triangular  area,  which  is  occupied  entirely  by  the  slates  I 
have  been  describing,  capped  by  an  immense  overlying  mass  of  olivinitic 
gabbro,  upwards  of  200  feet  in  thickness.  The  whole  height  of  Thunder 
Cape  is  over  1,000  feet,  from  700  to  800  feet  of  which  must  be  occupied  by 
the  slates,  which  thus  rise  entirely  across  the  horizon  of  the  sandstones 
mentioned,  and  several  hundred  feet  higher  than  they  do.  Such  a  relation 
can  only  be  explained  by  supposing  a  great  fault,  as  Logan  did,  by  which 
these  slates  are  brought  up;  or  by  supposing  an  erosion  to  intervene  be- 


ANIMIKIE  EOCKS  OF  THUNDEE  CAPE.  377 

tween  the  slates  and  the  overlying  white  sandstones.  The  latter  appears 
to  me  beyond  doubt  the  true  explanation,  since  all  along  the  east  coast  of 
Thunder  Bay,  where  these  sandstones  are  found,  the  slates  would  naturally 
occur  in  large  thickness,  to  judge  from  the  exposures  on  the  north  and  west 
sides  of  the  bay.  Such  an  iinconformity  is  further  strongly  suggested  by 
the  fact  that  only  a  few  miles  to  the  east  of  Thunder  Bay,  on  Black  Bay, 
the  sandstones  are  found  lying  directly  against  the  older  gneisses,  without 
the  intervening  slate.  This  unconformity  has  been  previously  suggested 
by  Hunt,-'  whom,  however,  I  cannot  follow  in  his  supposition  that  both 
the  slate  and  overlying  sandstone  are  newer  than  the  Keweenawan. 

The  capping  rock  of  Thunder  Cape  is,  as  already  indicated,  an  olivin- 
itic  gabbro,  not  in  any  respect  differing  from  the  rock  described  before  as 
occurring  in  a  number  of  places  within  the  Keweenawan,  and  within  the 
formation  now  under  description,  as  dikes  and  overflows.  It  is  medium- 
grained  to  coarse-gi'ained,  light-gray  to  dark-gray,  and  very  highly  ciys- 
talline.  The  diallage  is  predominant  in  large  areas,  often  including  a  num- 
ber of  feldspars;  the  olivine  is  verj'  plenty  and  coarse,  and  often  fresh,  but 
as  often  altered  to  an  ocherous  product ;  while  the  plagioclase  is,  as  usual  in 
these  olivinitic  rocks,  anorthite,  as  shown  by  a  large  number  of  optical 
measurements.  The  rock  is  close  to  the  coarse  olivine-gabbros  of  Bad 
River,  Wisconsin,  standing  between  them  and  the  coarser  luster-mottled 
melaphyrs,  which,  as  shown  heretofore,  are  to  be  regarded  as  merely  a 
finer  phase  of  the  olivinitic  gabbros.  This  rock  has  been  described  by 
Macfarlane  under  the  name  of  hyperite,  with  feldspar  and  hypersthene  as 
the  chief  constituents,  and  hornblende  and  magnetite  as  accessories;  a  des- 
cription which  illustrates  well  the  unreliableness  of  rock  determinations 
made  without  the  use  of  the  microscope,  even  when  the  observer  is  a  skilled 
one  like  Macfarlane. 

The  slates  at  Thunder  Cape  are  quite  arenaceous,  in  some  layers  so 
much  so  as  to  have  received  tlie  name  of  sandstone  from  the  Canadian 
geologists.  They  are  from  very  fine-grained  to  aphanitic,  in  the  more 
shaly  layers,  and  vary  from  a  dark-gray  to  black  color.  The  layers  are 
commonly  very  thin,  and  show  a  fine  subordinate  lamination.     I  have  not 

'Second  Geological  Survey  of  Pennsylvania,  Azoic  Eocks,  Part  I,  p.  239. 


378       COPPEE-BEAEING  EOCKS  OP  LAKE  SUPEEIOE. 

made  any  section  of  them.  From  Macfarlane's  analyses/  however,  it  is 
evident  that  the  more  arenaceous  layers  are  made  up  chiefly  of  quartz- 
grains,  along  with  some  argillaceous  and  calcareous  matter,  while  the  shaly 
layers  contain  more  of  the  argillaceous  substance,  which,  from  the  presence 
of  alkalies,  is  plainly  in  the  nature  of  decomposed  and  partly  decomposed 
feldspars.  Macfarlane  also  considers  that  the  presence  of  some  carbona- 
ceous matter  is  included  in  the  loss  on  heating.  The  shaly  layers  never 
contain  any  lime  or  magnesia  carbonates,  but  these  carbonates  are  occa- 
sionally present  in  the  more  compact  layers  up  to  20  per  cent.,  or  more. 

Bounding  Thunder  Cape,  the  slates  are  found  continuing  as  far  as 
Silver  Islet  Landing,  at  which  place  they  are  overlain  by  the  white  and 
red  sandstones  of  the  Keweenaw  Series.  Here,  again,  they  are  only  a  short 
distance  above  the  water,  an  amount  of  sinking  being  thus  indicated  which, 
the  very  flat  southeastward  dip  will  not  account  for,  but  which,  as  already 
shown,  must  be  attributed  to  an  intervening  erosion. 

Between  Thunder  Cape  and  Silver  Islet  a  large  number  of  dikes  are 
seen  cutting  the  slates.  Only  one  of  these  dike  rocks,  which  appear  for 
the  most  part  to  be  the  same  as  those  which  form  the  numerous  dikes  of 
the  southwest  shore  of  Thunder  Bay,  already  described,  was  examined. 
This  is  the  rock  which  forms  the  dike  at  Silver  Islet.  It  is  a  nearly  black, 
rather  fine-grained  rock,  distinctly  composed  of  a  greenish-black  and  a 
white  mineral,  the  latter  being,  of  course,  the  feldspar.  According  to 
Macfarlane,  its  specific  gravity  is  2.7,  and  its  silica  content  53.34  per  cent. 
It  contains  5.02  per  cent,  of  water,  an  amount  indicating  a  considerable 
alteration;  and  this  indication  is  fully  borne  out  by  a  microscopic  study 
of  the  thin  section.  This  shows  tabular  plagioclases,  with  some  ortho- 
clases,  as  predominating  ingredients.  These  feldspars  are  all  much  dulled 
by  alteration,  and  are  often  penetrated  by  secondary  quartz.  In  many 
places  the  larger  feldspars  have  between  them  a  mass  of  smaller,  much 
crushed,  and  always  highly  altered  feldspars.  The  augitic  ingredient  is  only 
partly  fresh,  being  commonly  much  altered  to  ocher  and  uralite,  with  which 
alteration  is  connected  the  formation  of  some  magnetite.  Rather  abundant 
titanic  iron,  for  the  most  part  altered  to  its  chai-acteristic  gray  decomposi- 

'  Canadian  Naturalist,  New  Series,  Vol.  IV,  p.  37. 


DIKES  AT  SILVER  ISLET.  379 

tion-prodiict,  and  sparse  apatite,  complete  the  resemblance  between  this 
rock  and  the  finer  orthoclase-gabbros  of  the  Keweenawan. 

The  dikes  appear  also  to  cut  the  overlying  sandstone  for  some  little 
distance  beyond  Silver  Islet  Landing,  but  as  to  whether  they  have  actually 
filled  fissures  in  the  sandstone,  or  have  had  the  sandstone  deposited  around 
them,  I  did  not  satisfy  myself.  At  Silver  Islet  Landing,  for  instance,  one 
narrow  dike  was  noticed  cutting  the  slate,  and  the  overlying  chert-conglom- 
erate and  sandstone.  On  one  side  of  the  dike  the  junction  of  the  sandstone 
and  slate  is  twenty  feet  .higher  than  on  the  other,  which  fact  might  point 
either  to  unconformity  or  to  faulting  on  the  line  of  the  dike. 

So  far,  then,  as  I  have  been  able  to  learn  by  original  observation,  and 
by  reading  in  the  light  of  the  observations  the  accounts  of  others,  the 
Animikie  rocks  of  the  Pigeon  River-Thunder  Bay  region  consist  of  a 
great  series,  probably  upwards  of  10,000  feet  in  thickness,  of  quartzites, 
which  are  often  arenaceous,  quartz  slates,  argillaceous  or  clay  slates,  mag- 
netitic  quartzites  and  sandstones,  thin  limestone  beds,  and  beds  of  a  cherty 
and  jaspery  material.  With  these  are  associated,  in  great  volume,  and  in 
both  interbedded  and  intersecting  masses,  several  types  of  coarse  gabbro- 
and  fine-grained  diabase,  all  of  the  types  being  well  known  in  the  Kewee- 
naw Series. 

Any  one  familiar  with  the  descriptions  of  the  Thunder  Bay  region  by 
Logan,  Macfarlane,  and  Bell,  will  see  at  once  that  in  the  statements  of  the 
last  paragraph  I  have  departed  widely  from  the  conclusions  of  these  geol- 
ogists as  to  both  the  thickness  and  composition  of  the  Animikie  Group. 
Their  descriptions  are,  however,  misleading  on  these  points,  being  based 
almost  exclusively  upon  what  is  seen  on  Thunder  Bay,  which  lies  where 
only  a  relatively  small  thickness  is  exposed,  and  where  the  rocks  sometimes 
come  nearer  to  being  "sandstones  and  shales" — the  terms  used  by  them — 
than  elsewhere.  Logan,  moreover,  evidently  took  as  Huronian  that  part 
of  the  Animikie  Group  which  occupies  "the  coast  for  a  distance  of  ten 
miles  immediately  below  the  mouth  of  the  Kaministiquia  River  on  the 
north  side,  leaning  in  a  narrow  strip  against  the  gneiss  of  the  older  series."^ 
Bell  includes  these  rocks  with  the  rest  of  the  Animikie  Group,  where,  as  I 
have  indicated  above,  they  unquestionably  belong. 

1  Geology  of  Canada,  p.  63. 


380       COPPEK-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 

The  following  is  Bell's  stratigraphical  scheme  for  the  Animikie  Group, 
or  "Lower  Group  of  the  Upper  Copper  Bearing  Rocks."  ^ 

Feet. 

1.  Conglomerates  composed  of  pebbles  of  quartz,  jasper,  and  greenish  slate,  in  a 

greenish  arenaceous  matrix.  Seen  on  the  north  shore  of  Thunder  Bay.  Es- 
timated thickness 70 

2.  Chert  layers,  mostly  thin  and  having  a  ribbon-like  appearance  in  cross-section. 

The  mass  is  generally  dart,  but  some  light-colored  layers  occur.  Thin  beds 
of  dolomite  sometimes  separate  the  chert  layers  from  one  another,  and  argil- 
laceous layers  are  also  occasionally  interstratified;  while  bands  of  dolomite, 
which  are  themselves  sometimes  separated  by.  argillaceous  beds,  are  inter- 
stratified with  the  foregoing.  The  chert  bands  contaiTa  iron  pyrites  in  specks, 
nodules,  and  thin  interrupted  layers.  A  mineral  resembling  anthracite  also 
occurs  in  the.rocks  of  this  and  the  following  division.  Seen  at  the  eastern 
extremity  of  Thunder  Bay,  and  near  the  five-mile  post  on  the  Eed  Eiver 
road.    Estimated  thickness 300 

3.  Darkly-colored  massive  argillites  and  flaggy  black  shales,  the  mass  being  charac- 

terized by  numerous  vertical  joints,  running  in  two  directions,  and  dividing  it 
into  blocks  of  a  very  symmetrical  character.  The  shaly  portions  hold  regu- 
larly formed  spheroidal  concretions  of  various  sizes.  Trap  beds  are  associ- 
ated with  these  rocks  along  the  north  shore  of  Thunder  Bay,  at  the  Thunder 
Bay  mine,  and  in  the  township  of  Mclntyre.  The  shales  are  seen  on  this  part 
of  the  Kaministiquia  Eiver,  especially  at  the  Grand  Falls,  and  along  the  coast 
of  Lake  Superior,  between  Fort  William  and  Pigeon  Eiver,  while  an  example 
of  the  massive  may  be  seen  in  the  workings  of  the  Thunder  Bay  mine.  Es- 
timated thickness 450 

4.  Grey  argillaceous  sandstones  and  shales,  mostly  thinly  and  evenly  bedded,  fine- 

grained, and  slightly  calcareous.  Examples  of  both  of  these  rocks  may  be 
observed  on  each  side  of  Thunder  Cape,  and  in  the  township  of  Mclntyre.  In 
the  southern  part  of  this  township,  and  at  the  northwestern  corner  of  Neebing, 
bands  of  sandstone,  supposed  to  belong  to  this  division,  occur,  containing  a 
large  percentage  of  magnetic  iron  ore.    Estimated  thickness 400 

This  scheme,  which  is  pretty  much  that  of  Logan,  is  obtained  by 
building  up  from  the  exposures  near  the  underlying  gneiss  in  the  northeast 
corner  of  Thunder  Bay.  But  gneiss  and  granite  evidently  do  not  always 
come  against  the  same  horizon  of  the  Animikie  slates,  so  that  such  a  scheme 
Tvoiild  be  unsatisfactory  in  any  case,  even  were  its  truth  not  distinctly  dis- 
proved by  the  occvirrences  along  Pigeon  River  and  thence  west  and  south, 
where  it  is  plain,  from  the  constant  southward  dip  and  broad  area  occupied, 
i;hat  there  is  a  thickness  of  fully  10,000  feet.     Even  at  the  Silver  Islet 

'  Geol.  Survey  of  Canada,  Eep't  for  1868-'69,  p.  318. 


BELL  ON  THE  ANIMIKIB  GROUP.  381 

mine  the  hard  quartzitic  slates  have  already  been  carried  down  1,000  feet 
or  more  below  the  lake  level,  a  distance  which  would  make  the  total  thick- 
ness actually  measurable  in  that  vicinity  far  beyond  Bell's  total. 

There  are  other  serious  objections  to  Bell's  stratigraphical  scheme. 
The  conglomerates,  which  according  to  him  form  the  base  of  the  series  in 
the  northeast  angle  of  Thunder  Bay,  seem  to  me  rather — unless,  as  does 
not  appear  probable,  I  did  not  visit  the  place  described  by  him — to  belong 
to  the  overlying  white  sandstones  of  the  Keweenawan.  Then  he  fails  to 
recognize  the  quartzitic  character  of  the  body  of  the  formation,  while  the 
magnetitic  arenaceous  rock  of  the  township  of  Neebing,  which  he  places 
near  the  summit  of  the  formation  and  as  equivalent  to  the  Thunder  Cape 
beds,  I  should  place  many  thousand  feet  below.  These  magnetitic  beds  are 
the  same  as  those  of  the  Mesabi  Range  of  the  Vermillion  Lake  region  of 
Minnesota,  of  Gunflint  Lake  on  the  national  boundary,  and  of  Pokegoma 
Falls  on  the  Mississippi  River,  and  belong  much  nearer  the  base  than  the 
summit  of  the  formation.  It  is  very  difficult  to  see  how  Bell  could  paral- 
lelize the  Thunder  Cape  beds,  manifestly  near  the  top  of  the  formation, 
with  beds  lying  twenty  miles  to  the  northwest,  when  he  at  the  same  time 
admits  a  general  southeasterly  dip  throughout  the  whole  area. 

Then,  again,  the  great  volume  of  included  beds  of  gabbro  and  diabase 
is  almost  entirely  ignored.  In  the  third  division  of  his  scheme  it  is  said 
that  "trap  beds  are  associated  with  these  rocks  along  the  north  shore  of 
Thunder  Bay,  at  the  Thiinder  Bay  mine,  and  in  the  township  of  Mc- 
Intyre,"^  and  yet  the  whole  volume  of  this  division  is  placed  at  only  450 
feet.  But,  as  seen,  all  the  way  from  Wauswaugoning  Bay  on  the  Minne- 
sota coast  to  the  south  side  of  the  Kaministiquia  Valley,  and  again  in  the 
Pigeon  River  country  of  Minnesota,  these  included  beds  must  aggregate 
over  a  thousand  feet,  while  they  may  be  much  more  than  this.  This  im- 
portant omission  is  probably  to  be  explained  by  Bell's  having  regarded 
all  of  these  beds  as  part  of  the  so-called  "crowning  overflow,"  which  is 
supposed  to  have  taken  place  after  the  accumulation  and  removal  of  the 
thousands  of  feet  of  newer  Keweenawan  or  copper-bearing  strata. 

The  only  evidence  of  any  such  general  overflow  consists  in  the  simi- 

lOp.  cit.,  p.  319. 


"382       GOPPEE-BBAEING  EOOKS  OF  LAKE  SUPEEIOE. 

larity  of  the  crystalline  rocks  fovind  capping  hills  in  different  parts  of  this 
region.  Not  only  is  it  much  more  in  accordance  with  the  geology  of  the 
entire  Lake  Superior  region  to  suppose  these  occurrences  to  represent  many 
different  flows,  but  there  is  distinct  evidence  that  they  do  so  in  many  cases. 
This  evidence  consists  in  part  in  actual  visible  interstratification  with  the 
slates,  in  some  places,  of  great  beds  of  olivinitic  gabbro,  identical  both 
macroscopically  and  microscopically  with  the  rock  capping  Thunder  Cape. 
Another  evidence  is  the  very  great  irregularity  of  level  that  this  supposed 
■flow  must  occupy,  the  height  at  which  it  is  found  varying  back  and  forth 
through  distances  of  several  hundred  feet.  A  yet  stronger  evidence  is 
found  in  the  general  structural  character  of  the  region,  by  virtue  of  which 
-each  heavy,  enduring,  crystalline  rock  layer  constitutes  a  ridge  with  a  long 
front  slope  and  a  precipitous  back  slope.  This  structure  is  especially  well 
marked  in  the  region  of  the  Pigeon  River,  Minnesota.^  Even  a  general 
•distant  view  of  the  belts  to  the  west  and  south  from  the  head  of  Thunder 
Bay  is  strongly  suggestive  of  this  structure. 

I  have  no  confidence,  then,  in  the  existence  of  any  one  crowning  over- 
flow, as  supposed  by  Logan,  Bell,  and  Macfarlane.  Bell  goes  so  far  as  to 
identify  all  exposures  of  rock,  lithologically  similar  (macroscopically)  to 
the  Thunder  Cape  gabbro  as  parts  of  the  crowning  overflow,  even  as  far 
north  as  the  region  about  Lake  Nipigon.  Lithological  similarity  is  no 
•evidence  of  original  continuity  in  this  case,  for  all  through  the  Keweenaw 
Series  of  the  North  and  South  Shores,  similar  olivinitic  gabbros  are  visibly 
interstratified  at  all  sorts  of  horizons. 

As  already  indicated,  the  Animikie  slates  have  been  traced  along  the 
national  boundary  as  far  as  Gunflint  Lake  by  Bell,^  and  more  recently  by 
N.  H.  Winchell.*  Thence  their  northern  boundary  extends  southwest  from 
Grunflint  Lake.  For  many  miles  farther  southwest  the  region  traversed  by 
these  rocks  has  not  been  examined  by  any  geologist,  but  their  continuance 
here  cannot  be  doubted,  for  in  the  Mesabi  Range,  in  T.  60,  R.  13  W., 
Minnesota,   they  show  in  full  force.      By  the  kindness  of  Prof.  A.   H. 

'N.  H.  Wiuchell,  in  Ninth  Annual  Report  of  the  Geological  and  Natural  History  Survey  of  Min- 
-nesota.    Minneapolis,  1881.    p.  76. 

» Report  of  the  Geological  Survey  of  Canada,  for  1872-'73,  pp.  92-94. 

=  Ninth  Annual  Report  of  the  Geological  and  Natural  History  Survey  of  Minnesota,  pp.  62,  83. 


ANIMIKIE  ROCKS  OF  THE  MESABI  IRON  RANGE.  383 

Chester,  I  have  a  full  collection  of  rocks  from  this  place.  He  describes  the 
Mesabi  Range  proper  as  a  backbone  of  red  granite,  south  of  which  occur 
the  Animikie  slates,  which  are  here  nearly  horizontal,  dipping  2°  or  3°  to 
the  southeast.  The  exposures  are  large,  presenting  often  a  bold  cliff  side 
of  nearly  horizontal  layers,  and  have  been  traced  by  Professor  Chester  a 
distance  of  some  six  miles  along  the  strike,  in  the  southwest  part  of  T.  60, 
R.  12  W.,  and  the  southeast  part  of  T.  60,  R.  13  W.  The  principal  rock 
is  an  arenaceous  gray  quartzite  or  quartz-slate,  impregnated  with  magnetite, 
which  is  often  aggregated  into  bands  of  some  richness.  The  specimens 
tindly  given  me  by  Professor  Chester  are  indistinguishable  from  much  of 
the  magnetitic  quartzite  of  the  Penokee  Range  of  Wisconsin.^ 
Professor  Chester  writes  me  with  regard  to  these  rocks: 

On  the  south  side  of  this  range  [the  Mesabi  Granite  Range]  magnetic  iron  is 
found  in  abundance,  particularly  in  T.  60,  R.  12  W.  and  13  W.  Here  the  ore  body 
is  of  some  magnitude,  sufQcient  to  induce  a  considerable  exploration  in  this  part,  but 
DO  large  bodies  of  ore  have  been  found  of  sufflcient  value  to  warrant  developments. 
The  ore  is  magnetite,  quite  similar  to  that  of  the  Penokee  Range.  It  lies  in  nearly 
horizontal  beds,  interstratified  with  quartzite,  so  as  to  make  it  quite  impossible  to  sort 
out  any  quantity  of  good  ore.  In  places  where  it  is  exposed  on  the  surface  for  some 
distance,  smoothed  and  polished  by  glacial  action,  at  first  glance  it  would  be  con- 
sidered valuable,  but  closer  inspection  shows  that  it  is  mixed  with  rock  so  as  to  have 
a  striped  appearance.  This  is  particularly  noticeable  in  the  sides  of  the  pits  sunk  down 
in  it.  A  layer  that  shows  the  best  ore  on  one  side  of  the  pit  is  white  quartzite  on  the 
other.  The  layers  of  good  ore  are  thin,  about  six  inches  being  the  thickest  found  that 
showed  good  ore  all  across,  and  would  average  60  per  cent,  of  iron.  A  cliff  in  Sec. 
17,  T.  60,  R.  12  W.,  shows  as  follows,  beginning  at  top:  Ore  of  about  50  per  cent., 
2  inches;  ore  with  some  bands  of  60  per  cent.,  1  foot  6  inches;  ore  much  mixed  with 
quartzite,  2  feet  6  inches;  ore  about  58  per  cent.,  2  inches;  ore  and  quartzite  poorer 
than  above,  10  feet  8  inches;  quartzite  with  very  little  ore,  10  feet.  A  great  many 
such  sections  were  examined,  and  about  two  feet  of  ore  was  the  thickest  layer  found. 
The  rocks  are  quartzites  and  slates,  more  or  less  altered.  They  lie  more  nearly  hori- 
zontally than  any  other  beds  I  have  ever  examined,  changing  dip  very  slightly  from 
point  to  point,  giving  the  idea  of  a  sort  of  undulating  surface,  yet,  on  the  whole,  dip- 
ping slightly  to  the  south. 

This  description,  save  as  to  the  horizontahty,  would  do  as  well  for 
bundreds  of  exposures  on  the  Penokee  Range  of  Wisconsin. 

Still  farther  southwest  from  here  the  same  magnetitic  rocks  are  exposed 
-on  the  Mississippi  River  at  Pokegoma  Falls  in  the  eastern  part  of  T.  55 

'See  Geology  of  Wisconsin,  Vol.  Ill,  pp.  118-136. 


384       COPPEE-BEAEING  EOCKS  OF  LAKE  SUPBEIOE. 

R.  26  W.,  and  again  on  Prairie  River  in  sections  33  and  34,  T.  56,  R.  25  W. 
The  rocks  at  these  points  are  described  as  arenaceous  quartzites  with  mag- 
netite, dipping  gently  southward,  by  Mr.  Bailey  Willis,  of  the  United  States^ 
Geological  Survey. 

The  line  drawn  from  the  Mesabi  Iron  Range  to  Pokegoma  Falls  indi- 
cates approximately  the  northern  limit  of  the  Animikie  rocks.  What  be- 
comes of  these  rocks  still  farther  west  is  unknown.  South  of  this  line^ 
howevei',  are  immense  exposures  of  slates  on  the  Saint  Louis  River,  begin- 
ning one  mile  above  Knife  Falls,  in  Sec.  14,  T.  49,  R.  17  W.,  and  contin- 
uing thence  down  stream  to  the  S.  W.  I,  Sec.  11,  T.  48,  R.  16  W.,  several 
miles  below  Thompson,  where  they  disappear  underneath  the  red  sand- 
stones, as  previously  described.  These  slates  dip  constantly  southward, 
more  often  at  a  higher  angle  than  45°  than  at  a  lower.  The  strike  is  not 
exactly  east,  but  more  or  less  north  of  east,  the  northing  increasing  in 
amount  to  the  eastward.  Throughout,  the  slate  is  affected  by  a  strong 
cleavage,  which  stands  vertically,  and  trends  with  the  strike  of  the  rocks. 
Across  these  slates  at  right  angles  to  the  trend  is  a  distance  of  several  miles,, 
which,  with  the  high  angle  of  dip,  would  indicate  an  enormous  thickness, 
unless  there  are  some  folds  here,  which,  from  the  presence  of  slaty  cleavage,, 
would  seem  probable.  South  and  west  from  the  Saint  Louis  the  slates  are- 
known  to  continue  for  some  miles,  but  their  extent  in  that  direction  has  not 
vet  been  worked  out.  These  slates  are  clay-slates,  light-  and  dark-gray  and 
greenish  being  the  prevailing  colors.  Often  they  merge  into  and  include 
beds  of  quartz-slate,  which  under  the  microscope  shows  the  quartz  largely 
as  a  fragmental  material,  but  also,  in  considerable  measure,  as  an  original- 
constituent.  Great  dikes  are  seen  at  several  points  cutting  the  slates,  trend- 
ing northward,  and  composed  of  a  moderately  coarse,  strongly  augitic  dia- 
base or  gabbro. 

The  Saint  Louis  River  slates  are  plainly  the  same  as  the  Thunder  Bay 
slates,  but  affected  by  a  slaty  cleavage.  They  are  lithologically  the  same, 
and  are  cut  by  the  same  great  dikes.  They  are  the  upper  portion  of  the 
series  of  which  the  Mesabi  iron  beds  form  the  lower  portion. 

The  rock  series  thus  described  under  the  name  of  the  Animikie 
Group — a  name  first  used  by  Hunt,^  and  referring  to  the  Indian  name  of 

'  Trans.  Amer.  Inst.  Mliing  Engineers,  I,  339. 


GEOLOGICAL  POSITION  OP  THE  ANIMIKIE  EOCKS.  385 

Thunder  Bay— has  been  placed  by  Logan^  and  BelP  at  the  base  of  the 
copper-bearing  series,  the  former  geologist,  in  his  Geology  of  Canada, 
speaking  of  it  as  the  "Lower  Group  of  the  Upper  Copper-bearing  rocks." 
Macfarlane^  and  Hunt,"  on  the  other  hand,  regard  the  Animikie  rocks  as 
altogether   newer   than   the   Keweenawan,    which   they   are   believed   to 
overlie  unconformably.     Hunt  says  that  they,  together  with  the  overlying 
sandstone  of  the  east  side  of  Thunder  Bay,  may  even  be  mesozoic  for  all 
the  evidence  there  is  to  the  contrary.^     There  can  be  no  doubt,  however, 
that  Logan  and  Bell  are  correct  in  placing  the  Animikie  Group  beneath 
the  copper-bearing  or  Keweenawan  rocks  proper.     This  seems  plainly  in- 
dicated by  the  exposures  the    east    side  of  Thunder  Bay,  and  thence  to 
Black  and  Nipigon  bays,  but  to  any  one  approaching  from  the  southwest 
along  the  Minnesota  coast  becomes  so  absolutely  certain  as  to  admit  of  no 
question  at  all.     Not  only  does  one  in  descending  the  lower  part  of  the 
Minnesota  coast  constantly  cross  the  Keweenawan  beds  in  descending  order 
until  the  Animikie  slates  are  reached  at  Grand  Portage  Bay,  but  on  the 
large  island  at  the  -mouth  of  this  bay  he  may  see  slates  underlying  Kewee- 
nawan diabases  in  a  continuous  cliff  exposure. 

So  far,  then,  I  agree  with  Logan  and  Bell;  but  from  their  view  that  the 
Animikie  rocks  are  but  a  downward  continuation  of  the  Keweenawan  I 
must  dissent  altogether.  The  erosion  which  intervened  between  the  two 
formations,  as  indicated  by  the  already  described  occurrences  on  the  east 
side  of  Thunder  Bay,  and  again  by  those  at  the  east  end  of  the  Minnesota 
coast,  and  the  pronounced  lithological  contrast  between  the  sedimentary 
beds  of  the  two  groups,  both  bear  heavily  against  any  such  view.  Moreover, 
the  essential  identity  between  the  Animikie  rocks  and  those  of  the  Penokee 
region  of  Wisconsin,  and  their  close  similarity  to  the  South  Shore  iron- 
bearing  schists  generally,  make  it  plain  to  me  that  we  are  dealing  here  with 
the  North  Shore  equivalents  of  the  South  Shore  iron-bearing  formation. 

We  have  in  the  Animikie  series,  as  in  the  South  Shore  Huronian,  sili- 
ceous  schists,  quartzites,  dolomites,  chert  beds,  and  magnetite-bearing  quartz- 

'  Geology  of  Canada,  1863.  " 

^  Eeport  of  the  Geological  Survey  of  Canada  for  1866-'69,  p.  318. 
=  Canadian  Naturalist,  New  Series,  III,  252,  IV,  38. 
<  2d  Geol.  Surv.  Pennsylvania,  Azoic  Eocks,  Part  I,  p.  240. 
"OiJ.  CT<.,p.  241. 
25  L  S 


386       COPPEE-BEAEING  EOOKS  OP  LAKE  SUPEEIOE. 

ites  and  quartz-slates.  In  the  Animilde  Group  these  alternate  with  great 
interbedcled  flows  of  diabase  and  gabbro,  and  are  intersected  by  great  num- 
bers of  dikes  of  the  same  rock,  the  equivalents  of  which  are  to  be  found  in 
the  interbedded  and  intersecting  diabases  of  the  South  Shore  Huronian.  The 
thickness  of  the  Animilde  rocks  is  a  great  one,  and  is  comparable  only  with 
that  of  the  South  Shore  Huronian.  The  affinity  of  the  Animikie  rocks  is 
especially  strong  with  the  schists  of  the  Penokee  Range  of  Wisconsin,  and 
these  I  have  elsewhere  shown  to  be  essentially  the  same  as  the  iron-bearing 
schists  of  Marquette.  The  Animikie  series  presents  the  smallest  number  of 
rock  kinds,  the  Penokee  series  a  lai'ger  number,  and  the  Marquette  and  Me- 
nominee Huronian  the  most.  Some  of  these  diflPerences  may  be  made  to 
disappear  on  closer  study  of  the  still  quite  imperfectly  known  Animikie  rocks; 
while  much  of  the  greater  variety  in  rock  kinds  on  the  South  Shore  is  the 
result  of  metasomatic  change  upon  the  included  eruptive  rocks.  Even  the 
"diorites"  of  the  South  Shore  Huronian  are  in  all  probability  merely  altered 
diabases.  The  greater  variety  in  the  kinds  of  schistose  rocks  is  directly 
connected  with  greater  amount  of  disturbance,  and  is  the  result  of  the  obscure 
and  ill-understood  process  known  as  metamorphism. 

THE    ORIGINAL    HURONIAN. 

The  original  Huronian  of  Logan  and  Murray  forms  the  north  shore  of 
Lake  Huron  from  the  Saint  Mary's  River  eastward.  Logan's  description  of 
1863^  makes  the  rocks  have  a  total  thickness  of  18,000  feet,  composed  as 
indicated  in  the  following  scheme,  which  is  given  in  ascending  order  :^ 

Feet. 

1.  Gray  quartzite,  thin  bedded  in  some  parts ;  the  thickness  is  very  doubtful,        500 

2.  Greenish,  red-weathering  chloritic  and  epidotic  slates,  interstratified  with 

trap-like  beds;  of  this  mass  also  the  thickness  is  very  doubtful, 2, 000 

3.  White  quartzite,  the  color  sometimes  passing  into  gray ;  the  rock  is  princi- 

pally fine  grained,  but  the  granular  texture  is  often  lost,  and  great 
masses  of  it  become  vitreous  quartzite.  The  rock  on  the  other  hand 
often  becomes  coarse  grained  and  assumes  the  character  of  a  conglom- 
erate from  the  presence  of  pebbles,  consisting  chiefly  of  white  quartz, 
varying  from  the  size  of  duck-shot  to  that  of  musket-balls.  The  beds, 
which  are  generally  massive,  are  frequently  separated  by  layers  of 
fine  grained  greenish-gray  siliceous  slate,  and  considerable  masses  of 
greenstone  are  frequently  intercalated  in  different  parts  of  the  whole 
thickness, 1, 000 

'  Geology  of  Canada,  1863,  p.  55.  ^ 


LOGAN  ON  THE  OEIGINAL  HUEONIAN.  387 

Feet. 
4.  Slate  conglomerate,  composed  of  pebbles  of  gueiss  and  syenite  held  in  an 

argillo-arenaceous  cement  of  a  graj^  or  more  frequently  of  a  greenish 
color,  the  latter  arising  apparently  from  the  presence  of  chlorite.  The 
pebbles,  which  are  of  reddish  and  gray  colors,  vary  greatly  in  size,  being 
sometimes  no  larger  than  swan-shot,  and  at  others  bowlders  rather 
than  pebbles,  measuring  upwards  of  afoot  in  diameter;  the  propor- 
tions of  these  also  vary  much ;  they  sometimes  constitute  nearly  the 
whole  mass  of  the  rock,  leaving  but  few  interstices  for  the  matrix,  and 
sometimes  on  the  contrary,  they  are  so  sparingly  disseminated  through 
considerable  masses  of  the  matrix  as  to  leave  spaces  of  several  feet 
between  neighboring  pebbles,  which  may  be  still  several  inches  in  dia- 
meter; with  the  pebbles  of  gneiss  and  syenite  are  occasionally  asso- 
ciated some  of  different  colored  jaspers  and  others  of  quartz.  The 
matrix  appears  to  pass  on  the  oue  hand  into  a  gray  quartzite  by  an 
increased  proportion  of  the  arenaceous  grains,  and  on  the  other  into  a 
thin  bedded,  dark  greenish,  fine  grained  slate,  which  is  sometimes 
very  chloritic.  A  third  form  assumed  by  the  matrix  is  one  in  which  it 
is  scarcely  distinguishable  from  fine  grained  greenstone.  In  the  slate 
the  stratification  is  often  marked  by  slight  differences  of  color,  in  the 
direction  of  which  it  is  occasionally  cleavable ;  the  bands  in  other  in- 
stances are  firmly  soldered  together,  but  in  both  cases  joints  usually 
prevail,  dividing  the  rock  into  rhomboidal  forms  which  are  sometimes 
very  perfect.  Very  heavy  masses  of  greenstone  are  generally  inter- 
stratified  in  the  rock,  which  do  not  seem  confined  to  any  one  strat- 
igraphical  place, 1, 280 

6.  Limestone,  usually  of  a  compact  texture,  but  sometimes  partially  granular; 
the  colors  are  green,  drab,  and  dark  gray,  the  latter  two  prevailing. 
Some  beds  are  occasionally  met  with  of  a  dull  white,  with  a  waxy  luster 
in  fresh  fractures;  these  weather  to  a  yellowish-brown  on  the  exterior, 
and  appear  to  be  dolomitic.  The  whole  band  is  in  general  thin  bedded, 
and  a  diversity  of  character  in  the  laj'ers,  probably  arising  from  the 
presence  of  more  or  less  siliceous  matter,  causes  the  surface  of  the 
weathered  blocks  to  present  a  set  of  bold  but  minute  ribs  of  various 
thicknesses,  which  when  the  beds  are  much  affected,  as  they  often  are 
by  diminutive  undulations,  contortions,  and  dislocations,  exhibit  on  a 
small  scale  a  beautiful  representation  of  almost  all  the  accidents  that 
occur  in  stratification,  affording  very  excellent  ready-made  geological 
models, 300 

6.  Slate  conglomerate  of  the  same  general  character  as  that  beneath  the  lime- 
stone, but  the  pebbles  are  not  so  large;  it  is  interstratified  with  beds 
of  reddish  and  gray  quartzite  and  layers  of  fine  grained,  greenish-black 
and  light  olive-green  siliceous  slate,  some  of  which  yield  hones  of  a 
very  fine  description;  considerable  masses  of  greenstone  are  inter- 
stratified in  various  parts  of  the  deposit, 3, 000 


388       COPPEE-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 

Feet. 

7.  Eed  quartzite,  inter  stratified  with  masses  of  greenstone;  tlie  quartzite  is  in 

general  granular  and  of  moderately  fine  texture,  but  it  occasionally  be- 
comes a  fine  conglomerate.  The  color  is  sometimes  only  a  light  tinge 
of  red  and  at  others  a  decided  red,  seemingly  derived  from  minute  and 
thickly  disseminated  spots  or  from  a  diffused  tinge  of  an  orange-red, 
probably  due  to  the  presence  of  iron,  but  the  spots  are  sometimes  of  a 
larger  size,  and  so  arranged  as  to  give  to  the  rock  a  speckled  aspect. 
The  rock  is  in  general  thick  bedded ;  some  of  the  beds  shew  oblique 
elementary  layers,  or  what  is  commonly  called  false  bedding,  and  the 
surfaces  of  other  beds  display  well  defined  rijjple-marks;  masses  of 
greenstone  are  interstratified  in  the  deposit,  some  of  them  of  great 
thickness, 2,300 

8.  Eed  jasper  conglomerates.    The  rock  is  sometimes  a  moderately  fine  grained 

white  quartzite,  often  with  a  vitreous  aspect,  but  it  very  commonly 
becomes  coarse  grained  and  assumes  the  character  of  a  conglomerate, 
the  pebbles  of  which  vary  from  the  size  of  a  duck-shot  to  that  of 
grape  and  canister;  these  pebbles  are  almost  entirely  either  of  opaque 
white  vitreous  quartz  or  various  colored  jaspers;  some  of  them  are 
lydian  stone,  some  hornstone,  and  other  varieties,  and  many  of  them 
are  banded,  shewing  their  derivation  from  a  more  ancient  stratified 
rock.  The  pebbles  are  often  displayed  at  the  top  or  bottom  or  in  the 
middle  of  fine  grained  beds;  they  are  sometimes  arranged  in  thick 
bands,  and  blood-red  jaspers  often  predominating  in  a  nearly  pure 
white  base  produce  a  brilliant,  unique,  and  beautiful  rock.  Consider- 
able masses  of  greenstone  are  intercalated  in  different  parts  of  the 
group,  - 2, 150 

9.  White  quartzite,  very  frequently  of  a  vitreous   aspect;   in  considerable 

thicknesses  of  the  rock  the  bedding  appears  sometimes  to  be  so  com- 
pletely obliterated,  and  the  whole  mass  presents  so  great  a  uniformity 
of  appearance,  that  it  becomes  quite  imisossible  to  ascertain  the  dip 
or  strike,  or  to  distinguish  joints  from  beds,  but  in  other  parts  massive 
beds  are  separaited  by  thin  siliceous  layers  resembling  chert,  and  green- 
stones occur  intercalated  between  different  masses  of  the  deposit, 2, 970 

10.  Yellowish  chert  in  thin  and  very  regular  beds,  interstratified  with  layers 

of  green,  buff',  and  gray  siliceous  limestone,  and  green  and  pale  drab 
compact  siliceous  slate,  with  a  stratum  of  red  and  yellowish  fine 
grained  sandstone  at  the  bottom, 400 

11.  White  quartzite,  frequently  of  vitreous  aspect,  and  occasionally  mottled 

with  lead-gray  patches, 1, 500 

12.  Yellowish  chert  and  impure  limestone,  similar  in  its  general  aspect  to  the 

previous  chert  band, 200 

13.  White  quartzite  imperfectly  examined, 400 

18, 000 


LOGAN  ON  THE  ORIGINAL  HUEONIAN.  389 

Of  the  interstratified  greenstone  flows  of  the  above  series,   Logan 
says :  ^ 

The  igneous  rocks  which,  as  overflows,  it  will  be  conveuient  to  consider  constitu- 
ent parts  of  the  stratified  series,  may  be  classed  as  a  whole  under  the  denomination 
of  greenstone  or  diorite.  The  masses  are  sometimes  very  great,  and  in  such  cases  the 
rock  usually  consists  of  a  greenish-white  feldspar,  and  dark  green  or  black  horn- 
blende. The  feldspar  is  sometimes  however  tinged  with  red,  and  the  diorite  then 
appears  to  pass  into  syenite  by  the  addition  of  a  sparing  amount  of  quartz.  These 
two  forms  of  the  diorite  are  almost  always  highly  crystalline,  and  in  general  not  very 
fine  grained.  Sometimes,  however,  the  greenstone  displays  a  fine  texture,  and  in  such 
cases  a  largo  amount  of  it,  more  particularly  in  the  lower  part  of  the  series,  frequently 
holds  much  disseminated  chlorite,  giving  a  very  decided  green  color.  Portions  of  it 
are  found  containing  so  great  a  proportion  of  this  mineral  as  to  yield  with  facility  to 
the  knife,  aifording  to  the  aborigines  an  excellent  material  for  the  manufacture  of  their 
calumets  or  tobacco-pipes.  In  addition  to  the  chlorite,  epidote  is  a  prevailing  mineral 
in  this  description  of  rock,  associated  with  which  an  amygdaloid,  already  alluded  to, 
is  in  one  place  seen,  some  of  the  cellules  of  which  contain  quartz,  others  calcspar  and 
bitterspar,  and  some  few  of  specular  iron.  The  amygdaloidal  trap  is  very  distinctly 
arranged  in  layers,  which,  though  they  do  not  exceed  two  or  three  in  number,  give, 
with  beds  of  porphyritic  greenstone  containing  large  crystals  of  feldspars,  occurring 
near  the  amygdaloid,  a  stratified  aspect  to  the  whole  of  the  mass  of  the  trap  associated 
with  them.  No  such  decided  appearances  of  stratification  have  been  met  with  in  the 
more  crystallinp  greenstones.  They  usually,  however,  display  parallel  planes  of  divis- 
ion in  several  directions,  and  it  frequently  happens  that  some  of  these  parallel  planes 
are  only  moderately  inclined;  but  there  have  not  been  observed  on  the  surfaces  or  in 
the  character  of  the  rock  any  distinct  evidences  of  stratification  or  of  successive 
deposit,  and  no  columnar  structure  at  right  angles  to  any  set  of  planes  such  as  some- 
times so  clearly  marks  an  overflow.  It  is  therefore,  in  most  instances,  only  by  a  refer- 
ence to  its  immediate  relation  to  the  sedimentary  rocks  on  each  side  that  the  general 
attitude  of  any  band  of  the  greenstone  can  be  made  out. 

Independent  of  the  overflows,  igneous  rocks  are  connected  with  the  formation  in 
intrusive  masses.  These  intrusive  masses  consist  of  greenstone  and  granite.  The 
intrusive  greenstones  do  not  seem  to  difter  much  in  mineral  character  from  those 
composing  the  overflows:  they  constitute  dikes  which  run  in  so  many  directions  that 
it  is  difficult  to  determine  the  prevailing  ones.  These  dikes  vary  in  breadth  from  a 
few  inches  to  several  hundred  feet;  they  cut  all  the  stratified  rocks  of  the  series, 
igneous  as  well  as  sedimentary,  splitting  into  branches  which  often  join  one  another 
and  inclose  great  fragments  and  masses  of  strata.  The  intrusive  granite,  in  so  far  as 
observed,  is  in  general  of  a  decided  red  color,  arising  from  the  presence  of  a  largely 
preponderating  quantity  of  red  feldspar,  which  is  mingled  with  translucent  white 
quartz :  mica  is  not  very  abundant,  and  hornblende  sometimes  accompanies  or  replaces 
it.  From  large  masses  of  the  rock  however  both  these  minerals  are  often  wholly 
absent,  but  epidote  in  general  forms  a  constituent,  sometimes  in  great  abundance. 

'  Geology  of  Canada,  1863,  p.  57. 


390       COPPEE-BEAEING  EOCKS  OF  LAKE  SUPEEIOE, 

In  regard  to  the  nature  of  these  greenstones,  it  is  to  be  said  that  they 
are  not  improbably  diabases,  no  microscopic  analyses  having  been  made  of 
them,  while  the  descriptions  of  kinds  carrying  much  red  feldspar  are 
certainly  suggestive  of  the  orthoclastic  gabbros  of  Chapter  III  of  this 
memoir. 

The  red  granitic  rocks  mentioned  as  intrusive  are  also  suggestive  of 
the  red  granitic  porphyries  and  augite-syenites  of  the  Keweenawan.  Sim- 
ilar granites  or  granitic  porphyries,  according  to  Norwood  and  Winchell,  are 
found  among  the  Animikie  slates  of  the  Thunder  Bay  country.  The  rocks 
of  Lake  Huron,  according  to  Logan's  sections,  are  bent  into  gentle  folds. 

The  whole  aspect  of  the  original  Huronian,  as  thus  described  by  Lo- 
gan, is  strongly  suggestive  of  the  Animikie  Group  of  the  North  Shore.  Both 
series  are  made  chiefly  of  quartzites  and  slates,  with  some  limestone  and  chert 
beds,  and  with  interbedded  greenstones,  along  with  intersecting  greenstones 
and  red  rocks.  Some  of  this  similarity  was  seen  by  Logan,  who,  however, 
could  not  have  realized  how  strong  it  was,  since  he  does  not  seem  to  have 
been  aware  that  the  Animikie  Group  was  prevailingly  quartzitic.  However, 
on  account  of  the  similarity  as  he  saw  it,  he  maintained^  for  many  years  the 
equivalence  of  the  Lake  Huron  rocks  with  the  native-copper-bearing  rocks 
of  Lake  Superior.  Subsequently  this  view  was  abandoned,  and  since  the 
Thunder  Bay  slates  were  regarded  as  merely  the  downward  continuation  of 
the  copper  rocks,  these  two  were  now  considered  as  newer  than  the  Huron- 
ian ^  Yet  so  striking  was  the  resemblance  then  made  out  between  some  of 
the  Animikie  beds  and  those  of  the  original  Huronian,  that  a  strip  of  rocks 
along  the  north  shore  of  Thunder  Bay,  which  are  most  plainly  part  of  the 
Animikie  slates,  was  separated  from  them  by  Logan  and  put  down  as  Hu- 
ronian.* 

With  my  present  knowledge  it  appears  to  me  very  probable  that  the 
original  Huronian  of  .Lake  Huron,  and  the  Animikie  slates  of  Thunder  Bay, 
and  thence  southwestward  to  the  Mississippi  River,  are  one  and  the  same 
formation.     The  Keweenawan  rocks,  as  shown  later,  are  newer  than  either. 

'  Report  of  Progress  of  Geol.  Survey  of  Canada,  for  1848,  p.  29. 

"SeeT.  S.  Hunt  in  Second  Geol.  Survey  of  Pennsylvania.     Special  Report  on  Trap  Dikes  and 
Azoic  Rocks  of  Pennsylvania,  Part  I,  p.  69. 
^Geology  of  Canada,  1863,  p.  63. 


THE  PENOKEB  HUEONLAJST.  391 


THE  PENOKEE  HTJBONIAN. 

In  the  third  volume  of  the  Geology  of  Wisconsin,  I  have  described 
in  some  detail  the  rocks  of  the  Penokee  region,  which  extends  from  the 
vicinity  of  Lake  Agogebic  in  Michigan  to  Lake  Numakagon  in  Wisconsin. 
The  following  is  the  succession  of  strata  as  I  have  given  it  in  that  volume: 

Feet. 
I.  Tremolitic  crystalline  limestone  90 

II.  (A)  Arenaceous  v/Mte,  quartzite,  often  brecciated,  35  feet;  (B)  magnetitic 

qwartz-scliist,  5  feet ■^O 

III.  Siliceous  slaty  schists;  including  quartzite,  "argillitic"  mica-schist,  and 
novaculite;  all  having  much  quartz,  and  none  ever  showing  any 

amorphous  material ^10 

rV,  Magnetic  belt;  including :  {a)  banded  magnetic  quartzite— gray  to  red 
quartzite,  free  from  or  lean  in  iron  oxides,  banded  with  seams,  from 
a  fraction  of  an  inch  to  several  inches  in  width,  of  pure  black  gran- 
ular magnetite,  only  rarely  mingled  with  the  specular  oxide;  (b)  mag- 
netitic quartzite,  the  magnetite  in  varying  proportions,  pretty  well  - 
scattered  throughout,  and  mingled  with  the  specular  oxide  in  pro- 
portions varying  from  nothing  to  a  predominating  quantity ;  (c)  mag- 
netitic quartz-slate,  the  magnetite  pervading  the  whole,  and  mingled 
with  the  specular  oxide  as  before;  {d)  slate  like  (c),  but  largely 
charged  with  tremolite  or  actinolite;  (e)  arenaceous  to  compact  and 
flaky  quartzite,  free,  or  nearly  so,  from  iron  oxides;  (/)  thin  lami- 
nated, soft,  black  magnetitic  slate;  {g)  hematitic  quartzite,  the  iron 
oxide  the  red  variety ;  (h)  garnetiferous  actinolite  schist,  or  eclogite; 
(t)  greenstone,  which  is  restricted  to  the  western  end  of  the  Huro- 
nian  belt.  Kinds  (a)  to  (d)  all  carry  much  pyrolusite  or  other  man- 
ganese oxide.  These  varieties  have  no  persistent  stratigraphical 
arrangement,  and  are  named  here  in  order  of  relative  abundance. 

Total  thickness,  about 780 

V.  Blaclc  feldspathic  slate;  consisting  of  orthoclase  grains  imbedded  in  a 

paste  of  biotite,  pyrite,  limonite  and  carbon 180 

VI.  Unknown,  always  drift  covered . .  880 

VII.  Dark-gray  to  black,  aphanitic  mica-slate,  having  a  wholly  crystalline 

base  of  quartz  and  orthoclase,  with  disseminated  biotite  scales 120 

VIII.  Unknown,  but  probably  in  large  part  the  same  as  VII 290 

IX.  Chloritic,  pyritiferous,  massive  greenstone 150 

X.  Black,  aphanitic  mica-s/:a<e  like  VII 25 

XI.  Covered,  btit  probably  mica-slate -- 280 

XII.  Blaclc  mica-slate;  aphanitic,  at  times  chiastolitic 225 

XIII.  Chloritic  greenstone 35 

XIV.  Blaclc  mica-slate,  like  XII,  often  chiastolitic  . .    375 

XV.  To  XVIII,  alterations  of  black  mica-slates,  with  quartzites  and  quartz- 
schists 675 


392       COPPEE-BBAEING  EOCKS  OF  LAKE  SUPEEIOE. 

Feet, 

XIX.  Greenstone ;  aphanitic 260 

XX.  Covered,  but  probably  like  XXI 525 

XXI.  Mica-schist;  from  aphanitic  to  medium-grained;  including  bands  of 
light- gray  quartz-schist,  the  mica  becoming  subordinate ;  all  varie- 
ties having  a  background  of  quartz ;  the  mica  wholly  biotite ;  pen- 
etrated by  veins  and  masses  of  very  coarse,  pink  to  brick-red  bio- 
tite-granite ;  total  on  Bad  Eiver,  4,960  feet ;  seen  further  east,  higher 
layers  2,500  feet,  in  all 7,460 

Total 12,800 

Later  investigations  have  shown  that  some,  at  least,  of  the  greenstones 
of  the  above  series  are  diabasic  rather  than  dioritic. 

Here  the  resemblance  to  the  Animikie  rocks  is  very  strong.  The 
magnetitic  quartzites,  other  quartzites  and  quartz-slates,  and  the  argilla- 
ceous slates  of  the  Animikie  and  Penokee  series  are  identical  in  character, 
while  much  of  the  upper  mica-schist  member  of  the  Penokee  Huronian  is 
very  close  to  the  mica-bearing  quartzite  of  the  Animikie.  In  each  of  the 
groups  the  magnetitic  quartzites  are  near  the  base  of  the  series,  and  in  both 
there  are  interstratified  greenstone  beds.  In  both,  the  same  relations  obtain 
to  the  newer  Keweenawan  and  older  gneisses.  The  two  groups  are  plainly 
enough  the  same. 

THE  MAEQUETTE  AND  MENOMINEE  HURONIAN. 

The  iron-bearing  schists  of  the  well  known  iron-regions  of  the  north- 
ern peninsula  of  Michigan  present  one  point  of  strong  contrast  with  the 
Animikie  Group  and  the  Penokee  Huronian,  in  that,  instead  of  dipping  uni- 
formly lakeward,  they  are  closely  folded  in  troughs  whose  sides  are  gneiss 
and  granite.  At  first  glance,  the  greater  number  of  rock  kinds  characterizing 
the  Marquette  and  Menominee  Huronian,  as  compared  with  that  of  the 
Penokee  region,  might  seem  a  further  difference.  There  is,  however,  so 
plainly  a  general  stratigraphical  equivalence  between  the  two  series,  as  I 
have  shown  elsewhere,^  that  there  can  be  no  doubt  of  their  belonging 
together.  There  is  probably  even  a  direct  connection  between  the  two. 
Moreover,  the  lithological  differences  in  a  large  measure  disappear  on  closer 
study.     According  to  Brooks^  the  rocks  making  up  by  far  the  greater  part 

'Geology  of  Wis.,  Vol.  Ill,  p.  163. 

!!  Geology  of  Wis.,  Vol.  Ill,  table  opp.  p.  446. 


THE  MAEQUETTE  AND  MEM)MINEB  HUEONIAN.  393 

of  the  Marquette  and  Menominee  Huronian  are  quartzites,  magnetitic 
quartzites,  rich  iron  ores,  limestones,  dolomites,  clay-slates,  mica-slates 
and  greenstones.  The  greenstones  are  for  the  most  part  diabases  with  rarer 
gabbro  and  peridotite.  From  Wichmann's  microscopic  descriptions  I  judge 
that  the  same  kinds  of  diabase  and  gabbro  are  to  be  found  here  as  in  the 
Animikie  Group,  though  for  the  most  part  more  altered,  i.  e.,  the  orthoclastic 
and  the  non-orthoclastic  kinds,  while  the  olivinitic  kinds  are  here  repre- 
sented by  the  altered  serpentinitic  peridotites. 

So  far  the  resemblance  to  the  Penokee  and  Animikie  rocks  is  striking. 
It  is  among  the  remaining  less  abundant  kinds  named  by  Brooks  and 
Wichmann,  that  the  seeming  lithological  differences  between  the  schistose 
systems  of  these  several  regions  are  found.  These  less  abundant  kinds 
are:  diorite,  among  the  greenstones;  syenite;  gneiss;  granite;  sericite-schist; 
jasper-schist  and  chert-schist;  amphibolites,  including  actinolite-schist,  mag- 
netitic actinolite-schist,  hornblende-rock  and  hornblende-schist;  augite- 
schist;  chlorite-schist ;  and  talc-schist.^ 

The  diorites  of  this  list  I  suspect  to  be  mainly  uralitic  diabases.  I 
suspect  this  on  account  of  the  frequency  of  a  uralitic  change  in  the  ortho- 
clastic  diabases  of  the  Lake  Superior  region  generally;  and  my  suspicion 
is  confirmed  by  the  fact  that  Wichmann  finds  both  augite  and  urahte  as  con- 
stituents of  his  diorites,  and  speaks  distinctly  of  a  gradation  between  the 
diabase  and  diorite.  Moreover,  his  descriptions  of  the  diorites  make 
them  in  other  points  very  similar  to  the  uralitic  orthoclase-diabases  and 
gabbros  that  I  have  examined  from  the  Keweenawan  and  Animikie  groups.'' 
These  points  are  the  presence  of  orthoclase  in  greater  or  less  quantity;  of 
titanic  iron  and  its  gray  decomposition-product;  of  a  little  quartz,  and  of 
very  abundant  apatite.  Moreover,  the  quartz  is  described  by  Wichmann  as 
occurring  in  such  a  way  as  to  "recall  the  lapis  Hebraicus";  a  mode  of 
occurrence  which  renders  its  secondary  origin  evident,^  and  thus  seems  to 
establish  the  complete  identity  of  these  diorites  and  my  uralitic  orthoclase- 
gabbros. 

The  syenite*  mentioned  by  Brooks  and  Wichmann  as  occurring  at  only 

■  Geology  of  Wisconsin,  Vol.  II,  p.  600. 
'Geology  of  Wisconsin,  Vol.  Ill,  p.  627,  et  aeq. 
3  Geology  of  Wisconsin,  Vol.  Ill,  p.  629,  $  91. 
<  Geology  of  Wisconsin,  Vol.  Ill,  pp.  523,  620. 


394       OOPPER-BEAEING  BOOKS  OP  LAKE  SUPEEIOE. 

one  point  near  Marquette,  their  descriptions  show  to  be  a  mere  phase  of  the 
diorite  rich  in  orthoclase,  and  therefore,  inferentially,  a  urahtic  orthoclase- 
gabbro  or  diabase.  Gneiss  may  be  exchided  from  the  list  as  being  more 
than  doubtfully  Huronian,  when  a  true  gneiss,^  or  as  being  a  mere  phase 
of  the  mica-schist  such  as  occurs  also  in  the  Penokee  upper  mica-schists. 
Granite  occurs  in  large  areas  only  in  the  Menominee  region,  where  it  has 
not  been  satisfactorily  shown  to  be  Huronian,  and  may  be  eruptive.  The 
sericite-schist  appears  to  be  very  close  to  some  of  the  rocks  of  formation  III 
in  my  descriptions  of  the  Penokee  Huronian,^  and  is  very  probably  repre- 
sented among  the  slates  of  the  Animikie  Group  on  Pigeon  River.  The  jas- 
per- and  chert-schists  are  of  course  found  also  both  in  the  Animikie  and  the 
original  Huronian.  Of  the  amphibolites  the  actinolite-schists  and  magnetitic 
actinolite-schists  are  known  also  in  the  Penokee  region.  Hornblende-rocks 
have  also  been  described  as  occurring  in  the  Penokee  region,^  but  a  schistose 
hornblende-rock  has  not  been  noticed  there.  Whether  any  of  the  massive 
hornblende-rocks  here  included  ai-e  altered  or  uralitic  diabases  I  am  at  pres- 
ent unable  to  say.  Augite-schist  is  also  mentioned  as  occurring  at  one 
point  in  the  Marquette  region.*  Whether  the  hornblende-schists  have  any 
relation  to  it  is  not  known.  The  chlorite-schists  of  the  Marquette  region 
belong  to  two  distinct  classes,  of  which  one  type  plainly  belongs  with  the 
greenstones  as  an  alteration  form,  while  the  other  may  possibly  be  con- 
nected with  the  micaceous  and  hornblendic  schists. 

It  thus  appears  that  the  Marquette  and  Menominee  iron-bearing  schists 
are  essentially  the  same,  lithologically,  with  those  of  the  Animikie  Group  of 
the  North  Shore.  Of  the  few  unusual  kinds  of  the  Marquette  and  Menom- 
inee regions  some  may  be  attributed  merely  to  metasomatic  changes,  while 
the  remaining  ones  are  possibly  to  be  attributed  to  the  processes  of  meta- 
morphism,^  which  in  turn  may  be  connected  with  the  complex  folding  of 

'  Geology  of  Wisconsin,  Vol.  Ill,  \).  529. 

°  Geology  of  Wisconsin,  Vol.  Ill,  p.  111. 

3  Geology  of  Wisconsin,  Vol.  Ill,  pp.  137,  252,  288. 

•■  Geology  of  Wisconsin,  Vol.  Ill,  p.  645. 

s  Brooks  (Geology  of  Wisconsin,  Vol.  Ill,  p.  521)  regards  these  greenstones  as  metamorphic,  but 
Wichmann  has  shown  plainly  (Ibid.,  p.  627)  that  this  position  is  untenable;  and  I  have  convinced 
myself  from  my  study  of  the  Keweeuawan  and  Animikie  greenstones  that  all  are  of  the  same  origin  and 
all  eruptive. 


CEYSTALLINE  SCHISTS  OF  DOUBTFUL  EELATIONS.  395 

the  rocks  in  these  districts,  as  compared  with  the  unfolded  condition  of  the 
Penokee  and  Animikie  beds. 

CRYSTALIjINE  schists  of  DOUBTFUIi  REIiATIONS. 

The  original  Huronian,  the  Animikie  slates,  the  Penokee  iron  rocks, 
and  the  iron-bearing  rocks  of  the  Marquette  and  Menominee  regions,  ap- 
pear to  me,  then,  in  all  probability  to  belong  together,  and  I  may  hence 
properly  call  them  all  Hui'onian.  In  each  of  the  regions  mentioned  the  areas 
of  Huronian  schists  are  limited  by  granite  and  gneiss.  Commonly,  when  a 
contact  of  the  schists  with  the  gneiss  and  granite  is  to  be  seen  there  is  more 
or  less  strong  evidence  of  unconformity,  and  in  all  cases — save  that  of  the 
so-called  Huronian  granite  of  Brooks  and  Wright,  in  the  Menominee  region 
of  Wisconsin — the  gneiss  and  granite  plainly  rise  from  beneath  the  schists. 
There  are,  however,  a  number  of  other  areas  and  belts  of  crystalline  schists, 
on  all  sides  of  Lake  Superior,  whose  relations  to  the  Huronian  and  to  the 
older  gneisses  are  in  greater  or  less  doubt.  The  doubt  arises  in  some  cases 
from  a  very  imperfect  knowledge  of  the  rocks  in  question;  but  in  others 
comes  either  from  the  structural  difficulties  involved  in  connecting  these 
areas  with  the  undoubted  Huronian;  or  from  greater  or  less  contrast  litho- 
logically  with  the  recognized  Huronian ;  or  from  the  difficulty  in  distin- 
guishing between  eruptive  and  non-eruptive  granites.  It  is  wholly  pos- 
sible that  some  of  the  granites  are  eruptive  arid  relatively  new,  while  others, 
and  especially  those  distinctly  connected  with  the  gneisses,  may  be  of  some 
sort  of  not  undei'stood  metamorphic  origin. 

The  whole  question  of  the  nature  and  relations  of  these  ancient  rocks 
is  in  great  confusion.  It  seems  to  be  an  open  question  as  to  whether  there 
are  schistose  rocks  belonging  with  the  so-called  Laurentian  gneisses  and 
granites  in  the  Lake  Superior  region  or  not.  The  later  Canadian  geologists 
— especially  Robert  Bell — have  worked  on  the  latter  view,  and  have  de- 
scribed and  mapped  as  Huronian  all  schistose  rocks  not  distinctly  gneissic, 
whether  in  appai'ent  conformity  to  the  gneiss  or  not.  Nearly  complete 
ignorance  as  to  the  true  mineralogical  nature  of  many  of  these  doubtful 
schists  adds  another  element  of  uncertainty  to  the  question.  The  diorites 
and  diorite-slates  described  by  Bell  as  occurring  in  the  Huronian  north 


396       COPPEK-BEAKING  EOCKS  OF  LAKE  SUPEEIOR. 

of  Thunder  Bay  are,  in  all  probability,  in  large  measure  cliabasic  ratlier  than 
dioritic  rocks,  if  we  may  judge  from  the  rarity  of  true  dioritic  rocks  gene- 
rally in  the  Lake  Superior  region.  I  have  already  spoken  of  the  urahtic 
nature  of  the  hornblende  of  the  Keweenawan  and  Huronian  greenstones. 
Recent  studies  by  myself  and  my  assistant,  Mr.  C.  R.  Vanhise,  of  a  number 
of  hornblendic  rocks  from  the  valley  of  the  Wisconsin  River  apparently 
plainly  connected  with  the  older  gneisses,  have  resulted  in  showing  that 
this  uralitic  change  is  found  in  many  other  kinds  of  rocks  than  the  green- 
stones, true  granites  and  even  hornblende-schists  frequently  showing  it.^ 
So  common  have  we  found  this  change  that  it  has  led  to  the  suspicion  that 
hornblende  does  not  occur  as  a  primary  constituent  in  any  of  these  ancient 
rocks.  While  such  a  generalization  is  of  course  unwarranted  from  the  rel- 
atively small  extent  of  our  studies,  our  experience  should  be  enough  to 
render  any  one  studying  hornblendic  rocks  very  watchful  for  indications  of 
the  secondary  origin  of  the  hornblende. 

I  proceed  to  give  brief  accounts  of  some  of  these  schistose  areas  of 
doubtful  relations.  The  positions  and  sizes  of  the  areas  mentioned  are  indi- 
cated on  the  accompanying  general  map  of  the  Lake  Superior  region. 

In  the  Thunder  Bay  region,  between  the  northern  limit  of  the  Animikie 
Grroup  and  the  vicinity  of  Dog  Lake,  schists  called  by  them  Huronian  have 
been  studied  by  Murray,  Logan  and  Bell.  These  schists  appear  nearly 
always,  if  not  always,  to  be  separated  from  the  fla,t-lying  Animikie  rocks  by 
a  belt  of  gneiss  and  granite,  which  is,  however,  at  times  very  narrow.  Iso- 
lated areas  of  gneiss  are  found  to  occur  within  the  schists,  which  are  bounded 
on  the  north  again  by  a  large  area  of  gneiss  and  granite.  Still  farther  west- 
northwest,  as  seen  about  the  west  end  of  Lake  Shebandowan,  are  other 
schists,  succeeded  in  turn  by  gneiss  and  granite.  These  so-called  Huronian 
rocks  consist,  according  to  Bell,  of  "slates,  some  of  them  dark-green  and  com- 
posed of  hornblende;  some  grayish-green  and  dioritic;  others  are  light- 
colored,  fine-grained,  quartzose,  somewhat  nacreous  micaceous  schists; 
while  dioritic  slate-conglomerates,  quartzites,  fine-grained  felsites,  massive 
diorites,  ribboned  jasper,  and  iron  ore  also  occur."^ 

These  descriptions  are  of  course  not  based  on  microscopic  study,  and 

'  Geology  of  Wisconsin,  Vol.  IV,  pp.  622-714. 

» Report  of  the  Geological  Survey  of  Canada,  1867-'69,  p.  326. 


CEYSTALLINB  SCHISTS  NOETH  OF  LAKE  SUPERIOR,  897 

are  to  be  taken  as  representing  the>  nature  of  the  rocks  only  in  the  most 
general  way.  Bell  describes  these  schists  as  always  standing  at  a  high 
angle,  with  a  strike  varying  between  N.  25°  E.  and  N.  80°  W.  The 
quartzite,  chert,  ribboned  jasper  and  iron  ore  of  these  rocks  certainly  have 
much  the  look  of  the  recognized  Huronian.  With  regard  to  the  other 
rocks  it  is  impossible  to  draw  any  conclusions  from  the  very  general  de- 
scriptions given;  while  there  is  no  evidence  presented  showing  that  part  of 
the  so-called  Huronian  rocks  might  not  really  belong  with  the  gneiss, 
which  in  some  places  seems  to  grade  into  mica-schist. 

From  the  statements  of  Bell,  N.  H.  Winchell,  and  Chester,  it  is  plain 
that  the  schistose  belts  (or  belt)  of  the  Thunder  Bay  region  continue  for 
over  two  hundred  miles  to  the  southwestward,  at  a  similarly  short  distance 
north  of  the  northern  boundary  of  the  Animikie  rocks,  and  similarly  in- 
volved with  gneiss  and  granite.  Still  farther  north,  in  the  vicinity  of  Eainy 
Lake,  other  like  schistose  bands  occm-,  as  shown  by  Bigsby,  Bell,  and 
other  geologists. 

The  band  of  schists  running  west  and  south  from  the  northern  part  of 
Saganaga  Lake,  on  the  national  boundary,  for  instance,  is  described  by 
BelP  as  consisting  of  "rusty,  brown,  altered  sandstone  containing  small 
white  quartz  pebbles;"  "soft  green  argillite;"  "dioritic  schist;"  "cherty 
felsitic  slate;"  "siliceous  schist;"  "chert-rock,"  which  "resembles  the  chert 
near  the  base  of  the  Upper  Copper-beaiing  Series,"  i.  e.,  the  Animikie 
Group;  "gray  granular  quartzite;"  and  "fine-grained  glossy  clay-slate." 
All  of  these  rocks  are  said  to  stand  nearly  vertically,  inclining  slighth'  on 
one  side  or  the  other,  and  to  strike  from  15°  to  80°  west  of  south. 

A  different  belt  of  schistose  rocks  was  crossed  farther  west  by  N.  H. 
Winchell  in  making  a  canoe  trip  from  Bois  Blanc  Lake,  on  the  national 
boundary,  to  Vermillion  Lake,  in  1879.  He  speaks  of  these  schists  as  soft 
greenish  slates,  siliceous  slates,  and  hornblende  rocks  and  schists  of  several 
kinds,^  trending  in  a  general  southwesterly  direction,  and  standing  always  at 
a  very  high  angle,  and  apparently  conformable  with  the  associated  gneiss. 
From  the  descriptions  given  by  Winchell,  and  from  specimens  sent  me  by 

'  Eeport  of  Progress  of  the  Geological  Survey  of  Canada,  for  1872-73,  p.  93. 

'Ninth  Annual  Report  of  the  Geological  and  Natural  History  Survey  of  Minnesota,  p.  91,  et  seq. 


398       COPPEE-BEARING  EOCKS  OF  LAKE  SUPERIOE. 

Professor  A.  H.  Chester  of  the  rocks  lying  east  and  north  of  Vermillion  Lake, 
I  suspect  that  these  greenish  rocks  are  very  close  to  schistose  material  inter- 
laminated  with  the  gneiss  at  Penokee  Gap,  in  Wisconsin. 

By  Professor  Chester's  kindness  I  am  also  in  receipt  of  a  large  number 
of  specimens  collected  by  him  around  the  east  and  south  sides  of  Vermillion 
Lake  for  several  miles,  and  thence  southward  to  the  Mesabi  Range  in  town- 
ship 59.  From  these  specimens  and  the  notes  accompanying  them,  and 
from  the  published  notes  of  N.  H.  WinchelV  I  gather  that  there  is  here 
a  broad  belt  of  schists  trending  in  a  general  way  something  south  of 
west,  with  a  common  very  high  dip,  a  little  on  one  or  the  other  side  of 
vertical,  and  intricately  folded.  Professor  Chester's  specimens  include: 
clay-slate,  which  he  says  is  largely  exposed  about  Vermillion  Lake  ;  silvery 
mica-schist,  running  into  greasy-surfaced,  a,phanitic,  siliceous  schist,  which 
often  suggests  the  rock  of  No.  Ill  of  the  Penokee  series,^  and  is  commonly 
very  much  altered  and  charged  with  hematite  to  such  an  extent  as  to  con- 
stitute beds  of  soft  hematite  analogous  to  those  of  the  Marquette"  region ; 
quartzites  of  several  kinds;  gray  crystalline  limestone  associated  with  white 
quartzite,  and  precisely  similax,  both  in  nature  and  in  this  association,  to 
beds  in  the  Penokee,  Marquette  and  Menominee  regions;  gray  cherty 
schists  like  those  of  the  Animikie  Group;  banded  jaspery  and  cherty  mag- 
netitic  schists;  banded  quartzite  and  magnetite;  lean  slaty  magnetites  pre- 
cisely like  those  of  the  Animikie  Group  at  the  Mesabi  Range  and  like 
those  of  the  Penokee  and  Marquette  regions ;  and  rich  specular  iron  ores. 
Between  these  schists  and  the  flat-lying  beds  of  the  Animikie  Group,  in 
the  region  south  from  Vermillion  Lake,  is  a  broad  belt  of  gneiss  and  gran- 
ite which  is  plainly  the  one  crossed  by  Bell  between  Gunflint  and  Saga- 
naga  Lakes. 

The  strong  lithological  similarity  of  the  Vermillion  Lake  iron-bearing 
rocks  to  those  of  the  Animikie  Group,  and  of  the  South  Shore  iron-bearing 
formation,  makes  it  almost  a  certainty  that  these  also  are  Huronian;  while 
the  connection  of  the  Vermillion  Lake  schists  with  the  schistose  rocks 
northeast  as  far  as  Thunder  Bay  renders  it  nearly  as  certain  that  the  latter 

'  Ninth  Annual  Report  of  the  Geoloofioal  and  Natural  History  Survey  of  Minnesota,  p.  96  et  acq. 
2  Geology  of  Wisconsin,  Vol.  Ill,  p.  Ill 


CEYSTALLDTE  SCHISTS  NORTH  OF  LAKE  SUPEEIOE. 


399 


^ 


crq 


of  relation  must  certainly  subsist  on  the  South  Shore  be-  » 

tween  the  eastern  extension  of  the  unfolded  rocks  of  the  ® 

Penokee  Iron   Range  and  the  highly  folded  iron  bearing  § 

schists  of  the  Menominee  region.^  tri 

On  the  South  Shore  the  relations  of  the  different  schistose  » 

— ■ — ■ . a 

a* 

1  This  was  -written  in  1881.     Since  then  (1882)  N.  H.  Wiuchell  has  published  5' 

(in  the  Tenth  Annual  Report  of  the  Geological  Survey  of  Minn.)  some  notes  on  P 
the  geology  of  the  vicinity  of  Guniiiut  and  Saganaga  Lakes  on  the  national  bound- 
ary, in  which  region  the  unfolded  Animikie  and  folded  schists  approach  each 
other  closely.    Winchell  notes  the  peculiar  lithological  similarity  here  obtaining 
between  these  two  sets  of  rocks,  and  suggests  their  possible  identity. 


too  are  Huronian  in  large  measure.     My  own  suspicion  is  that  in  all  of  this 
region  there  are  two  distinct  kinds  of  schists — the  iron-bear- 
ing schists  of  the  Huronian,  and  the  schistose  greenish  phase 
of  the  older  gneiss.    These  having  been  taken  together  by  the  3 
Canadian  geologists,  and  the  whole  region  being  relatively  w 
so  poorly  known,  it  is  impossible  now  to  separate  them.         ^ 
But  the  decision  that  both  the  flat-lying  Animikie  slates  S 
and  some  of  the  more  northern  folded  iron-bearing  schists  5^ 
are  Huronian  renders  it  necessary  that  I  should  present  » 

o 

some  suggestion  as  to  the  structural  relations  of  the  two.  I"" 
Though  my  knowledge  of  the  region  and  of  the  nature  of  g. 

o 

the  older  rocks  is  yet  too  limited  to  allow  of  very  confident  S. 
generalization,  I  have  but  little  doubt  that  the  relation  is 
some  such  as  indicated  in  the  accompanying  diagram.  That  *| 
this  is  the  true  relation  is  rendered  probable  not  only  by  the  s 
lithological  similarity  and  present  attitudes  of  the  two  groups,  17 
but  also  by  the  fact  that  at  several  points  a  curving  upwards  | 

O 

of  the  otherwise  very  flat  Animikie  beds,  where  they  come  o 
into  contact  with  the  underlying  granite  and  gneiss,  has  been  S^ 
observed.  I  saw  this  curving  myself  on  the  north  shore  of  ^ 
Thunder  Bay,  where  the  two  formations  come  together ;  N.  I 
H.  Winchell,  if  I  have  understood  him  correctly,  observed  q 
something  of  the  kind  on  the  national  boundary ;  and  Mr.  | 
G.  W.  Stuntz  again  at  the  Mesabi  Range.     The  same  sort  o 


<^ 


'<-:-- 


\\ 


'!/;/ 


400       COPPER-BEAEmG  EOOKS  OP  LAKE  SUPERIOR. 

rocks  have  been  more  satisfactorily  worked  out,  though  there  are  many  doubt- 
ful minor  points,  and  probably  some  schistose  rocks  have  been  called  Huron- 
ian  that  are  not  certainly  so.  However,  it  seems  certain  that  here  we  have 
genuine  schists  interbedded  with  the  older  gneiss  in  such  a  way  as  to  admit 
of  no  doubt  of  their  being  subordinate  to  it.  Brooks  has  observed  this  in 
the  Menominee  region,  and  I  have  found  it  so  in  the  Penokee  region  and  the 
region  of  the  Wisconsin  Valley.  In  the  Wisconsin  Valley,  in  the  neighbor- 
hood of  Wausau,  is  found  a  southwestward  extension  of  the  Menominee 
Huronian  in  the  shape  of  siliceous  schist,  chert-schist,  siliceous  lime- 
stone, mica-schist,  quartzite,  quartz-porphyry  and  greenstone  (including 
diabase,  gabbro  and  peridotite).  Genuine  hornblendic  schists  are  un- 
known in  this  region  in  the  Huronian,  while  they  are  frequently  associated 
with  the  gneiss.  Hornblende-rocks  have  been  heretofore  regarded  as  char- 
acteristic of  the  Huronian  Series.  But  I  have  already  shown  good  reason 
for  suspecting  that  all  of  the  so-called  diorites,  syenites  and  hornblende- 
rocks  of  the  Marquette  and  Menominee  regions  are  in  all  probability  but 
tiralitic  diabases,  while  the  hornblende-schists  may  be,  in  part  at  least, 
altered  augitic  schists.  So  for  as  our  present  knowledge  of  the  microscopic 
characters  of  the  rocks  of  the  two  systems  goes,  hornblende  is  very  much 
more  characteristic  of  the  older  gneisses  than  of  the  Huronian. 

On  the  east  shore  of  Lake  Superior,  Murray,  Logan,  and  Bell  have 
described  rocks  which  they  refer  to  the  Huronian.  To  the  south  of  Batch- 
ewanung  Bay,  for  instance,  is  a  large  development  of  rocks  which  have  a 
typical  Huronian  aspect,  and  are  composed  chiefly,  according  to  Macfar- 
lane,-"  of  pyroxenic  greenstones,  slates,  slate-conglomerates,  quartzites, 
and  jaspery  iron-ores.     The  rocks  are  much  folded. 

Further  north,  according  to  Logan,  Huronian  rocks  are  "spread  over 
what  appears  to  be  a  triangular  area,  extending  along  the  shore  from  eight  to 
nine  miles  on  each  side  of  Michipicoten  River,  at  the  mouth,  and  about  the 
same  distance  up  the  stream.  A  little  further  west  it  presents  a  very  narrow 
strip  running  about  twelve  miles  along  the  coast,  and  another  one  of  eight 
miles  about  five  miles  south  from  Otter  Head."^ 

'Geological  Survey  of  Canada,  Eeport  of  Progress,  1863-1866,  p.  123. 

'^  Geology  of  Canada,  1863,  p.  63.    See  general  geological  map  of  the  Lake  Superior  region,  accom- 
panying this  memoir.  ' 


CEYSTALLINE  SCHISTS  OF  THE  EAST  COAST.  401 

In  another  place^  the  same  writer  gives  a  section  of  the  "conglomerate 
or  pebbly  slates"  of  the  Huronian  "exposed  at  the  mouth  of  the  river 
Dor^,  near  Gros  Cap,  about  five  miles  above  the  mouth  of  the  Michipicoten 
River."  The  section  measures  some  1,700  feet.  A  further  thickness,  not 
measured,  is  said  to  underlie  this  at  this  place.  "The  strike  of  the  rock  is 
very  regular,  being  about  east  and  west,  while  the  dip  is  very  highly  in- 
clined, the  beds  being  not  more  than  from  ten  to  fifteen  degrees  from  a 
vertical  attitude;  but  the  slope  is  for  part  of  the  distance  to  the  north,  and 
for  the  remainder  to  the  south;  there  is  not,  however,  supposed  to  be  any 
repetition  of  the  measures,  *  *  *."^  I  have  not  been  able  to  find  any 
thing  further  of  interest  in  the  present  connection  with  regard  to  the  schistose 
rocks  in  the  neighborhood  of  Michipicoten  Island  and  River. 

In  the  extreme  northeastern  corner  of  the  lake,  in  the  region  of  the 
Pic  River,  are  again  rocks  as  to  the  Huronian  age  of  at  least  some  of 
which  there  can  be  no  doubt.  Logan  and  Bell  have  described  these  rocks. 
Logan  even  doubtfully  refers  part  of  them  to  the  copper-bearing  series, 
though  he  maps  all  of  them  as  Huronian.     He  says: 

They  occupy  the  coast  for  about  seven  miles  on  each  side  of  the  New  Pic  Elver, 
while  an  interval  from  this  to  a  point  two  miles  beyond  the  Old  Pic  Eiver,  including 
the  coast  of  Peninsula  Bay  and  Harbor  and  Pic  Island,  is  composed  of  trap.  Beyond 
this  the  chloritic  slates  occuj)y  about  fifteen  miles  of  the  coast,  extending  to  the  neigh- 
borhood of  the  deep  cove  which  receives  the  Pike  Eiver.  It  appears  probable  that  the 
slates  thus  flanking  the  trap  on  either  hand  may  be  the.  sides  of  a  trough  converg- 
ing to  a  point  inland,  the  distance  of  which  from  the  coast  has  not  been  ascertained. 
The  Slate  Islands  are  nearly  on  the  strike  of  the  northwestern  side  of  the  trough,  and 
they  may  probably  derive  their  name  from  being  composed  of  slate  rock; — but  the 
islands  have  yet  to  be  examined.' 

With  regard  to  the  trap,  Logan  says  in  another  place:  "In  a  straight 
line  across  from  one  side  to  the  other  on  the  coast,  it  occupies  a  space  of 
about  fourteen  miles.  No  rocks  of  a  sedimentary  character  have  been 
observed  to  be  associated  with  it;  but  its  stratification  is  very  distinctly 
marked,  with  a  dip  southwesterly  of  about  twelve  degrees.  Its  character 
differs  in  difi'erent  places;  but  no  jDortion  of  it  was  observed  to  be  amyg- 
daloidal,  except  one  bed,  which  exhibited  a  transverse  columnar  structure."^ 

'Geology  of  Canada,  1863,  p.  5:3.  ^ geology  of  Canada,  1863,  p.  80. 

'Geology  of  Canada,  1863,  63. 
26  L  S 


402       COPPEE-BEAEING  EOCKS  OF  LAKE  SUPEEIOR, 

Bell  has  much  more  recently  examined  this  region,  his  explorations 
having  extended  far  into  the  interior.^  He  carries  the  Huronian  far  beyond 
the  area  indicated  by  Logan,  including,  however,  in  the  Huronian  all  the 
schistose  rocks  met  with,  and  even  large  areas  of  gneiss  and  granite.  It  seems 
thus  evident  that  there  are,  in  the  area  outlined  by  Bell  and  copied  on  the 
accompanying  general  map  of  the  Lake  Superior  region,  true  Huronian 
schists  and  older  rocks. 

RELATIONS  OP  THE  KEWEENAW  SERIES  TO  THE  HURONIAN". 

Having  given  thus  an  outline  account  of  the  rocks  of  the  Lake  Supe- 
rior region  which  are  to  be  referred  with  greater  or  less  confidence  to  the 
original  Huronian  of  Murray  and  Logan,  I  have  next  to  consider  the  rela- 
tions sustained  by  the  Keweenawan  or  copper-bearing  series  to  the  Huro- 
nian, to  which  consideration  the  foregoing  discussion  was  indeed  prepara- 
tory. The  data  at  hand  upon  which  I  have  to  base  my  conclusions  as  to 
this  disputed  question  are,  in  the  first  place,  the  lithological  similarities 
and  dissimilarities  of  the  two  systems,  and,  in  the  second  place,  the  struct- 
ural relations  between  the  two  where  they  are  found  in  proximity  to  each 
other. 

If  the  basic  eruptive  rocks  of  the  two  groups  are  alone  considered, 
the  lithological  similarity  is  very  close.  The  vesicular  or  amygdaloidal 
character  so  common  in  the  Keweenawan  rocks  is  generally  wanting  in 
the  Huronian,  as  are  also  the  pseud-amygdaloids,  and  the  peculiar  fine- 
grained brownish  and  purplish  diabases  that  are  so  common  in  the  upper 
part  of  the  copper  series.  The  more  massive  rocks  are,  however,  identical 
in  the  two  groups,  and  the  classification  of  the  Keweenawan  basic  rocks 
given  in  Chapter  III  would  cover  those  of  the  Huronian  as  well,  so  far 
as  microscopic  investigation  has  gone.  The  so-called  diorites  of  the 
Huronian  being  taken  as  uralitic  diabases,  all  the  basic  eruptive  rocks  of 
the  two  groups  fall  into  the  augite-plagioclase  class,  ranging  between  the 
orthoclase-bearing  more  acid  kinds,  and  the  more  basic  kinds  holding  no 
orthoclase,  much  olivine,  and  having  the  feldspar  anorthite,  or  near  it. 

Even  with  the  very  similar  basic  rocks  there  are,  however,  structural 

'  Geological  Survey  of  Canada;  Eeport  of  Progress  for  1870-71,  p.  322. 


EELATIOI^S  OF  THE  KEWEENAWAN  AND  HURONIAN  ROOKS.  403 

differences.  These  rocks  occur  in  both  groups  in  great  interbedded  flows, 
but  the  amygdaloidal  or  vesicular  upper  portions  are  wanting,  or  nearly  so, 
in  the  Huronian,  while  the  occurrences  as  dikes  or  cutting  masses  is  far 
more  common  in  the  Huronian  than  in  the  Keweenawan.  This  is  a  fact 
evidently  indicating,  as  shown  before — since  the  dike  rocks  are  often  iden- 
tical with  and  commonly  very  closely  related  to  those  that  occur  as  flows 
in  the  Keweenawan — that  in  the  fissures  now  filled  by  the  Huronian  dikes 
we  find  the  vents  through  which  came  both  Huronian  and  Keweenawan 
flows. 

Passing  from  the  basic  eruptive  rocks  of  the  two  groups  to  the  acid 
kinds,  the  similarity  does  not  hold.  We  look  in  vain  in  the  Huronian 
throughoutthe  entire  Lake  Superior  region  for  the  great  flows  of  red  felsite  and 
quartziferous  porphyry  which  constitute  so  marked  a  feature  of  the  copper 
series.  Nearly  the  same  may  be  said  for  the  red  augite-syenite  and  granitic 
porphyry  which  form  so  great  masses  in  the  Lower  Keweenawan,  although 
it  appears  probable  from  Norwood's  statements^  that  similar  rocks  on  a 
smaller  scale  occur  in  the  Huronian  (Animikie)  of  the  Pigeon  River  region 
of  the  North  Shore.  Logan's  descriptions  of  the  west  shore  of  Lake  Huron 
suggest  also  the  possibility  of  the  occurrence  of  such  rocks  there. 

Between  the  distinctly  bedded  or  sedimentary  rocks  of  the  two  groups 
the  contrast  is  strong.  The  Keweenawan  red  sandstones  and  shales  have 
nothing  in  common  with  the  quartz  -  slates  and  quartz -schists  of  the 
Huronian,  even  when  the  latter  are  hardly  more  than  sandstone  and  clay- 
shale.  The  two  groups  are  thus  linked  by  the  basic  rocks  of  each,  the 
basic  eruptions  having  apparently  begun  in  the  later  Huronian  and  con- 
tinued uninterruptedly  into  the  Keweenawan.  They  'are  separated  by  the 
absence  or  relative  rarity  of  the  acid  eruptives  in  the  Huronian,  and  by 
the  strong  contrast  between  the  sediments  of  the  two  groups.  That  the 
separation  was  sufficiently  great  a  one  to  allow  of  some  intervening  alter- 
ation is  suggested  by  the  occasional  presence  of  pebbles  in  the  Keweenawan 
conglomerates  which  may  be  regarded  as  derived  from  the  Huronian.  On 
Thunder  Bay  the  Keweenawan  sandstones  carry  chert  and  jasper  pebbles 
from  the  underlying   Animikie   Group,   and  on  the    Montreal    River,   in 

'Owen's  Geological  Survey  of  Iowa,  Wiscousin,  and  Minnesota,  p.  408  ct  seq. 


404       COPPEE-BBAEING  EOCKS  OP  LAKE  SUPEEIOE. 

Wisconsin,  quartzite  pebbles  apparently  from  the  underlying  Penokee 
Huronian.  But  these  occurrences,  and  the  kinds  of  rocks  involved,  are  too 
few  to  alloM^  of  our  very  confidently  concluding  from  them  the  general 
a,lteration  of  the  Huronian  schists  prior  to  the  time  of  Keweenawan  depo- 
sition. Pebbles  of  the  older  granite  and  gneiss  occur  more  frequently  in 
the  Keweenawan  conglomerates,  so  that  tliose  rocks  must  have  reached 
their  present  conditions  before  the  formation  of  the  conglomerates. 

But  for  the  relations  of  the  Keweenawan  and  supposed  Huronian  crys- 
talline schists  in  the  basin  of  Lake  Nipigon,  subsequently  noted,  the  structural 
relations  of  the  two  groups  broadly  viewed  would  lead  us  to  the  conclusion 
that  there  was  a  time  interval  between  the  groups  sufficiently  long  to  allow 
of  ei'osion  to  some  extent,  but  not  great  enough  to  produce  a  true  uncon- 
formity. On  the  South  Shore,  for  all  the  miles  between  Lake  Agogebic, 
in  Michigan,  and  Numakagon  Lake,  of  Wisconsin,  the  Keweenawan  beds 
present  a  general  parallelism  in  trend  and  dip  to  those  of  the  underlying 
Huronian,  all  being  tipped  up  towards  the  north  at  angles  which  are  nearly 
always  very  high.  So  far,  the  appearance  is  one  of  complete  conformity. 
Looking  the  other  way  in  this  region  are  the  following  facts,  as  I  have 
shown  in  my  discussion  of  the  structure  of  Northern  Wisconsin  for  the 
Wisconsin  Geological  Reports: 

"In  the  Penokee  country,  the  uppermost. beds  of  the  Huronian  are 
gradually  cut  out,  as  we  trace  them  westward,  by  the  gabbro  that  forms 
the  base  of  the  Keweenaw  Series — a  fact  which  appears  to  me  best  ex- 
plained by  the  supposition  that  the  gabbro  covers  and  conceals  these  miss- 
ing beds.  West  of  Lake  Numakagon,  the  diabases  and  other  eruptive 
rocks  of  the  Kewee'naw  Series  appear  to  completely  cover  the  Huronian, 
in  a  great  overflow.  Nevertheless,  the  approach  to  conformity  in  Wisconsin 
is  close,  and  were  we  to  draw  our  conclusions  from  this  region  only,  the 
nonconformity  could  hardly  be  regarded  as  proven.  There  are  no  such 
undulations  in  the  Huronian  of  the  Penokee  district  as  in  Michigan,  the 
subordinate  members  making  long  and  regular  bands  conforming  to  the 
general  trend  of  the  formation,  and  also,  in  a  general  way,  to  the  trend  of 
the  several  belts  of  the  Keweenaw  Series.     Moreover,  the  lessening  in  dip 


EELATIONS  OF  THE  KEWEENAW  AN  AND  HURONIAN  ROOKS.  405 

towards  the  west,  already  noted  as  aflfecting  the  latter  rocks,  is  observed 
also  in  the  underlying  Huronian,  so  far  as  can  be  traced  westward." 

So  far  as  this  region  is  concerned,  the  unconformity  is  only  one,  at  most, 
of  intervening  erosion,  without  intervening  disturbance. 

On  the  North  Shore,  again,  from  the  Saint  Louis  to  Thunder  Bay,  a 
distance  of  some  two  hundred  miles,  there  are  just  the  same  relations  be- 
tween the  Animikie  Huronian  and  the  overlying  Keweenawan  as  in  the 
Penokee  region.  Both  formations  are  exposed  here  on  so  large  a  scale, 
and  both  He  for  the  most  part  so  flat,  that  there  can  be  no  doubt  at  all  as 
to  their  relations.  The  parallelism  in  bedding  between  the  two  is  complete. 
At  the  eastern  end  of  the  Minnesota  coast,  for  instance,  both  incline  lake- 
ward  at  an  angle  of  1 0°,  the  contact  of  the  two  being  in  sight,  with  some 
hundreds  of  feet  in  thickness  of  each.  The  same  perfect  paralleHsm  at  a 
still  flatter  angle  is  to  be  observed  on  the  east  side  of  Thunder  Bay,  where 
the  overlying  Keweenawan  has  at  its  base  several  hundred  feet  of  sand- 
stone. 

Indicative  of  an  intervening  erosion  on  the  North  Shore  is  the  absence 
of  the  sandstone  just  mentioned  only  a  few  miles  to  the  southwest,  where, 
at  Grand  Portage  Bay,  the  two  formations  come  together;  while  the  rela- 
tion of  these  sandstones  to  the  underlying  slates  at  Thunder  Cape  amounts 
to  a  demonstration  of  this  intervening  erosion.  Along  the  east  side  of 
Thunder  Bay  the  Animikie  slates  may  be  seen  for  miles  lying  beneath 
the  Keweenawan  sandstones,  but  when  Thunder  Cape  is  reached  the  slate 
suddenly  rises  entirely  across  the  horizon  of  some  six  hundred  to  seven  hun- 
dred feet  of  the  sandstone.  Logan  explains  this  peculiar  behavior  by  sup- 
posing the  Thunder  Cape  rocks  to  be  separated  from  those  around  them  by 
a  fault.  But,  as  I  have  shown  on  a  previous  page,  a  much  simpler  and 
more  satisfactory  explanation  is  reached  by  supposing  an  intervening 
erosion  between  the  slate  and  sandstone.^  The  truth  of  this  view  is  further 
confirmed  by  the  occurrence  of  ledges  of  the  overlying  sandstone  on  the 
flanks  of  the  Thunder  Cape  elevation,  and  by  the  occurrence  in  the  sand- 
stone of  pebbles  derived  from  the  underlying  slates.^ 

'As  first  suggested  by  T.  S.  Hunt,  Second  Geological  Survey  of  Pennsylvania,  Report  E,  Part 

I,  p.  239. 

=  T.  Macfarlane,  Canadian  Naturalist,  New  Series,  Vol.  IV,  p.  459. 


406       COPPEE-BEAEING  KOCKS  OF  LAKE  SUPEEIOE. 

In  the  next  chapter  I  shall  attempt  to  show  that  the  whole  of  the  Lake 
Superior  basin  is  a  synclinal  trough  in  the  Keweenawan  beds.  From  the 
relations  of  the  Huronian  and  Keweenawan  in  the  western  half  of  the  basin 
it  is  plain  that  they  also  partake  in  this  synclinal — a  fact  in  itself  strongly 
indicating  that  they  here  underlie  the  Keweenawan  without  true  uncon- 
formity. 

Thus  far,  however,  I  have  considered  the  structural  relations  of  the 
Keweenawan  rocks  to  the  unfolded  Huronian  only.  In  the  case  of  the 
folded  Huronian  the  problem  to  solve  is  the  time  when  this  folding  took 
place.  Did  it  entirely  precede  the  Keweenawan  depositions  and  outflows? 
was  it  contemporaneous  with  them?  or  was  it  wholly  subsequent?  The 
solution  of  this  problem  is  not  easy  to  reach  without  more  extended  knowl- 
edge of  the  Huronian  than  we  now  possess. 

On  the  South  Shore,  the  folded  Huronian  beds  and  the  Keweenawan 
are  never  in  close  proximity.  They  are  nearest  to  each  other  in  the  Ke- 
weenaw Point  and  Marquette  districts.  On  the  east  shore  of  the  lake  the 
Keweenawan  beds  are  found  near  to  folded  Huronian,  but  unfortunately 
our  knowledge  of  that  coast  is  too  meager  to  base  any  conclusions  upon  as 
to  the  relations  of  the  two  systems  of  rocks.  The  only  other  portion  of  the 
Lake  Superior  basin  in  which  the  Keweenawan  and  folded  Huronian  ap- 
proach each  other  is  that  district  lying  northward  from  Thunder  and  Nipi- 
gon  bays.  Here  again,  unfortunately,  our  ignorance  as  to  the  Huronian 
leaves  us  in  a  good  deal  of  doubt;  for  while  it  seems  certain  that  folded 
Huronian  schists  exist  in  this  regidn,  it  is  also,  as  already  indicated,  a  matter 
for  grave  doubt  as  to  how  far  the  schists  called  by  Bell  Huronian,  belong 
in  fact  to  an  older  formation. 

The  relations  subsisting  between  the  Keweenawan  rocks  of  Keweenaw 
Point  and  the  folded  Huronian  to  the  southeast  form  the  subject  of  an 
article  by  T.  B.  Brooks,  published  in  1876,  in  which  he  urges  the  exist- 
ence of  a  true  unconformity  between  the  two  rock  systems.^  The  same 
writer,  along  with  Pumpelly,  had  only  a  few  years  previously  held  to  a 
complete  conformity  between  the  two  systems.^     To  this  last-named  view 

'Amer.  Jour.  Soi.,  March,  1876,  "On  the  Youngest  Huronian  Eocks  South  of  Lake  Superior." 
'Geol.  Surv.  of  Mich.,  Vol.  I,  Part  II,  pp.  1-6. 


EBLATIONS  OF  THE  KBWEBiSrAWAK  AKD  HUEOXIAN  EOCKS.  407 

he  and  Pumpelly  had  been  led  by  the  strong  appearance  of  conformity 
between  the  Keweenawan  and  unfolded  Huronian  of  the  Penokee  region. 
Working  myself  a  few  years  later,  I  was  led  to  acquiesce  in  this  view.^ 
To  Brooks's  later  view  I  again  in  a  measure  acceded,  in  my  discussion 
of  the  structure  of  Northern  Wisconsin  for  the  third  volume  of  the  Geol- 
ogy of  Wisconsin;  though  still  maintaining  an  intervening  erosion  without 
true  unconformity  for  the  western  half  of  the  Lake  Superior  basin.  The 
whole  history  of  this  discussion  is  but  a  new  illustration  of  the  danger  of 
generahzing  as  to  the  structural  relations  of  a  system  of  rocks  before  the 
whole  ground  has  been  looked  over. 

So  far  as  Brooks's  argument  is  based  on  lithological  grounds,  it 
has  been  sufficiently  considered  in  foregoing  paragraphs.  The  structural 
grounds  for  his  conclusions  are  (1)  the  unfolded  condition  of  the  Kewee- 
nawan beds  as  contrasted  with  the  frequently  folded  Huronian,  and  (2)  the 
absence  of  patches  of  the  Keweenawan  rocks  in  the  numerous  deep  syn- 
clinals of  the  Huronian  in  the  Marquette  region.  The  first  of  these  points 
merits  no  consideration,  for  the  very  point  at  issue  is  the  time  of  folding  of 
the  Huronian.  With  regard  to  the  second  point  it  may  be  said  that  it  is 
plainly  to  be  seen  that  the  absence  of  patches  of  the  Keweenawan  rocks 
overlying  the  Huronian  in  the  Marquette  region  is  quite  as  difficult  of 
explanation,  or  even  more  difficult,  on  an  hypothesis  of  complete  uncon- 
formity as  on  one  of  complete  conformity.  On  Keweenaw  Point  the  Ke- 
weenawan rocks  have  a  thickness  measured  by  miles,  ending  to  the  south- 
ward in  a  bold  escarpment  beyond  which  no  patch  of  these  rocks  is  to 
be  found.  The  absence  of  such  outliei's,  especially  of  such  hard  rocks, 
would  be  incredible  and  contrary  to  all  experience  on  any  hypothesis  of 
former  extension  southward.  It  has  already  been  shown  that  the  escarp- 
ment on  the  south  side  of  Keweenaw  Point  must  be  a  fault  line,  as  Fos- 
ter and  Whitney  long  since  urged;  and  their  view  is  strongly  confirmed 
by  the  very  absence  of  outliers  to  the  southward.  The  Keweenawan  rocks 
are,  then,  not  to  be  found  in  the  Marquette  region  for  the  simple  reason 
that  they  never  extended  so  far;  and  but  for  some  features  in  the  geology 

'  "On  the  Age  of  tlie  Copper-Bearing  Rocks  of  Lake  Superior,"  Ainer.  Jour.  Sci.,  June,  1874. 


408 


OOPPEE-BEASING  ROCKS  OP  LAKE  SUPEEIOR. 


of  the  Nipig'on  Lake  region  as  described  by  Bell,  precisely  the  same  reason 
might  be  advanced  for  their  absence  in  the  region  of  folded  Huronian  schists 
north  of  Lake  Superior. 

The  Keweenawan  rocks,  or  Nipigon  Series,  as  they  are  termed  by  him, 
are  represented  by  Bell  as  lying  in  the  Nipigon  Lake  basin,  directly  athwart 
the  course  of  several  belts  of  folded  schists,  and  with  a  general  horizontal 
attitude.  These  schists  Bell  regards  as  Huronian,  and  maps  them  as 
directly  continuous  with  the  schistose  belts  of  the  country  immediately 
north  of  Thunder  Bay.  (See  Fig.  34.)  If  these  rocks  are  indeed  Huro- 
^ortTz.  nian,   and  their  relation  to  the 

Keweenawan  is  as  stated  by 
Bell,  no  doubt  can  remain  as  to 
the  existence  of  a  genuine  un- 
conformity between  the  two  sys- 
tems. Bell's  detailed  descrip- 
tions, however,  do  not  fully  bear 
out  his  reference  of  the  schistose 
rocks  in  question  to  the  Huro- 
nian, nor  his  general  statement 
as  to  the  horizontal  attitude  of 
the  Keweenawan  rocks  them- 
selves. 

In  conclusion,  then,  it  is  to 
be  said  that  the  Huronian  and 
Keweenawan  rocks  are  linked 
together  by  the  lithological  affin- 
ities of  their  basic  eruptive  mem- 
bers, and  by  the  close  approach 

Fig.  35. — Outline  geological  map  of  the  Nipigon  Lake  /•  •  t_  j.i 

Region,  after  a  manuscript  map  by  R.  Bell.  tO      COnlorUUty      between       them 

which  obtains  throughout  the  whole  western  half  of  the  Lake  Superior  basin; 
and  that  they  are  separated  from  one  another  by  the  lithological  contrast 
between  their  sedimentary  members;  by  an  intervening  erosion,  which  has 
plainly  taken  place,  even  where  there  is  a  close  approach  to  conformity; 


EELATIONS  OF  THE  KEWEENAWAN  AND  HUEONIAN  EOCKS.  409 

and  possibly  also  by  an  intervening  period  of  alteration  and  folding.  The 
Huronian  sediments  are  metamorphic,  whatever  the  nature  of  the  meta- 
morphosing process  may  have  been — and  the  metamorphism  has  always  been 
greatest  where  the  folding  has  been  greatest — while  the  Keweenawan  sedi- 
ments are  unaltered.  The  metamorphism  and  folding  may  have  taken 
place  before  or  during  the  period  of  Keweenawan  eruptions  and  depositions, 
or  both.  Our  present  knowledge  of  the  Huronian  is  too  incomplete  to 
allow  of  a  very  firm  opinion  as  to  this  point. 


CHAPTER    IX. 

STRUCTURE  OF  THE  LAKE  SUPERIOR  BASIN. 

Foster  and  Whitney's  views  as  to  the  existence  of  a  synclinal  between  Isle  Royale  and  Keweenaw 
Point. — Westward  extension  of  the  Isle  Royale-Keweenaw  Point  synclinal;  first  shown  to 
exist  in  1873. — Course  and  structure  of  the  synclinal  as  shown  by  the  work  of  the  Wisconsin 
Geological  Survey. — Structure  and  extent  of  the  synclinal  as  now  worked  out. — Parallelism 
between  the  courses  -of  the  Keweenawau  belts  of  the  North  and  South  Shores ;  and  of  the  coast 
lines  with  these  belts. — Explanation  of  Plates  XXVIII  and  XXIX. — Nature  of  the  bottom  of 
the  synclinal. — Complications  of  the  synclinal  by  faulting. 

Foster  and  Whitney  first  pointed  out  the  probability  that  a  synclinal 
depression  exists,  underneath  the  waters  of  Lake  Superior,  between  Isle 
Royale  and  Keweenaw  Point,^  being  led  to  this  view  by  the  lakeward  in- 
clination of  the  rocks  on  both  Isle  Royale  and  the  Point,  and  by  the  fact 
that,  on  the  lakeward  side  of  each,  sandstone  and  conglomerate  prevail, 
while  on  the  side  away  from  the  lake  the  rock  beds  in  each  case  are  pre- 
vailingly crystalline. 

During  my  first  season's  work  on  Lake  Superior,  for  the  Wisconsin 
State  Geological  Survey,  in  1873,  I  collected  facts  in  the  Bad  River  country 
of  Wisconsin  going  to  show  that  to  the  westward  the  Isle  Royale-Kewee- 
naw Point  synclinal  runs  on  to  the  South  Shore,  the  rocks  being  found  there 
dipping  both  ways.  The  observations  of  my  assistant,  Mr.  E.  T.  Sweet, 
made  during  the  same  season  on  the  Copper  Range  of  Douglas  County, 
Wisconsin,  tended  very  strongly  to  confirm  this  conclusion,  since  the  rocks 
of  that  range  were  found  to  dip  southward.  My  conclusions  from  the  data 
then  in  hand  were  published  in  1874,  along  with  an  outline  map  and 
section.^  The  subsequent  work  of  the  Wisconsin  Survey,  during  the  years 
from  1874  to  1878,  by  Sweet,  Strong,  Chamberlin  and  myself,  served  to 
place  beyond  question  the  truth  of  the  main  points  of  my  conclusions  of 

•  Report  on  Lake  Superior  Land  District,  Part  I,  p.  109. 

2 "  On  the  Age  of  the  Copper-Bearing  Rocks  of  Lake  Superior ;  and  on  the  Westward  Continuation 
of  the  Lake  Superior  Synclinal."    American  Journal  of  Science  and  Arts,  Vol.  VIII,  July,  18?4. 
410 


UNITED  STATES   GEOLOGICAL  SURVEY 


COPPER -BEARING  ROCKS  OF  LAKE  SUPERIOR   PL  XW[: 


M.\P  OF  THE  L.VI^   SFPI-RIOI!  BASIN  DKSICXED  TO  SH(AV  THE  STRUCTl^RE  .\ND  EXTENT  OF  THE    ias\VEENA\V:.\N   TROTTGH. 


THE  LAKE  SUPERIOR  SYNCLINAL.  411 

1873,  while  at  the  same  time  modifying  them  in  some  respects,  and  develop- 
ing a  number  of  new  facts  with  regard  to  the  structure  and  course  of  the 
synclinal. 

The  most  important  modification  was  that  with  regard  to  the  supposed 
occurrence  of  horizontal  Potsdam  or  unconformably  overlying  sandstone 
in  the  trough  of  the  synclinal.  On  my  map  of  1874  I  had  marked  such 
a  sandstone  as  occurring  along  the  upper  Saint  Croix,  as  indicated  by  the 
descriptions  of  Dr.  D.  D.  Owen,^  but  this  sandstone  was  subsequently 
shown  by  Sweet ^  to  belong  in  the  Upper  Division  of  the  Keweenaw  Series, 
it  being  in  fact  but  the  westward  continuation  of  the  south-dipping 
sandstones  of  White  and  Bad  rivers.  The  upper  Saint  Croix  was  again 
further  examined  by  Sweet  and  Strong  in  1876,  and  the  sandstone  in 
question  found  to  be  underlain  conformably  by  fine-grained  diabases 
and  melaphyrs  with  interbedded  conglomerate  and  sandstone.  My  map 
and  section  of  1874  had  also  shown  horizontal  sandstones  filling  the 
trough  of  the  synclinal  in  the  Bad  River  country.  This  conclusion  was 
based  on  an  observation  by  Dr.  I.  A.  Lapham,  which  subsequent  ex- 
amination by  myself  failed  to  verify.  There  then  remained,  to  indicate 
the  presence  of  this  newer  sandstone  in  the  trough  of  the  synclinal,  only  an 
exposure  of  flat  sandstone  on  the  shore  of  Lake  Superior  at  Clinton  Point, 
four  miles  west  of  the  mouth  of  Montreal  River;  and  this,  as  shown  on  a 
previous  page,  is  rather  to  be  regarded  as  the  eastern  termination  of  the 
horizontal  sandstone  of  the  Apostle  Islands  and  of  the  coast  of  Bayfield 
County. 

The  chief  new  developments  as  to  the  structure  and  course  of  the  syn- 
clinal, resulting  from  the  later  work  of  the  Wisconsin  Survey,  were  (1)  the 
connection  by  Strong  of  the  Keweenaw  Range  of  north-dipping  rocks  with 
the  similar  rocks  of  the  Saint  Croix  by  exposures  all  across  the  previously 
wholly  unexamined  interval  between  that  river  and  Numakagon  Lake: 
(2)  the  determination  of  the  comparative  flatness  of  the  northward  dip 
across  this  interval;  (3)  the  determination  by  Chamberlin  of  a  curve  to  the 
southward  of  the  belts  of  this  range,  with  a  flat  westerly  dip,  in  the  imme- 

'  Geological  Survey  of  Wisconsin,  Iowa,  and  Minnesota,  p.  161. 

'"Notes  on  the  Geology  of  Northern  Wisconsin,"  by  E.  T.  Sweet.  Trans.  Wis.  Acad.  Science, 
Vol.  Ill,  1876. 


412       COPPEE-BEAEING  EOOKS  OF  LAKE  SUPERIOE. 

diate  vicinity  of  the  Dalles  of  the  Saint  Croix,  of  the  sinailar  southerly- 
course  with  high  easterly  dip  of  the  Keweenawan  beds  of  Snake  River, 
Minnesota,  and,  as  a  consequence,  of  the  southerly  direction  of  the  axis  of 
the  synclinal  near  its  final  termination;  and  (4)  the  determination  by  Sweet 
of  the  existence  of  a  southward  dip  in  the  Saint  Louis  River  slates,  and  the 
consequent  probability  that  the  Huronian  rocks  form  the  bottom  beds  of 
the  synclinal. 

In  Vol.  Ill  of  the  Gleology  of  Wisconsin,  published  in  1880,  I  embod- 
ied these  points  in  a  brief  discussion  of  the  structure  of  Northern  Wis- 
consin, accompanied  by  a  map,  which  I  now  modify  only  as  to  the  exact 
extent  of  the  upper  sandstones  of  the  Keweenawan,  and  as  to  the  western 
extension  of  the  horizontal  sandstone  of  the  lake  shore,  which,  on  the  map 
of  1880,  was  made  by  misprint  to  extend  to  the  north  side  of  the  Saint 
Louis  River  at  Duluth.     No  such  sandstone  is  to  be  seen  near  Duluth. 

At  the  beginning  of  my  study  for  the  present  memoir.  North  Wisconsin 
had  been  shown  to  be  traversed  by  a  broad  synclinal  in  the  Keweenawan 
rocks,  possibly  also  in  the  Huronian,  which  was  presumably  the  continua- 
tion of  the  Isle  Royale-Keweenaw  Point  depression.  The  exact  nature 
and  position  of  the  western  termination  of  the  synclinal,  the  relation  to  the 
synclinal  of  the  rocks  of  the  Minnesota  coast,  and  of  the  Porcupine  Mount- 
ains, and  the  behavior  of  the  depression  to  the  eastward  of  Isle  Royale, 
were  all  points  left  in  doubt,  though  it  appeared  exceedingly  probable  that 
the  entire  western  half  of  the  Lake  Superior  basin  is  a  synclinal  depression, 
affecting  both  Huronian  and  Keweenawan  rocks. 

Now,  however,  I  feel  able  to  announce  with  confidence  that  the  entire 
lake  basin,  including  not  only  the  western  half,  but  the  eastern  half  as  well, 
is  a  synclinal  depression;  that  this  depression  certainly  afi'ects  the  Kewee- 
nawan rocks  throughout  their  entire  extent;  that  it  as  certainly  afi'ects  in 
very  large  measure  the  underlying  Huronian  rocks,  which,  while  they  are 
greatly  folded  where  extending  without  the  limits  of  the  depression,  within 
its  limits  form  without  folds  its  bottom  layers;  that  the  axis  of  the  depres- 
sion has,  like  the  lake  itself,  at  first  a  northwesterly  and  then  a  southwest- 
erly direction,  with  minor  bends  corresponding  to  the  several  bends  in  the 
axis  of  the  lake;  that  the  eastern  termination  of  the  depression  is  bui-ied 


THE  LAKE  SUPEEIOE  STNOLINAL.  413 

beneath  the  newer  formations  in  the  vicinity  of  the  Sault  Saint  Marie ;  that 
the  western  extension  passes  on  to  the  south  shore  of  Lake  Superior  with 
"a  course  curving  more  and  more  to  the  southwest  until,  at  the  termination 
in  the  Saint  Croix  Valley — and  therefore  without  the  present  hydrographic 
basin  of  Lake  Superior — it  becomes  nearly  due  south,  the  exact  termina- 
tion here  again  being  buried  beneath  the  newer  horizontal  Cambrian  for- 
mations; and  that,  in  the  region  of  the  Porcupine  Mountains  of  Michigan, 
and  the  Douglas  County  Copper  Range  of  Wisconsin,  there  are  minor  folds 
superinduced  upon  the  grand  synclinal,  accompanied  in  the  former  case  at 
least,  by  further  comphcations,  due  to  faulting. 

The  evidence  upon  which  these  conclusions  are  based  is  to  be  found  in 
(1)  the  nearly  constant  dip  inwards  of  the  Keweenawan  strata  towards  the 
middle  of  the  basin;  (2)  in  the  frequently  similar  dip  of  the  Huronian;  (3)  in 
the  constant  order  of  Upper  Keweenawan,  Lower  Keweenawan,  Huronian, 
and  gneiss  with  granite  and  folded  crystalline  schists,  met  with  on  all  sides  on 
going  from  within  the  supposed  trough  outwards;  and  (4)  in  the  parallelism 
between  the  courses  of  the  Keweenawan  belts  of  the  North  and  South 
Shores,  and  of  the  shore  line  with  these  belts. 

The  details  of  the  evidence  under  the  first  three  of  these  heads  are 
given  in  Chapters  VI  and  VII,  and  on  the  maps  and  sections  of  Plates  I, 
XVII,  XVIII,  XXII,  XXIII,  XXVI,  and  XXVII,  and  need  not  therefore 
be  repeated  here.  That  under  the  fourth  head,  however,  needs  some  fur- 
ther remarks.  In  the  first  place  it  is  to  be  observed  that  the  drawings,  from 
which  the  accompanying  maps  of  Lake  Superior,  Plates  I  and  XXVIII,  are 
reduced,  are  much  more  accurate  than  any  previously  made  with  geological 
data,  being  compiled  directly  from  the  maps  of  the  United  States  Lake  Sur- 
vey, from  Captain  Bayfield's  chart,  and  from  the  United  States  land-office 
plats;  and  that,  consequently,  correct  ideas  may  be  obtained  from  them  as 
to  the  courses  of  the  coast  and  other  topographical  fines,  and  of  rock  belts. 
Directly  north  of  the  east  and  west  portion  of  Keweenaw  Point,  be- 
tween Agate  Harbor  and  Copper  Harbor,  with  its  east  and  west  rock  behs, 
dipping  north,  we  find  the  east  and  west  part  of  Isle  Saint  Ignace  again  made 
up  of  east  and  west  rock  belts,  which  now,  however,  dip  to  the  south.  West- 
ward from  Agate  Harbor  and  Eagle  Harbor,  on  Keweenaw  Point,  the  coast 


414       COPPEE-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 

line  of  the  Point  and  the  course  of  the  constituent  rock  belts  swing  around 
to  the  southwest.  Correspondingly,  we  find,  on  the  North  Shore,  a  southwest 
trend  (participated  in  by  rock  belts,  coast  lines,  and  lines  of  islands),  begin- 
ning in  the  western  part  of  Isle  Saint  Ignace,  and  continuing  through  the 
peninsula  which  forms  the  south  side  of  Black  Bay,  through  the  adjoining 
islands,  and  through  Isle  Royale. 

Isle  Royale  does  not  lie  on  a  straight  course,  but  on  a  curving-  one, 
its  outlines,  projecting  points,  ridges  and  rock  belts  at  the  western  extremity 
trending  10°  to  12°  more  to  the  south  than  at  the  eastern  extremity.  This 
curvature  to  the  westward  is  continued  to  a  nearly  due  westerly  direction 
in  the  rock  belts,  projecting  points  and  other  topographical  features  of  the 
Minnesota  coast  between  Pigeon  River  and  Grand  Marais,  although  the 
coast  line  in  this  distance  trends  as  a  whole  some  20°  south  of  west.  The 
counterpart  of  this  swing  to  the  west  is  found  on  the  south  shore  of  Lake 
Superior  in  the  course  of  the  Main  Trap  Range  and  its  constituent  rock 
belts,  and  of  the  coast  line  between  Fourteen-mile  Point  and  Black  River. 

West  of  Black  River,  the  Main  Trap  Range  of  the  South  Shore  and  its 
rock  beds  curve  again  to  the  south  of  west,  and  as  Bad  River  is  neared  the 
direction  is  only  some  30°  west  of  south.  The  corresponding  curvature  on 
the  North  Shore  is  to  be  found  in  the  distance  between  Grand  Marais  and 
Split  Rock  River.  For  much  of  this  distance  the  coast  line  follows  the 
trend  of  the  strata,  until  the  latter  comes  around  to  only  a  few  degrees 
west  of  south,  when  the  rock  belts  depart  from  the  coast,  and  run  with  a 
eastward  curvature  over  to  the  South  Shore.  Still  further  west,  both  sides 
of  the  synclinal  are  on  the  South  Shore,  the  strata,  and  with  them  many 
topographical  features,  on  both  sides,  trending  at  first  well  around  to  the 
west,  and  then  more  and  more  towards  the  south,  until  the  termination  is 
reached  in  the  Saint  Croix  Valley. 

Beyond  Copper  Harbor  to  the  eastward,  on  Keweenaw  Point,  the  point 
and  its  strata  begin  to  swing  aroiind  to  the  south  of  east,  and  this  direction 
is  continued  on  Manitou  Island,  and  in  Stannard's  Rock,  which  is,  as  pre- 
viously shown,  a  mass  of  quartzless  porphyry.  Parallel  to  this  curving 
course  is  the  coastline  of  the  lake  between  Huron  Bay  and  Marquette. 
Now  on  the  North  Shore,  in  the  line  of  islands  lying  south  of  Nipigon 


THE  LAKE  SUPERIOR  SYNCLINAL.  415 

Bay,  and  in  the  rock  belts  composing  them,  a  similar  curvature  to  the  south 
of  east  is  begun.  That  this  continues  until  it  becomes  nearly  or  quite  a 
southerly  course  is  shown  by  the  trend  of  the  northeast  coast  line  of  the 
lake,  which  is  composed  of  the  older  rocks,  between  the  Pic  and  Michipi- 
coten  Island,  where  the  Keweenawan  rocks  again  appear.  The  parallelism  of 
the  northeast  coast,  of  the  line  marked  out  by  the  eastern  end  of  Keweenaw 
Point  and  Stannard's  Rock,  and  of  the  south  coast  between  Keweenaw 
Bay  and  Marquette,  looks  also  the  same  way. 

Still  further  to  the  east,  the  South  Shore  shows  only  rocks  newer  than 
the  Keweenawan,  but  at  the  east  end  of  the  lake  a  continuous  belt  of  the 
latter  rocks  is  marked  by  Michipicoten  Island,  Capes  Choyye  and  Gargantua, 
Pointe  Aux  Mines,  the  peninsula  of  Mamainse,  the  coast  of  Batchewanung 
Bay,  and  Gros  Cap,  the  beds  always  dipping  lakeward.  The  most  striking 
thing  about  this  belt  is  its  parallelism  to  the  lake  coast  behind  it,  and  the 
consequent  abrupt  turn,  at  more  than  right  angles,  in  the  Michipicoten 
bight. 

In  the  map  of  Plate  XXVIII  and  the  accompanying  sections  of  Plate 
XXIX,  I  have  attempted  to  summarize  the  facts  bearing  upon  the  subject 
of  this  chapter,  and  to  generalize  from  them  to  the  structure  of  the  syn- 
clinal. The  spaces  between  the  red  Hues  of  this  map  are  each  supposed 
to  represent  2,500  feet  of  rock  thickness,  the  spaces  being  narrow  where 
the  dip  is  high,  and  correspondingly  broad  where  it  is  low.  The  lines  were 
constructed  by  first  platting  out  the  spaces  in  those  districts  where  actual 
measurements  had  been  made  of  strike  and  dip,  the  width  of  each  space 
being  made  to  correspond  to  the  width  of  the  surface  outcrop  of  a  2,500- 
feet  thickness  at  the  measured  angle  of  dip.  Where  actual  thinning  on  a 
large  scale  had  been  proved  by  careful  measurement  to  exist — e.  g.,  on 
Keweenaw  Point — the  lines  were  approached  on  this  account  also  to  the 
determined  amount.  Then  the  broken  connecting  lines  were  sketched  in, 
taking  into  account  the  general  lithological  characters  of  different  horizons 
— often  recognized  for  over  a  hundred  miles — the  relations  of  the  belts  to 
the  junction  with  the  Huronian  below  and  to  the  line  between  the  Upper 
and  Lower  Divisions  of  the  Keweenawan,  and  the  angles  of  inclination  and 
trends  indicated  by  the  nearest  exposures. 


416       COPPEE-BEAEING  EOOKS  OF  LAKE  SUPEEIOE. 

The  spaces  between  the  lines  do  not,  of  course,  represent  single  con- 
tinuous beds,  or  even,  in  many  cases,  groups  of  beds,  for  in  their  course 
around  the  lake  such  beds  must  constantly  thin  out  and  be  replaced  by 
others.  The  spaces  are,  however,  designed  to  cover  the  same  general 
horizons,  so  far  as  practicable.  Of  course  there  must  be  many  imperfections  in 
such  a  map,  under  the  very  best  of  circumstances,  for  not  only  do  the  courses 
of  the  belts  under  the  lake  have  to  be  hypothetical,  but,  from  the  general 
similarity  of  the  beds  of  the  Lower  Division  at  very  different  horizons,  there 
must  always  be  more  or  less  doubt  as  to  the  correctness  of  the  connecting 
lines,  even  on  the  land.  Then,  again,  the  map  is  very  irregular  as  to  accu- 
racy in  those  places  where  the  courses  and  inclinations  of  the  beds  can  be 
marked  out.  On  Keweenaw  Point,  for  instance,  the  detailed  measurements 
of  Pumpelly  and  Marvine  make  it  possible  to  locate  the  courses  of  the 
2,500-feet  spaces  with  far  greater  minuteness  of  detail  than  it  is  possible  to 
show  on  svich  a  map  as  this.  From  this  downwards  there  is  every  degree 
of  accuracy  to  cases  where  the  lines  are  purely  hypothetical. 

Notwithstanding  all  these  defects,  the  general  correctness  of  the  struct- 
ure of  the  great  synclinal  indicated  by  the  red  lines  appears  to  me  to  be 
beyond  question.  One  objection  that  I  anticipate  to  this  map  is  that  it  is 
an  attempt  to  apply  the  methods  used  in  studying  sedimeutaiy  beds  to  a 
series  largely  formed  of  eruptive  ones;  to  which  I  have  to  answer  beforehand 
that  this  series  is  just  as  much  made  up  of  layers  as  any  sedimentary  one, 
and  that  in  a  sedimentary  series  beds  thicken  and  thin  and  disappear  just 
as  here. 

The  map  and  sections  do  not  show  the  nature  of  the  bottom  of  the 
trough.  I  believe  this  bottom  to  be  made  up  of  Huronian  slates  below, 
resting  upon  the  older  gneiss,  and  of  Keweenawan  strata  above,  but  both 
greatly  thinned,  since  the  eruptive  rocks,  which  constitute  so  large  a  part 
of  these  groups  around  the  edge  of  the  basin,  appear  to  me  to  have  reached 
the  surface  there. 

The  simplicity  of  the  synclinal  has  been  further  complicated  by 
faulting.  The  fault  to  the  south  of  the  Keweenaw  Point  Range,  it  seems 
probable,  may  have  been  connected  with  a  sudden  change  in  the  dip  of  the 
strata  from  a  flat  to  a  steep  lakeward  inclination.     It  seems  a  plausible 


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•  L      .s  ;  , 

17 

North    aii.l     sulHIi     Uiioui>l(     Mi-  In  piroten     Island                                                                                                                         l^,tr 

Ma^ndtian.       Trmioii'  and.  GndanaU-                             „ 
u                      1                    ,,                    ft               '    EasUrn      softdstone              \  Umtstont'      \                 froupt                 ;                      Jna^ara-  group 

Brldtr^tr^      gmafi 

Giieiss,   gramu    ttt 

lif.,rfj:<u      .<rrir:s.     uf>i"i      lUvi^iuii                                                                                                                                                              JTai-eenaif    serirs                                                                                                       Mu-oninn 

loH-ei-   4t\i.tu>n                                                                                                                Irr'n  beanng  ttfyrt. 

' 

.^...^-. ' 

GKNKKAI.IZKI)    (;K()I.O(;irAL  SKCTION'S    OF  TIIK    lAKK    Sri'KHlOU    liASIN 


SiMli-   soOOTfTi   or  1  ini  li  -  7nui   i 


THE  LAKE  SUFEEIOR  SYKOLINAL. 


417 


speculation  that  this  fault  is  met  at  a  large  angle  by 
another,  coming  from  the  southeast,  at  a  point  behind 
the  line  from  the  end  of  Keweenaw  Point  to  Stannard's 
Rock,  the  junction  of  the  two  being  the  lowest  part  of 
the  dislocation.  Behind  the  Porcupine  Mountains,  as 
previously  shown,  is  again  a  fault,  of  much  smaller  ex- 
tent, which  is  again  connected,  with  a  fold,  though  a  sub- 
ordinate one.  I  have  also  already  indicated  the  prob- 
ability of  the  existence  of  a  fault  on  the  north  side  of  the 
Douglas  County  Copper  Range  of  Wisconsin.  The 
connection  of  the  belts  of  this  range  with  those  of  the 
North  Shore  is  one  of  the  least  satisfactory  parts  of  the 
map  of  Plate  XXX.  It  is  evident,  however,  from  the 
trends  on  the  North  Shore,  and  in  the  Douglas  County 
rock  belts,  that  some  su.ch  connection  must  exist,  though 
whether  with  so  much  of  a  fold  as  I  have  indicated  is 
not  so  plain. 

The  relation  of  the  Huronian  to  the  synclinal  is  a 
point  of  great  interest.  Beyond  question,  in  the  western 
half  of  the  Lake  Superior  basin,  it  bottoms  the  great 
trough,  for  its  beds  are  found  dipping  inwards  on  both 
sides;  on  the  North  Shore  at  a  low  angle,  and  on  the 
south  generally  at  a  high  one.  It  appears  highly  prob- 
able that  the  eastern  part  of  the  trough  is  similarly  bot- 
tomed by  the  Huronian.  The  Huronian  beds  are,  how- 
ever, here  found,  just  without  the  rim  of  the  synclinal, 
folded  in  a  complicated  manner;  for  instance,  beyond 
the  western  end  of  the  trough  in  Minnesota,  in  the  iron 
regions  of  Michigan,  on  the  east  shore  of  Lake  Supe- 
rior, and  about  the  head  of  Lake  Huron.  Other  folded 
schists,  which  possibly  belong  with  the  Huronian, 
occur  in  Canada,  north  of  Lake  Superior.  The  con- 
nection of  these  folded  beds  with  the  unfolded  is  a 
structural  problem  still  needing  investigation.  So  far 
27  LS 


i\^ 


418       COPPBR-BEAEIKG  ROOKS  OP  LAKE  SUPERIOR. 

as  present  knowledge  will  allow,  it  has  been  discussed  in  the  previous  chap- 
ter. 

In  the  preceding  generalized  hypothetical  section  of  the  Lake  Superior 
basin,  which  may  be  looked  on  as  taken  across  from  the  Pigeon  River  region 
of  the  North  Shore,  through  Ontonagon,  the  South  Range,  and  the  Meno- 
minee region  of  Michigan  and  Wisconsin,  but  not  on  a  straight  line,  and 
not  drawn  to  any  scale,  I  have  attempted  to  bring  out  the  following  points: 
(1)  the  synclinal  structure  of  the  lake  basin ;  (2)  the  partial  unconformity 
of  the  Keweenawan  to  the  unfolded  Huronian;  (3)  the  supposed  relations 
of  the  folded  and  unfolded  Huronian ;  (4)  the  limitation  of  the  Keweenawan 
outwards  by  the  higher  Huronian  land ;  and  (5)  the  origin  of  the  Keweenawan 
eruptive  rocks  through  fissures  arranged  around  the  rim  of  the  trough.  If 
this  sketch  represents  actual  conditions,  then  the  downward  bowing  of  the 
great  trough,  which  subsequently  was  filled  with  the  Keweenawan  accumu- 
lations, was  begun  in  the  Huronian  and  continued  through  the  Keweenawan. 
Accompanying  this  downward  bowing  was  a  crumpling  of  the  Huronian  to 
either  side  of  the  broader  bow — and  this  crumpling,  so  far  as  this  sketch  is 
concerned,  may  have  taken  place  in  large  measure  before  the  Keweenawan 
— and  an  extravasation  of  molten  matter  around  the  rim  of  the  trough. 


CHAPTER  X. 

THE  COPPER  DEPOSITS. 

No  special  investigation  made  of  the  copper  deposits. — Different  kinds  of  copper  deposits. — Cupriferona 
sandstones  and  conglomerates. — Cupriftrous  amygdaloids. — Epidote  belts. — Transverse  veins. — 
Similarity  in  origin  of  the  several  forms  of  copper  deposit. — Source  of  the  copper,  and  cause  of 
its  precipitation ;  different  views. — Rules  to  guide  the  explorer  for  copper. — Portions  of  the 
Keweenaw  Series  favorable  and  unfavorable  to  the  occurrence  of  copper. — Prospects  of  future 
developments  of  copper  without  the  present  producing  districts:  in  the  Bad  River  country  of 
Wisconsin ;  between  Bad  River  and  the  Saint  Croix ;  in  the  Saint  Croix  Valley  ;  on  the  Douglas 
County  Copper  Range ;  on  the  Minnesota  side  of  Lake  Superior ;  on  Isle  Royale. 

A  special  study  of  the  copper  deposits  of  the  Keweenaw  Series 
formed  no  part  of  the  plan  of  the  investigation  upon  which  this  memoir  is 
based.  These  deposits,  were,  of  course,  already  the  best  known  things 
about  the  series,  and  any  study  made  with  the  hope  of  adding  materially 
to  the  facts  collected  by  the  numerous  geologists  who  have  hitherto  written 
upon  them  would  have  occupied  far  more  than  the  whole  time  at  com- 
mand. For  the  sake  of  rounding  off  the  subject,  however,  I  may  appropri- 
ately offer  a  general  account  of  the  structural  and  genetic  relations  of  these 
deposits,  adding  a  few  general  considerations  of  an  economic  bearing  as  a 
guide  to  the  future  explorer  for  copper,  both  within  and  without  the  present 
producing  districts. 

All  the  workable  deposits  of  copper  heretofore  discovered  in  the  Lake 
Superior  region  fall  into  one  or  other  of  two  classes,  which  we  may  term 
belt  or  bed  deposits,  and  transverse  vein  deposits.  The  first  class  includes 
the  cupriferous  conglomerates  and  sandstones,  the  cupriferous  amygdaloids, 
and  most,  if  not  all,  of  the  so-called  veins  carrying  much  epidote  and  coin- 
ciding with  the  bearing  of  the  formation ;  the  second  class  includes  those 
veins,  which  traverse  the  formation  in  a  direction  more  or  less  nearly  at 
right  angles  to  the  bedding.  No  copper  has  ever  been  observed  in  connec- 
tion with  the  acid  eraptives  of  the  series,  nor  have  any  workable  deposits 
been  discovered  in  the  massive  non-vesicular  diabase  beds,  except  as  dis- 
tinctly subordinate  to,  and  directly  connected  with,  the  amygdaloid  deposits 
or  epidote  courses,  and  always  accompanied  with  an  extreme  degree  of 
alteration. 

419 


420       COPPEE-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 

The  conglomerate  and  sandstone  deposits  are  simply  portions  of  the 
beds  of  these  rocks,  in  all  respects  of  the  ordinary  character,  save  that  they 
are  impregnated  with  the  native  copper.  Cupriferous  deposits  of  this  char- 
acter are  for  the  most  part  confined  to  the  thin  conglomerate  beds  which  are 
interstratified  with  the  ordinary  diabase  flows;  but  one  cupriferous  bed  of 
sandstone  is  known  within  the  upper  or  purely  detrital  division  of  the  Ke- 
weenaw Series,  and  separated  from  the  nearest  trappean  flow  beneath  it  by  a 
thickness  of  many  hundred  feet  of  sandstone  layers.  This  is  the  belt  of 
dark  colored  sandstone  and  shale  in  which  occurs  the  Nonesuch  copper  bed 
of  the  Porcupine  Mountains.  This  belt  has  been  traced  from  Keweenaw 
Point  to  Bad  River,  a  distance  of  about  150  miles;  and  has  been  found  to 
contain  copper  at  a  number  of  points  in  the  vicinity  of  the  Porcupine 
Mountains,  and  again  on  the  Montreal  Rivei-,  the  boundary  line  between 
Michigan  and  Wisconsin. 

In  the  cupriferous  conglomerates  and  sandstones  the  copper  occurs  as 
a  cementing  material,  and  as  a  replacer  of  the  constituent  grains,  being  in  all 
cases  plainly  of  secondary  origin,  and  a  result  of  deposition  from  an  aque- 
ous solution.  Moreover,  the  cementing  copper  itself,  i.  e.,  that  which  is 
to  be  seen  in  the  thin  section  between  the  constituent  grains  molding 
itself  sharply  around  their  contours,  is  often  also  plainly  a  replacer  of  still 
smaller  constituent  particles.  In  the  case  of  the  Nonesuch  sandstone  of  the 
Porcupine  Mountain  region  a  large  proportion  of  the  particles  of  cementing 
copper  have  within  them  a  core  of  magnetite.  It  is  indeed  not  improbable 
that  in  all  cases,  the  cementing  copper  is  not  a  deposit  in  the  original  inter- 
spaces of  the  fragmental  particles,  but  is  always  a  replacer. 

In  the  thin  sections  of  these  cupriferous  conglomerates  the  larger  par- 
ticles of  porphyry  matrix,  and  fragments  of  the  feldspars,  are  found  to  be 
replaced  by  copper  in  varying  degrees,  the  metal  in  the  case  of  the  feldspar 
fragments  tending  to  follow  the  cleavage  directions.  In  the  famous  con- 
glomerate of  the  Calumet  and  Hecla  mine  in  the  Portage  Lake  region  the 
copper  has  not  only  saturated  the  matrix,  but  has  also  entered  into  and 
more  or  less  completely  replaced  large-sized  pebbles  ajid  even  bowlders 
several  inches  to  a  foot  or  more  in  diameter.  Hundreds  of  such  bowlders 
are  picked  each  day  from  the  heaps  of  rock,  before  it  is  taken  to  the  stamps 


CUPEIFBROUS  CONGLOMERATES  AND  AMYGDALOIDS.         421 

In  these  bowlders  the  copper  has  replaced  both  the  matrix  and  the  porphy- 
ritic  feldspars,  occurring  in  the  latter,  when  the  replacement  has  not  been 
carried  very  far,  often  along  the  cleavage  lines  only.  Pumpelly  has  shown 
that  the  deposition  of  this  copper  has  always  followed  other  great  changes 
in  the  condition  of  the  porphyry  fragments,  and  notably  the  replacement 
of  both  matrix  and  feldspars  by  chlorite  and  epidote;  these  minerals  hav- 
ing in  turn  been  replaced  by  the  copper.  This  relation,  between  copper, 
epidote  and  chlorite,  is  one  which  exists  also  in  the  altered  amygdaloids; 
and  the  source  of  the  constituents  of  these  minerals  may  be  found  either  in 
the  particles  of  amygdaloid  matrix  and  other  basic  materials  which  not 
iinfrequently  occur  in  the  conglomerates  themselves — in  the  Nonesuch  sand- 
stone forming  a  predominating  quantity — or  in  the  overlying  trappean  beds, 
from  which  they  may  have  descended  along  with  the  infiltrating  carbon- 
ated waters. 

The  ordinary  cupriferous  amygdaloids,  such  as  those  which  are  so  largely 
mined  about  Portage  Lake,  are,  as  Pumpelly  was  the  first  to  show,  simply 
the  moi'e  or  less  completely  altered  and  copper-saturated  upper  vesicular 
portions  of  the  old  lava  flows,  and  are  neither  independent  la5^ers,  nor 
"veins"  parallel  with  the  formation.  The  copper  has  been  introduced  into 
these  amygdaloids  during  one  of  the  later  stages  of  a  long  chain  of  replace- 
ments, whose  history  has  already  been  briefly  outlined,  as  worked  out  by 
Pumpelly,  on  a  previous  page.  Several  paragraphs  of  his  descriptions  may 
appropriately  be  quoted  again  in  the  present  connection. 

Considerable  portious  of  the  bed  have  lost  every  semblance  of  an  amygdaloid, 
and  consist  now  of  chlorite,  epidote,  calcite,  and  quartz,  more  or  less  intimately  asso- 
ciated, or  forming  larger  masses,  of  the  most  indefinite  shapes,  and  merging  into  each 
other.  Sometimes  portions  of  partially  altered  prehnite  occur.  In  places,  considera- 
ble masses  of  rich  brown  and  green  fresh  prehnite  filled  with  copper  occur;  but^  as  a 
rule,  this  mineral  has  given  way  to  its  products. 

To  this  process  the  copper-bearing  beds  of  Portage  Lake — wrongly  called  lodes 
— owe  their  origin.  Considerable  portions  of  these  beds  are  but  partially  altered 
amygdaloids,  containing  amygdules  of  prehnite,  chlorite,  calcite  or  quartz,  with  more 
or  less  copper;  other  portions  are  in  the  condition  described  above. 

In  the  still  amygdaloidal  portions,  the  copper  was  deposited  in  the  cavities  and 
in  cleavage-planes  of  some  minerals,  and  replaced  calcite  amygdules,  etc.  But  in  the 
confused  and  highly  altered  parts  of  the  bed  it  crystalized  free,  where  it  had  a  chance; 
more  generally  it  replaced  other  minerals  on  a  considerable  scale.    It  formed,  ia 


422       COPPEE-BBAEING  EOCKS  OP  LAKE  SUPEEIOR. 

ealcite  bodies,  those  irregular,  solid,  branching  forms,  that  are  locally  known  as  horn- 
copper,  often  many  hundred  pounds  in  weight;  in  the  epidote  quartz,  and  prehnite 
bodies,  it  occurs  as  thread  and  flake-like  impregnations ;  in  the  foliaceous  lenticular 
chloritic  bodies,  it  forms  flakes  between  the  cleavage-planes  and  oblique  joints,  or  in 
places — and  this  is  more  particularly  true  of  the  fissure  veins,  which  we  are  not  now 
considering — it  replaces  the  chloritic,  selvage-like  substance  till  it  forms  literally 
liseudomorphs,  sometimes  several  hundred  tons  in  weight.' 

The  copper  in  these  deposits  is  not  restricted  to  that  portion  of  the  bed 
which  was  originally  vesicular,  but  runs  from  it  downward  irregularly  into 
the  originally  compact  portions,  following  always  a  great  alteration  of  the 
rock.  The  copper,  however,  tends  always  to  be  very  irregular  in  distribu- 
tion, and,  even  in  the  longest  worked  and  most  reliable  amygdaloids,  has 
frequently  to  be  searched  for  through  many  feet  of  barren  rock.  In  this 
search  the  diamond  drill- is  now  extensively  used,  the  miners  being  guided 
in  its  use  by  the  occurrence  of  seams  of  ealcite  and  epidote,  and  other 
alteration  forms,  which,  when  followed  up  with  the  drill,  are  often  found  to 
lead  to  pockets  containing  much  copper. 

In  one  class  of  amygdaloids,  those  of  the  ashbed  type, — which  I  agree 
with  Wadsworth  in  regarding  as  merely  very  highly  scoriaceous  and  open 
lava  flows,  into  whose  interstices  the  intermingled  detrital  material  has  sub- 
sequently been  washed — the  distribution  of  the  copper  is  sometimes  more 
uniform  than  in  the  ordinary  cupriferous  amygdaloids,  so  that  the  whole  of 
the  bed  may  be  broken  down  and  taken  to  the  stamps,  as  is  done  for 
instance  at  the  Atlantic  mine. 

The  copper  deposits  of  the  Ontonagon  region  have  not  had  the  study 
given  to  them  that  has  of  late  years  been  devoted  to  those  of  the  Kewee- 
naw Point  and  Portage  Lake  districts:  so  that  it  is  not  possible  to  be  quite 
so  positive  in  our  statements  in  regard  to  them.  The  copper  of  this  region 
never  occurs  in  transverse  fissures,  but  either  lies  in  irregular  accumula- 
tions— often  solid  masses  many  tons  in  weight — associated  with  much 
epidote  and  ealcite,  distributed  along  the  course  of  diabase  beds,  or  else 
occurs  with  more  persistent  and  vein-like  aggregations  of  epidote  and  eal- 
cite. The  latter  coincide  always  with  the  bearing  of  the  formation,  and 
commonly  also  with  its  dip,  but  in  some  cases,  as  for  instance  in  the  once 
famous  Minnesota  mine,  dip  at  a  h'gher  angle  than  that  of  the  formation. 


EPIDOTE  GOUESBS.— TEANSVEESE  VEINS.  423 

which  they  consequently  slowly  traverse  in  depth.  According  to  Foster 
and  Whitney,  deposits  like  that  of  the  Minnesota  mine  show  another  indica- 
tion of  a  vein-like  character  in  the  shape  of  slickensided  and  generally 
sharply  defined  walls  The  "vein"  at  the  National  mine  is  also  peculiar 
in  lying  at  the  base  of  -one  of  the  great  lava  flows,  and  immediately  above 
a  conglomerate  bed,  while  coinciding  with  them  in  both  bearing  and  dip." 

It  is  evident,  even  with  our  present  knowledge  of  the  deposits  of  the 
Ontonagon  district,  that  their  history  has  been  essentially  the  same  as  that 
of  the  Portage  Lake  deposits.  In  the  case  of  that  copper  which  occurs 
irregularly  distributed,  along  with  epidote  and  calcite,  throughout  certain 
of  the  trappean  beds,  the  process  of  replacement  has  gone  on  irregularly, 
because  of  some  irregularity  of  texture  in  the  original  rock.  Deposits  like 
that  of  the  Minnesota  mine  may  have  resulted  from  the  deflection  of  the 
altering  waters  along  the  course  of  a  pre-existing  but  not  open  fissure;  the 
"vein"  being  in  this  case,  as  before,  a  replacement,  at  least  in  large  mea- 
sure, of  original  rock  substance. 

The  transverse  veins  have  been  mined  for  copper  on  Keweenaw  Point 
only,  where  they  are  found  varying  in  width  from  mere  seams  to  10  and 
even  20  and  30  feet.  For  the  most  part,  however,  they  do  not  exceed 
one  to  three  feet  in  width,  the  expanded  portions  being  met  with  where 
they  traverse  the  amygdaloidal  or  otherwise  open  textured  portions  of  the 
flows.  The  same  veins  which,  in  the  amygdaloid  and  looser  textured 
diabases,  are  expanded  and  often  rich  in  copper,  will,  when  in  the  more 
compact  and  massive  beds,  such  as  the  well-known  Greenstone,  contract 
to  mere  seams  without  metallic  contents;  and  the  same  is  in  large  measure 
true  of  their  intersections  with  the  sandstone  belts.  The  veins  lie  always 
very  nearly  at  right  angles  to  the  trend  of  the  beds  which  they  traverse, 
standing  always  very  near  the  perpendicular.  Quartz,  calcite  and  prehnite 
make  up  the  common  veinstone,  but  they  are  mingled  with  more  or  less  of 
the  wall  rock  of  the  vein,  which  frequently  predominates  greatly  over  any 
true  veinstone.  The  veins  are  in  fact  for  the  most  part  not  sharply  defined 
from  the  surrounding  rock,  but  consist  in  each  case  of  a  network  of  smaller 
seams  traversing  the  shattered  wall  rock.  Veins  composed  almost  wholly 
of  calcite  are  not  unknown,  but  they  are  never  productive  of  copper.     The 


424       COPPER-BEARmG  EOGKS  OF  LAKE  SUPEEIOE. 

copper  in  these  veins  occurs  both  in  smaller  fragments  and  minute  particles 
intimately  mixed  with  veinstone,  and  again  in  masses  many  tons  in  weight. 
The  larger  masses  frequently  are  found  to  contain  within  them  portions  of 
the  wall  rock. 

Nearly  all  of  the  productive  mines  based  on  these  transverse  veins  are 
working  directly  beneath  the  Grreenstone,  the  layer  which  is  described  in 
a  previous  chapter  as  constituting  so  prominent  a  feature  in  the  geology 
and  topography  of  Keweenaw  Point.  This  position  of  the  mines  is  one 
not  due  to  the  non-occurrence  of  copper  elsewhere  on  the  course  of  these 
veins,  but  results  from  the  fact  that  further  south  they  become  buried  be- 
neath a  heavy  coating  of  drift,  while  to  the  northward  they  pinch  out  and 
become  barren  in  the  broad  Greenstone  belt. 

These  veins,  on  account  of  their  transverse  position  to  the  bedding  of 
the  formation,  of  their  often  slickensided  walls,  and  from  their  carrying 
often  a  true  veinstone,  have  commonly  been  regarded  as  "true  fissures." 
That  they  are  on  the  hues  of  pre-existing  fissures  or  transverse  cracks  in  the 
formation  there  can,  I  think,  be  no  doubt;  but  they  are  not  true  fissure 
veins  in  the  sense  that  the  veinstone  and  metallic  matter  occupy,  along 
with  wall-rock  fragments,  original  fissure  space.  I  see  in  them  simply 
the  results  of  a  rock  alteration  entirely  analogous  to  that  which  has 
brought  about  the  deposition  of  copper  and  its  associated  veinstone 
minerals  within  the  cupriferous  amygdaloids.  They  are  alteration  zones 
which  traverse,  instead  of  following,  the  bedding,  simply  because  the 
drainage  of  the  altering  waters  has  been  given  this  direction  by  the  pre- 
existing fissures.  All  of  the  phenomena  of  these  veins  coincide  completely 
with  this  view:  the  common  occurrence  of  wall  rock  within  the  vein,  or 
rather  the  embracing  of  the  wall  rock  masses  by  the  vein;  the  replacement 
of  wall  rock  by- copper  masses;  the  occurrence  of  wall  rock  within  these 
masses;  the  expansion  of  the  veins  and  their  greater  richness  where  travers- 
ing the  more  readily  alterable  amygdaloids  and  looser  textured  diabases; 
their  contraction  and  barrenness  within  the  compact  and  less  readily 
changeable  Greenstone;  and  the  coincidence  of  the  paragenesis  of  the 
vein  minerals  with  that  of  the  cupriferous  amygdaloids,  are  all  facts  better 
explicable  on  this  view  than  on  any  other. 


OEIGIN  OF  THE  COPPER  DEPOSITS.  425 

Thus  the  differences  in  origin  of  the  several  classes  of  copper  deposits 
— conglomerate  beds,  cupriferous  amygdaloids,  epidote  veins  parallel  to  the 
bedding,  and  "fissure"  veins  transverse  to  it — which  at  first  sight  seem  to 
be  great,  on  closer  inspection  for  the  most  part  disappear.  They  are  all 
the  result  of  the  percolation  of  carbonated  waters,  which,  in  the  lines  of 
fissure,  the  open  textured  amygdaloids,  and  the  nearly  equally  open  con- 
glomerates, found  the  least  resistance  to  their  passage,  and  at  the  same  time 
the  greatest  susceptibility  to  their  altering  power.  This  susceptibility  de- 
pended partly  upon  the  very  openness  of  these  different  rocks,  but  also,  in 
the  case  of  the  amygdaloids,  in  the  presence  of  a  large- proportion  of  glass 
basis,  the  most  readily  alterable  substance  among  rock  constituents. 

The  source  and  the  cause  of  the  arrest  of  the  copper  which  was  carried 
in  with  the  altering  waters  are  other  and  more  difficult  questions.  Its 
home  has  commonly  been  regarded  as  being  within  the  mass  of  the  trap- 
pean  flows  themselves,  with  which  it  is  supposed  to  have  come  to  the  sur- 
face. Another  view  is  that  it  was  originally  deposited  in  a  sulphuretted 
form  along  with  the  detrital  members  of  the  series,  from  which  it  was  sub- 
sequently leached,  partly  in  the  shape  of  a  sulphate,  but  principally  as  a 
carbonate  and  silicate.  The  latter  is  the  view  which  Pumpelly  has  elabo- 
rated;^ to  whom  also  is  due  the  credit  of  having  advanced  the  only  satis- 
factory view  as  to  the  cause  of  arrest  of  the  copper  in  the  places  where  it  is 
now  found.  He  has  shown  the  existence  of  an  intimate  relation  between 
the  precipitation  of  the  copper  and  the  peroxidation  of  the  ferrous  oxide  of 
the  augitic  constituent  of  the  basic  rocks;  a  relation  so  constant  as  to  ren- 
der irresistible  the  conclusion  that  in  this  ferrous  oxide  is  to  be  found  the 
precipitating  agent  of  the  copper.  To  this  I  would  add  that  the  ferrous 
oxide  of  the  magnetite,  and  of  the  unindividualized  magma  of  the  vesicular 
layers,  has  also  been  concerned  in  this  reaction. 

While  this  explanation  of  the  precipitation  of  the  copper  seems  satis- 
factory, we  have  too  little  to  go  upon  in  deciding  between  the  two  views 
above  referred  to  as  to  the  source  of  the  metal.  Too  few  signs  have  been 
observed  of  the  existence  of  copper  in  the  upper  sandstones  of  the  series, 
such  as  would  be  expected  were  this  its  home,  to  allow  of  an  easy  acqu-i- 

'  Geology  of  Michigan,  Vol.  I,  Part  III,  p.  43. 


426       COPPBE-BEAEING  EOCKS  OF  LAKE  SUPEEIOE. 

escence  in  Pumpelly's  view.  On  the  other  hand,  the  trappeau  rocks  them- 
selves are  for  the  most  part  devoid  of  copper,  except  such  as  is  plainly 
secondary.  Copper  in  a  sulphureted  form  I  have,  however,  observed  in 
the  coarse  gabbros  of  Duluth,  in  the  green  uralitic  gabbro  of  Mount 
Bohemia,  and  in  similar  coarse  rocks  in  one  or  two  places  on  the  north 
shore  of  Lake  Superior.  It  is  commonly  said  that  copper  occurs  in  the 
conglomerates  and  sandstones  only  where  it  could  have  leached  directly 
downwards  from  an  overlying  trappean  mass;  and  with  one  exception  the 
statement  is  undoubtedly  correct.  The  exception  is  that  of  the  Nonesuch 
cupriferous  sandstone,  which  is,  however,  a  very  important  exception,  since 
this  rock  not  only  has  no  overlying  diabase,  but  is  separated  from  the  near- 
est trappean  flow  beneath  it  by  many  hundred  feet  of  detrital  material. 
As  previously  shown,  this  sandstone  is  unusual  for  its  large  proportion  of 
basic  detritus.  Its  copper  can  only  be  connected  with  a  trappean  source 
by  supposing  it  to  have  formed  part  of  this  detritus  in  the  sulphuretted  con- 
dition, and  afterwards  to  have  been  dissolved  and  redeposited  in  a  native 
state.  This  is  a  supposition  which  would  seem  on  the  whole,  however,  to 
be  rather  more  violent  than  to  regard  the  copper  as  having  come  from  the 
overlying  sandstones,  and  as  having  been  arrested  in  its  descent  on  meeting 
a  layer  so  rich  in  basic  detritus  as  to  be  able  to  furnish  the  requisite  supply 
of  precipitating  agent. 

From  the  facts  and  theoretical  considerations  thus  given,  may  be  for- 
mulated a  few  simple  rules  to  guide  the  exploiter  for  copper  in  the  regions 
traversed  by  the  Keweenaw  Series.  Thus  the  explorer,  should  he  be 
searching  for  transverse  veins,  should  bear  in  mind  that  epidote,  prehnite 
and  chlorite  are  the  favorite  associates  of  the  copper ;  that  veins  carrying  a 
greatly  predominating  quantity  of  calcite  are  not  likely  to  be  cupriferous ; 
that  laumontitic  veins  have  hitherto  not  proved  to  be  sufficiently  rich  for 
exploitation;  that  a  vein  which  may  be  very  rich  and  wide  in  the  amygda- 
loidal  or  other  soft  and  easily  decomposed  rocks  will  pinch  to  a  mere  seam 
and  become  barren  within  the  massive  and  more  compact  layers ;  that, 
hence,  the  intersection  of  a  vein  with  such  amygdaloidal  or  other  soft  beds 
should  always  be  searched  for ;  that  the  copper  occurs  in  these  veins  with 
extreme  irregularity;  and  finally,  that  a  vein  found  traversing  decomposed 


EXPLORING  FOE  OOPPEE.  427 

amygdaloid  beds  with  the  favorable  veinstone,  even  though  it  show  only  a 
little  copper  at  surface,  is  worthy  of  examination. 

Should  our  explorer  be  looking  for  cupriferous  belts,  he  should  see  that 
they  are  well  defined  ;  that  they  present  evidence  of  much  alteration  such 
as  is  above  indicated ;  and  that  one  or  more  of  the  favorite  associate  min- 
erals of  the  copper  are  present.  These  favorable  indications,  along  with  a 
more  or  less  well  preserved  amygdaloidal  character  to  the  rock,  and  the 
presence  of  some  copper  at  surface,  are  sufficient  to  warrant  further  exam- 
ination. In  searching  for  these  belts  care  should  be  taken  not  to  be  misled 
by  the  occurrence  of  seams  of  native  copper  without  veinstone  along  the 
joint  cracks  of  an  unaltered  massive  diabase,  and  of  isolated  pockets  of 
epidotic  and  calcitic  material  carrying  some  copper. 

In  the  case  of  sandstone  and  conglomerate  deposits  the  explorer  is  to 
bear  in  mind  that  thus  far  they  have  been  found  only  where  a  thin  seam  of 
conglomerate  is  directly  overlain  by  a  trappean  mass ;  or  if  away  altogether 
from  the  trappean  beds,  only  in  sandstone  which  is  very  rich  in  basic  det- 
ritus. Beyond  this,  there  is  nothing  to  guide  him  except  the  finding  of  the 
copper  itself.  Any  one  of  the  numerous  conglomerate  seams  which  from 
Keweenaw  Point  to  Minnesota  are  everywhere  interbedded  with  the  pre- 
vailing basic  flows,  might  become  cupriferous  at  any  point  along  its  course. 

Large  portions  of  the  Keweenaw  Series  may  be  thrown  out  of  the  ques- 
tion in  considering  the  possibilities  of  future  discovery  of  copper  in  the  Lake 
Superior  region.  Thus  the  whole  extent  of  country  occupied  by  the  Upper 
Division  of  the  series,  with  the  one  exception  of  the  Nonesuch  sandstone 
belt,  appears  to  be  non-cupriferous.  The  extent  of  the  Upper  Division  is 
indicated  on  the  accompanying  maps.  Again,  all  of  the  belts  and  areas  of 
acid  eruptive  rocks,  such  as  the  central  area  of  the  Porcupine  Mountains, 
and  the  great  spread  of  red  rock  in  the  Bruld  Lake  country  in  Minnesota, 
are  without  copper.  The  same  is  true  also  of  all  belts  and  areas  of  coarse- 
grained basic  rocks,  such  as  the  great  area  of  coarse  gabbro  in  the  Bad 
River  country  in  Wisconsin  and  the  similar  area  which  occupies  so  large  a 
belt  of  country  between  Duluth  and  Brul^  Lake  in  Minnesota.  The  favor- 
able phase  of  the  formation  for  the  existence  of  copper  in  any  form  of  deposit 
is  the  thin  and  regularly  bedded  one,  with  well-developed  amygdaloids. 


428       COPPER-BEAEIN^G  EOCKS  OF  LAKE  SUPEEIOE. 

Thus  far  native  copper  mining  has  proved  profitable  within  the  hmits 
of  the  State  of  Michigan  only,  and  it  seems  to  be  true  also  that  all  or  nearly 
all  of  the  producing  deposits  have  been  opened  on  and  Avorked  by  the 
ancient  miners,  whose  attention  was  of  course  attracted  by  those  deposits 
which  by  the  accidents  of  erosion  had  been  left  prominently  exposed.  It 
is  incredible  that  even  in  the  long-settled  districts  of  Michigan  all  of  the 
workable  deposits  of  copper  have  been  discovered.  Thus  on  Keweenaw 
Point  the  valley  south  of  the  Greenstone  Range,  in  which  lie  buried 
beneath  a  surface  coating  of  drift  the  equivalents  of  the  Portage  Lake 
cupriferous  beds,  has  never  been  explored  by  trenching  or  mining  opera- 
tions. The  same  is  in  a  measure  true  of  the  Bohemian  Range  of  Keweenaw 
Point. 

Without  the  boundaries  of  the  State  of  Michigan,  the  attempts  at 
copper  mining  have  been  but  feeble,  and  utterly  inadequate  to  prove  or 
disprove  the  existence  of  workable  copper  deposits.  In  Wisconsin  native 
copper  has  been  met  with  all  along  the  course  of  the  southern  Keweenawan 
belt  from  Montreal  River  to  the  Saint  Croix.  Running  from  the  Montreal, 
in  Sec.  2,  T.  47,  R.  1  E.,  southwest  and  west,  is  a  belt  of  distinctly  bedded 
and  often  amygdaloidal  diabases  in  which  copper  has  been  seen  in  greater 
or  smaller  quantity  both  in  crossing  veins  and  in  altered  diabase  belts,  at 
the  crossing  of  each  stream,  the  intervening  areas  being  drift  covered.  At 
the  crossing  of  Montreal  and  Bad  rivers  this  belt  is  worthy  of  further  exam- 
ination.-' Beyond  Bad  River,  to  the  southwestward,  float  copper  is  exceed- 
ingly common,  and  traces  of  it  are  here  and  there  met  with  in  the  ledges 
themselves.  Unfortunately  the  country  is  one  covered  with  heavy  drift 
accumulations,  through  which  only  the  harder  and  more  enduring,  and 
therefore  non-cupriferous,  beds  ordinarily  project.  The  indications  are 
that,  but  for  the  overlying  sheet  of  drift,  this  region  would  be  as  productive 
in  copper  as  that  of  Keweenaw  Point. 

Rounding  the  turn  at  the  western  end  of  the  great  Keweenawan  syn- 
clinal, in  the  Saint  Croix  Valley,  we  find  the  drift  covering  lighter,  and 
here,  in  the  vicinity  of  Snake  and  Kettle  Rivers,  and  thence  northeastward 
into  Douglas  County,  in  Wisconsin,  are  found  plainly  bedded  diabases  and 

'See  Vol.  Ill,  Geol.  of  Wis.,  pp.  205,206. 


COPPER  IN  WISCONSIN  AND  MINNESOTA.  429 

amygdaloids  carrying  copper  with  interbedded  cupriferous  conglomerates. 
The  region  is  one  which  in  the  early  days  of  mining  excitement  on  Lake 
Superior  was  so  remote  and  inaccessible  that  the  flood  of  copper  hunters 
which  at  that  time  spread  west  from  Keweenaw  Point  failed  to  reach  it.  It 
still  lies  almost  wholly  unexplored,  while  promising  more  to  the  copper 
hunter  than  any  other  portion  of  the  entire  extent  of  the  formation  outside 
of  the  State  of  Michigan. 

Further  north  and  east  from  the  district  last  described  hes  the  Copper 
Tlange  of  Douglas  County,  Wisconsin.^  This  range  has  already  been  fully 
described  on  a  previous  page  as  to  its  position  and  structural  characters. 
Copper  has  been  found  along  its  course  in  a  number  of  places,  chiefly  in 
epidotic  altered  amygdaloids,  and  the  general  structural  characters  are  such 
as  to  indicate  the  possibility  of  the  occurrence  of  copper  in  quantity  along 
this  belt.  Some  little  mining  has  been  done  at  several  points,  but  not 
enough  to  lead  to  any  satisfactory  conclusions. 

On  the  Minnesota  coast  of  Lake  Superior,  copper  has  been  met  with 
at  only  two  or  three  points.  Of  the  five  subordinate  groups  into  which  I 
have  divided  the  rocks  of  this  coast,  only  two,  the  Agate  Bay  and  Tem- 
perance River  groups,  are  of  such  a  nature  as  to  encourage  the  expectation 
that  copper  might  be  found  in  them.  The  great  thickness  which  makes  up 
the  other  three  groups— and  the  same  is  true  of  considerable  portions  of  the 
two  groups  named — is  for  the  most  part  composed  of  very  massive  compact 
beds  such  as  have  never  yielded  copper  on  the  South  Shore.  The  beds  of  the 
Agate  Bay  and  Temperance  Eiver  groups  are  often  thin,  much  altered,  and 
Tiighly  amygdaloidal,  and  might  perhaps  be  found  to  carry  here  and 
there  workable  deposits  of  copper.  The  distribution  of  the  rocks  of  these 
two  groups  is  approximately  shown  on  Plate  XXVI  of  this  volume,  from 
which  it  will  be  seen  that  the  extent  of  country  within  which  there  is  any 
likehhood  of  the  discovery  of  copper  in  this  region  in  the  future  is  a  small 
■one,  lying  for  the  most  part  in  the  immediate  vicinity  of  the  lake  shore. 
It  is  also  to  be  observed  that  the  most  probable  mode  of  occurrence  for 
copper  within  this  restricted  area  is  the  amygdaloid  belt,  the  form  in  which 
occurs  the  copper  of  French  River,  where  the  metal  is  associated  with 

'See  Chapter  VI,  p.  250.     See  also  Geo],  of  Wis.,  Vol.  Ill,  pp.  357,  362. 


430       COPPEE-BEAEING  EOCKS  OP  LAKE  SUPEEIOE. 

much  prehnite;  that  such  amygdaloid  belts  will  dip  towards  the  lake  at  a 
very  flat  angle;  and  that  cuprifei'ous  conglomerates  are  not  to  be  looked 
for. 

Isle  Royale  is  the  only  remaining  portion  of  the  copper-bearing  rocks 
within  the  territory  of  the  United  States.  It  has  long  been  known  to  be 
cupriferous  ;  the  copper  occurring  here  in  the  three  forms  of  transverse 
veins,  epidote  courses  and  amygdaloid  belts.  Thus  far  mining  has  never 
prospered  on  the  island.  It  can,  however,  hardly  be  said  that  the  ground 
has  as  yet  been  sufficiently  tested. 


]S^  O  T  E  S. 


NOTE    I 


(Page  13,  line  1.) 
N.  H.  WESrCHELL  ON  THE  GEOLOGICAL  POSITION  OF  THE  COPPEE  SERIES. 

Since  this  was  printed,  E".  H.  Winchell  has  published  (Tenth  Annual  Eeport  of 
the  Geological  and  Natural  History  Survey  of  Minnesota,  pp.  123-126)  a  more  definite 
statement  of  his  views  as  to  the  geological  position  of  the  Copper-Bearing  Series, 
which  he  places,  as  before,  as  the  equivalent  of  the  Potsdam  Sandstone  of  New  York, 
regarding  the  Eastern  Sandstone,  however,  as  in  part  newer  than,  and  unconformably 
superposed  upon,  the  Copper-Bearing  Series.  He  had  not  before  stated  definitely  his 
views  as  to  the  position  of  the  Eastern  Sandstone.    See  further  as  to  this,  Note  22. 

NOTE   2. 

(Page  14,  foot-note.) 

LITERATUEE  LIST. 

Mr.  M.  B.  Wadsworth's  bibliography  of  Lake  Superior  Geology,  referred  to  in  this 

foot-note,  covers  references,  not  only  to  the  Copper-Bearing  Series,  but  to  all  other 

formations  represented  in  the  Lake  Superior  region.    It  is  arranged  alphabetically, 

and  brought  down  to  1880.    The  references  in  the  literature  list  of  this  volume  are 

all  taken  directly  from  the  originals,  my  indebtedness  to  Mr.  Wadsworth's  bibliography 

consisting  in  its  having  led  me  to  a  number  of  references  which  might  otherwise  have 

been  overlooked. 

NOTE   3. 

ADDITIONS  TO  LITERATURE  LIST. 

(Page  18.) 

Insert,  as  last  reference  under  1854 : 
Whitney,  J.  D.    Metallic  Wealth  of  the  United  States,    Philadelphia,  1854,  510  pp. 

(Page  21.) 

Insert  under  the  year  1871,  as  the  second  work: 

Kloos,  J.  H.    Geologische  Notizen  aus  Minnesota.    Zeit.  der  deutsch.  geol.  Gesell., 

1871. 

431 


432  COPPER  BEAEING  ROCKS  OF  LAKE  SUPERIOR. 

(Page  23.) 

Insert  under  1880  the  following  additional  reference: 
iBViNCr,  R.  D.    The  Mineral  Resources  of  Wisconsin.    Trans.  Am.  Inst.  Min.  Eng., 
1880,  viii,  478-508. 

(Page  23.) 

The  following  works,  referring  more  or  less  fully  to  the  Copper-Bearing  Eocks, 
have  appeared  since  the  literature  list  was  printed: 

1882. 

Powell,  J.  W.  Report  of  the  Director  of  the  U.  S.  Geological  Survey,  for  the  year 
ending  June  30,  1881.  Contains  a  brief  preliminary  announcement  (pp.  xxxi- 
xxxix)  of  some  of  E.  D.  Irving's  most  important  results. 

WiNCHELL,  !N".  H.  Tenth  Annual  Report  of  the  Geol.  and  Kat.  Hist.  Survey  of  Min- 
nesota, 254  pp. 

1883. 

Chambeklin,  T.  C.    Geology  of  Wisconsin,  Vol.  I,  Part]  I.    General  Geology,  1-300. 
Ikving,  R.  D.    Mineralogy  and  Lithology  of  Wisconsin.    Geology  of  Wisconsin,  Vol. 
I,  Part  II,  309-361. 

NOTE   4. 

(Page  32.) 

CHEONOLOGICAL  EELATIONS  OF  THE  DIFFERENT  CLASSES  OF  EEUPTIVES  OF  THE 

KEWEENAW  SEEIES. 

The  facts  upon  which  is  based  the  statement  of  page  32,  to  the  effect  that  no  such 
chronological  relations  are  found  to  obtain  between  the  Keweenawan  eruptives  of  dif- 
ferent degrees  of  acidity  as  are  said  to  hold  true  in  so  many  Tertiary  and  Post-Tertiary 
volcanic  regions,  are  given  in  various  places  in  subsequent  pages  of  the  memoir.  It 
will,  however,  be  convenient  to  summarize  and  classify  them  here  briefly,  including  the 
results  of  some  analytical  determinations  made  since  the  memoir  has  been  in  type. 

The  chronological  relations  referred  to  as  obtaining,  according  to  the  geologists 
who  have  examined  them,  in  many  Tertiary  and  Post-Tertiary  volcanic  regions,  and 
notably  in  those  of  the  western  Cordilleras,  consist  in  this,  viz,  that  the  eruptives  of 
different  acidity  have  followed  one  another  in  a  certain  unvarying  order.  The  earliest 
eruptives  of  any  one  district  are  found  to  have  been  of  rocks  of  intermediate  acidity. 
Next  in  order  have  come  rocks  of  high  acidity;  whilst  last  of  all  have  been  erupted 
those  of  low  acidity,  ordinarily  known  as  "  basic. "  Among  the  Keweenawan  eruptives 
there  has  been  no  such  chronological  relation.  The  facts  upon  which  this  conclusion 
is  based  may  be  classed  under  the  following  heads :  (1)  the  positions  of  the  different 
kinds  of  eruptives  in  the  stratigraphy  of  the  series;  (2)  the  occurrence  of  acid  flows 
directly  and  visibly  superposed  upon  basic  flows ;  (3)  the  occurrence  of  flows  of  in- 
termediate acidity  overlying  acid  flows ;  (4)  the  occurrence  of  flows  of  intermediate 
acidity  overlying  porphyry-conglomerates;  (5)  the  occurrence  of  flows  of  intermediate 
acidity  overlying  basic  flows ;  (6)  the  intersection  of  basic  by  acid  rocks.  On  the  other 
hand  we  may  cite  as  showing  not  only  the  absence  of  the  ordinary  Tertiary  order,  but 
the  failure  of  order  of  any  kind;  (7)  the  occurrence  of  basic  overlying  acid  flows;  (8) 


NOTES.  433 

the  occurrence  of  basic  flows  superposed  upon  intermediate  flows;  and  (9)  the  inter- 
section of  acid  by  basic  rocks. 

(1.)  The  position  of  ilie  different  hinds  of  eruptives  in  the  general  stratigraphy  of  the 
series. — Acid  rocks,  while  on  the  whole  decidedly  affecting  low  horizons  in  the  series, 
here  and  there  occur  at  quite  high  ones,  as,  for  instance,  in  the  Porcupine  Mountain 
region  (pp.  155,  206—224,  and  Plates  XIX,  XX,  and  XXI),  and  on  Michipicoten  Island 
(pp.  155,  342,  343).  In  the  latter  case  the  acid  rocks  occur  at  the  summit  of  a  series 
of  over  18,000  feet  of  plainly  bedded  eruptive  flows,  both  intermediate  and  basic, 
with  interstratified  conglomerates  and  sandstones.  But,  wherever  occurring,  at  low 
horizons  or  high,  the  beds  of  acid  rocks  have  commonly  above  and  below  them  basic 
beds.  The  instances  of  such  an  occurrence  of  acid  rocks  are  altogether  too  numer- 
ous for  all  to  be  here  referred  to.  I  may  merely  mention  the  following  cases:  Mount 
Houghton,  on  Keweenaw  Point  (pp.  181-183,  Plates  XVII,  XVIII),  is  a  mass  of  red 
felsite,  having  both  on  the  south,  or  below  it,  and  on  the  north,  or  above  it,  great 
thicknesses  of  basic  flows.  The  bolt  of  red  porphyry  which  forms  so  prominent  a 
feature  to  the  west  of  the  Ontonagon  Eiver,  and  as  far  as  the  Bad  River  in  Wisconsin, 
has  in  a  similar  manner  basic  flows  both  above  and  below  it  (pp.  199,  209,  220,  231, 
Plates  XXII,  XXIII.  See  also  Geol.  of  Wis.,  Ill,  pp.  195, 198,  Plate  XVI,  and  Atlas, 
Plate  XXII).  On  the  Minnesota  coast  the  larger  number  of  flows  of  acid  rocks  occur 
in  the  subordinate  series  of  beds  which  I  have  called  the  Beaver  Bay  Group  (pp.  298- 
323) .  Both  above  and  below  the  Beaver  Bay  Group  occur  great  thicknesses  of  basic 
flows  (Agate  Bay  Group,  Temperance  Eiver  Group,  pp.  267,  268,  and  Plate  XXVI)  j 
besides  which,  within  the  Beaver  Bay  beds  themselves,  the  acid  rocks  are  found  over- 
lain and  underlain  by  basic  flows. 

But  not  onlj  is  it  plain  from  their  stratigraphical  relations  that  the  acid  and  basic 
eruptives  altei'nated  with  one  another  in  formation ;  the  flows  of  intermediate  acidity 
also  evidently  alternated,  as  to  time  of  formation,  Avith  both  basic  and  acid  flows,  for 
the  beds  of  intermediate  acidity  occur  at  many  horizons  throughout  the  series.  For 
instance,  the  peculiar,  resinous-looking  diabase-porphyrites  of  the  south  side  of  Michi- 
picoten Island  (pp.  86,  87,  343),  having  about  60  per  cent,  of  silica,  lie  above  many 
thousand  feet  of  basic  flows.  Numerous  instances  of  the  occurrence  of  sub-basic 
diabase-porphyrites  on  the  Minnesota  coast  might  also  be  cited  in  this  connection,  as, 
for  instance,  the  brown  diabase-porphyrite,  with  strongly-developed  amygdaloid,  which 
forms  the  shore  cliff  one  mile  below  the  mouth  of  Silver  Creek,  NE.  ^  Sec.  22,  T.  53, 
E.  10  W.,  Minnesota  (pp.  80, 84,  285).  An  interesting  case  of  the  interstratification  of 
a  rock  of  intermediate  acidity  with  basic  kinds  is  furnished  by  the  trap  belt  which 
overlies  the  felsitic  porphyry  of  the  Porcupine  section  (pp.  209,  214,  217,  Plates  XIX^ 
XX,  XXI).  This  belt  has  a  surface  width  of  one-fourth  to  one-third  miles,  and  a  thick- 
ness of  from  300  to  500  feet.  Towards  the  middle  of  the  thickness  a  porphyry-con- 
glomerate is  included,  with  a  thickness  of  over  60  feet.  The  rocks  of  this  belt  include 
diabases  and  diabase-amygdaloids  of  the  ordinary  types,  and  luster-mottled  melaphyrs. 
Interstratified  with  these,  both  above  and  below  the  intermediate  conglomerate,  are 
layers  of  a  diabase-porphyrite  which  is  distinctly  of  jptermediate  acidity,  containing, 
80  far  as  determined,  about  60  per  cent,  of  silica.  Clearly,  then,  we  have  here  an  acid 
porphyry  succeeded  by  flows  of  a  wholly  basic  material,  following  which  come,  in  as- 
cending order,  a  rock  of  intermediate  acidity,  others  which  are  completely  basic,  a  por- 


434  COPPEE  BEAEING  EOCKS  OF  LAKE  STJPEEIOR. 

phyry-conglomerate,  basic  flows,,  again  more  rocks  of  intermediate  acidity,  more  basic 
rocks,  and  finally  a  great  thickness  of  porpliyry-conglomerate  and  sandstone.  N'o 
possible  explanation  of  this  section  can  be  offered  by  which  a  succession  of  intermedi- 
ate, acid,  and  basic  eruptions  can  be  made  out,  for  even  if  the  acid  porj)hyry  lying  at 
the  base  of  the  section  should  be  taken  as  intrusive,  and  therefore  possibly  subsequent 
to  the  overlying  rocks,  there  remains  the  intervening  porphyry-conglomerate  to  prove 
the  existence  of  acid  porphyries  prior  to  the  eruption  of  both  the  intermediate  and 
basic  rocks  of  this  belt;  whilst  the  diabase-porphyrite  cannot  in  any  way  be  made  out 
as  antecedent  to  all  of  the  basic  rocks  with  which  it  is  immediately  associated. 

It  should  be  said  that  in  all  cases  here  cited  of  the  occurrence  of  rocks  of  inter- 
mediate acidity,  care  has  been  taken  to  refer  only  to  those  in  which  the  intermediate 
acidity  is  plainly  an  original  character,  and  not  in  any  measure  one  due  to  a  subse- 
quent infiltration  of  secondary  quartz. 

The  question  might  arise  in  some  minds  as  to  whether  the  cases  here  cited  of  the 
indiscriminate  stratification  of  acid,  intermediate,  and  basic  rocks  might  not  be  due  to 
the  subsequent  intrusion  in  the  form  of  sheets  of  all  of  the  basic  rocks  concerned. 
That  some  of  the  basic  rock  beds  of  the  series,  and  especially  those  formed  of  coarse- 
grained rocks,  may  be  of  an  intrusive  nature,  has  been  indicated  in  the  memoir  (pp.  27, 
144),  though  definite  evidence  of  this  is  lacking.  However  this  may  be,  in  the  present 
connection,  carcjhas  been  taken  to  consider  only  those  basic  rocks  which  are  furnished 
with  well-developed  amygdaloids  and  are  consequently  the  results  of  flowage  at 
the  then  existing  surface  in  each  case.  Indeed,  should  we,  for  the  sake  of  argument, 
admit — what  I  do  not  at  all  believe — that  all  of  the  basic  and  intermediate  beds  which 
are  not  furnished  with  amygdaloids  are  intrusive,  those  furnished  with  amygdaloids 
being  always  taken  as  surface  flows,  we  should  immediately  find  ourselves  at  the  same 
result,  namely,  that  there  has  been  no  definite  order  among  the  eruptions  of  different 
acidity. 

Many  more  instances  than  are  here  mentioned  might  be  cited,  but  it  is  thought 
that  those  given  are  sufficient  for  the  sake  of  the  argument. 

The  Huronian,  beneath  the  Keweenawan,  contains  many  beds  of  eruptive  mate- 
rial. True  acid  rocks  are  extremely  rare,  if  indeed  they  occur  at  all,  but  basic  and 
intermediate  eruptives  are  plenty.  There  is  much  doubt,  with  our  present  knowledge 
of  them,  as  to  how  far  these  eruptives  are  intrusive,  and  though  it  is  not  deemed  prob- 
able, some  of  them  may  be  intrusive  sheets,  contemporaneous  with  the  surface  flows 
of  the  Keweenawan.  Many  of  these  beds  partake  of  the  folds  of  the  folded  Huro- 
nian and  hence  antedate  the  folding.  On  the  whole,  it  now  seems  probable  that  by  far 
the  greater  part  of  the  Huronian  eruptives  preceded  all  of  the  Keweenawan  eruptives, 
acid  and  basic. 

(2.)  The  occurrence  of  acid  flows  directly  and  visibly  superposed  upon  basic  flows. — 
Acid  rocks  directly  overlying  basic  flows  are  met  with  in  several  places  on  the  Min- 
nesota coast,  but  in  the  case  of  the  Great  Palisades,  fully  described  in  the  memoir  (pp. 
146-148,  314-318,  Figs.  23  and  24),  the  occurrence  is  so  striking  and  conclusive  that  no 
others  need  be  cited.  To  the  descriptions  given  in  the  memoir,  I  may  merely  add  here 
that  analytical  determinations  made  since  these  descriiitions  were  in  type  show  that 
the  diabases  (or  rather  diabase-porphyrites,  since  they  contain  much  non-polarizing 
matter)  underlying  the  quartz-porphyry  of  the  Palisades  belong  with  the  more  basic  of 


NOTES.  435 

the  basic  rocks,  having  less  than  48  per  cent,  silica.  Of  course  the  presence  of  strongly- 
marked  amygdaloids  and  of  intervening  red  shaly  seams  proves  that  these  diabases 
succeeded  each  other  regularly  as  flows  at  the  then-existing  surface,  and  that  the  last 
of  them  was  succeeded  in  turn  by  the  flow  of  the  quartz-porphyry  which  makes  up  the 
mass  of  the  Palisades. 

(3.)  The  occurrence  of  flows  of  intermediate  acidity  immediately  overlying  acid  flows.— 
The  porphyry  of  the  Palisades  just  alluded  to,  or  another  flow  closely  like  it,  at  the 
mouth  of  Baptism  Eiver,  passes  under  a  series  of  beds  seen  in  a  single  cliff,  in  which 
the  succession  is  as  follows,  beginning  below:  (1)  brown,  aphanitic  diabase-porphyrite, 
with  52.5  per  cent,  of  silica,  thickness  not  measured,  but  under  fifty  feet;  (2)  black 
oUvine-diabase,  with  crowning  amygdaloid,  and  containing  50.76  per  cent,  of  silica;  and 
(3)  brown  diabase-porphyrite,  with  57.87  per  cent,  of  silica,  and  also  furnished  with  a 
crowning  amygdaloid.  Thus,  overlying  a  quartz-porphyry,  we  have  m  order  a  sub- 
basic,  a  basic,  and  an  intermediate  flow. 

(4.)  The  occurrence  of  flows  of  intermediate  acidity  overlying  porphyry-conglomer- 
ates.—InteTmedisite  flows  are  of  course  often  met  with  at  horizons  in  the  series  higher 
than  occupied  by  beds  of  conglomerate.  In  the  case  of  the  inner  trap  belt  of  the  Por- 
cupine Mountains  (pp.  214^217)  a  diabase-porphyrite,  with  60  per  cent,  of  silica,  very 
closely  overlies  a  porphyry-conglomerate,  a  small  thickness  of  basic  flows  separating 

the  two.  . 

(5).  The  occurrence  of  flows  of  intermediate  acidity  immediately  superposed  upon  baste 
flaws.— A  number  of  occurrences  of  this  kind  are  to  be  met  with  among  the  Agate  Bay 
and  Lester  River  beds  of  the  Minnesota  coast  (pp.  267,  279-294),  but  the  only  case  in 
which  the  silica  contents  of  the  adjoining  rocks  have  been  determined  is  that  of  the 
clitt"  side  one  mile  below  the  mouth  of  Baptism  River,  cited  in  the  last  paragraph,  in 
which  case  an  olivine-diabase,  with  50  per  cent,  of  silica,  is  overlain  by  a  diabase-porphy- 
lite  with  58  per  cent,  of  silica,  both  rocks  being  plainly  surface  flows,  since  both  are 
furnished  with  well-developed  amygdaloids. 

(6.)  The  intersection  of  basic  by  acid  rocfe.— Coarse  olivine-gabbro  is  intersected  by 
granite  in  a  number  of  places  in  the  Bad  River  region  of  Wisconsin  (Geol.  of  Wis., 
Ill,  pp.  168,  183-193;  this  vol.,  p.  125).  The  coarse  orthoclase-bearing  gabbro  of  Du- 
luth  (49  per  cent,  silica)  is  intersected  by  granitic  and  other  acid  porphyries  (pp.  270- 
272).  Granite-like  rocks,  apparently  intersecting  basic  flows,  occur  at  several  points 
on  the  Minnesota  coast  (pp.  303,  305,  310,  329).  On  the  Bohemian  Mountain,  on  the 
north  side  of  Lac  La  Belle,  Keweenaw  Point,  red  granitic  porphyry  apparently  inter- 
sects a  melaphyr  (p.  184). 

(7.)  The  occurrence  of  basic  flows  overlying  acid  roefcs.— The  quaj-tziferous  porphyry 
of  Baptism  River,  already  several  times  cited,  is  closely  overlain  by  a  basic  flow,  and  a 
number  of  other  instances  are  to  be  met  with  on  the  Minnesota  coast.  In  the  region  of 
the  Ontonagon  River  and  the  Porcupine  Mountains  (pp.  206-225),  and  again  in  the  Bad 
River  region  of  Wisconsin  (Geol.  of  Wis.,  pp.  195-198,  and  Atlas,  Plate  XXII)  are  sim- 
ilar occurrences. 

(8.)  The  occurrence  of  basic  flows  overlying  those  of  intermediate  acidity.— A.  number 
of  cases  of  this  occur  among  the  Agate  Bay  beds  of  the  Minnesota  coast  (pp.  284-294), 
and  the  same  thing  is  met  with  in  the  inner  trap  belt  of  the  Porcupine  Mountains,  as 
already  described. 


4.3  G  COrPER  BEAEIIfG  ROCKS  OF  LAKE  SUPERIOR. 

(9.)  The  intersection  of  acid  by  basic  rocl<s. — Dikes  of  basic  rocks  are  to  be  observed 
cutting  acid  porpliyries  at  a  number  of  places  on  the  north  shore,  as  for  instance  near 
Lester  River  (p.  283),  at  Beaver  Bay  (p.  307),  at  Grand  Marais  (p.  320),  near  Red  Rock 
Bay  (p.  322).  In  the  last  cas€J  the  dike-rock  is  a  diabase-porphyrite  with  only  45.8 
per  cent,  of  silica,  whilst  the  rock  intersected  is  a  typically  developed  quartz-porphyry. 

NOTE    5. 

(Page  32.) 

COMPARISON  BETWEEN  TERTIARY  AND  KEWEENAWAN  ERUPTIVES. 

In  a  first  brief  announcement  of  the  results  of  my  study  of  the  Keweenaw  Series 
(Report  of  the  Director  of  the  United  States  Geological  Survey  for  the  year  ending 
June  30, 1881,  p.  xxxiii),  Director  Powell  says  that  the  acid  eruptives  of  the  Kewee- 
nawan  "  are  regarded  by  Irving  as  ancient  rhyolites  and  trachytes,  from  the  degrada- 
tion of  which  the  conglomerates  of  the  series  have  resulted."  In  the  notes  I  furnished 
the  Director  as  to  my  results,  I  had  not  meant  to  convey  the  meaning  that  I  regarded 
the  Keweeuawan  acid  eruptives  as  lithologically  identical  or  equivalent  with  the  Ter- 
tiary rhyolites  and  trachytes — though  his  words  perhaps  might  be  so  understood — 
but  merely  to  indicate  that  they  occupied  the  same  general  position  as  to  acidity  and 
general  lithological  characters  among  the  Keweenawan  eruptives  as  are  occupied 
among  the  Tertiary  eruptives  by  the  rhyolites  and  trachytes.  My  acquaintance  with 
the  Tertiary  eruptives  is  too  limited  to  allow  of  my  passing  an  opinion  on  the  accuracy 
of  the  view  commonly  accepted  bj^  the  most  eminent  lithologists  of  the  day,  viz,  that 
the  Tertiary  eruptives  are  always  distinct  and  deserving  of  separate  names  from  those 
that  preceded  Tertiary  time. 

NOTE    6. 

(Page  32,  eighth  line  from  bottom ;  also  page  138,  last  paragraph.) 

A^OLCANIC  ASH  IN  THE  KEWEENAW  SERIES. 

Mr.  A.  R.  C.  Selwyn  has  published  (Science,  Vol.  I,  No.  1,  February  9, 1883 ;  also, 
Vol.  I,  No.  8,  March  30, 1883),  since  this  volume  was  in  type,  a  statement  that  volcanic 
ash  exists  in  the  Keweenawan  rocks  of  Michipicoten  Island,  but  he  has  not  yet  pub- 
lished any  description  of  this  material.  Macfarlane,  in  his  descriptions  of  the  Geology 
of  Michipicoten  Island  (Geol.  Surv.  Canada:  Report  of  Progress,  1863-'6G,  p.  138  et 
seq.),  speaks  of  "  breccias,"  which,  as  I  understand  him,  are  the  rocks  to  which  Selwyn 
refers.  Macfarlane  describes  the  breccia  in  one  case  as  consisting  of  "  small  fragments 
of  melaphyr,  some  fresh  looking,  but  the  greater  part  bleached  to  a  reddish-gray  color, 
inclosed  in  a  reddish-brown  earthy  matrix,  consisting  most  probably  of  finely  commi- 
nuted melaphyric  material,  as  it  is  readily  fusible  before  the  blow-pipe."  In  another 
case  he  speaks  of  "a  trap  breccia,  composed  of  fragments  of  dark-brown  melaphyr, 
cemented  together  by  a  brownish-red  trappean  sand."  I  have  not  observed  anywhere 
on  the  south  or  north  shores  of  the  lake  any  rocks  which  resemble  these,  if  I  under- 
stand Macfarlane's  descriptions  correctly,  unless  they  are  somewhat  like  the  Nonesuch 
sandstone  of  the  Porcupine  Mountains.  I  have  said  in  the  text  that  this  sandstone  may 
be  in  part  of  volcanic  ash  nature.     Though  it  is  not  impossible  that  the  Michipicoten 


NOTES.  4S7 

breccias  may,  in  part  at  least,  have  originated  as  volcanic  asli,  I  imagine  that  it  would 
be  exceedingly  difficult  to  prove  such  an  origin  for  tbeui.  Constant  angularity  of  tlie 
particles  might  perhaps  point  that  way,  but  a  genuine  vesicular  character  to  each 
fragment  would  be  about  the  only  proof  of  such  an  origin,  and  in  such  ancient  rocks, 
so  profoundly  altered  as  these  must  be,  it  would  be  extraordinary  if  angularity  and 
vesicular  character  should  be  preserved.  Dr.  T.  S.  Hunt  writes  me  that  he  observed 
nothing  on  Michipicoten  Island  that  reminded  him  of  the  typical  volcanic  ash  of  such 
regions  as  those  of  Vesuvius  and  the  Eifel,  and  suggests  that  these  brecciated  rocks  may 
be  due  to  the  disintegrating  effect  of  waters  upon  material  extravasated  beneath  the 
sea.  Such  an  origin  is,  of  course,  possible,  and  it  may  be  true  in  a  measure  also  of 
the  common  red  sandstones  of  the  Keweenaw  Series,  though  these  are  very  plainly  in 
large  measure  composed  of  water-rolled  and  water-worn  fragments.  But,  whatever 
the  origin  of  the  Michipicoten  breccias  may  be,  I  have  never  met  with  anything  just  like 
them  on  either  the  north  or  south  shores  of  Lake  Superior,  if  I  understand  Macfar- 
lane's  descriptions  correctly. 

NOTE   7.. 

(Page  39.) 

PLAGIOCLASTIC  INGREDIENTS  OF  THE  KEWEENAWAN  BASIC  EEUPTIVES. 

Since  this  volume  has  been  in  type,  a  number  of  separations,  by  Thoulet's  specific 
gravity  method,  of  the  plagioclastic  ingredients  of  the  several  kinds  of  Keweenawan 
basic  rocka  have  been  made  by  my  assistant,  Mr.  C.  E.  Vanhise.  These  separations 
were  undertaken  with  the  view  of  determining  whether  only  one  plagioclase  feldspar, 
as  indicated  by  the  optical  method,  or  more  than  one,  is  concerned  in  the  make-up  of 
the  Keweenawan  basic  eruptives.  The  investigation  is  not  yet  completed,  but  so  far 
as  it  has  gone  it  has  tended  to  strengthen  the  conclusion  already  arrived  at,  namely, 
that  only  one  plagioclase  feldspar  is  ordinarily  present,  except  in  some  of  the  por- 
phyritic  kinds,  in  which  the  porphyritic  plagioclases  are  different  from  the  microliths 
•of  the  groundmass.  Especially  in  the  case  of  the  coarse  olivine-gabbros  and  olivine- 
free  gabbros  did  the  experiment  confirm  the  conclusions  before  arrived  at,  namely,  that 
only  one  plagioclase  is  concerned,  and  that  commonly  lies,  in  its  silica  content,  between 
anorthite  and  labradorite.  The  silica  determinations  made  upon  the  separated  plagi- 
oclases in  no  case  showed  less  than  46  per  cent.  But  if  we  accept,  as  it  seems  almost 
necessary  that  we  should  do,  Tschermak's  theory  of  the  nature  of  the  intermediate 
plagioclase  feldspars,  this  high  silica  content  is  easily  explicable  on  the  view  that  only 
one  feldspar  is  concerned.  Tschermak's  view  is  still  further  confirmed  by  the  fact  that, 
in  every  instance  where  the  optical  measurements  stood  near  the  border  between 
anorthite  and  labradorite,  the  silica  content  also  more  nearly  approached  that  of 
labradorite. 

See  also  Kote  10  for  the  results  of  experiments  made  with  the  coarse  anorthite- 
rock  of  the  Minnesota  coast. 


438 


COPPEE  BEARING  EOCKS  OF  LAKE  SUPEEIOR. 


NOTES. 

(Page  46.) 

NATURE  OF  THE  FELDSPATHIC  INGREDIENT  OF  GABBRO  FROM  CLOQUET  RIVER. 
The  feldspar  of  specimen  1103,  separated  from  the  other  ingredients  by  Thonlet's 
method,  gave  52.40  per  cent,  of  silica,  or  almost  exactly  that  of  labradorite,  the  feld- 
spar indicated  by  the  optical  measurements,  as  will  be  seen  by  reference  to  the  table 

on  page  46. 

NOTE    9. 

(Page  60.) 
NATURE  OF  THE  FELDSPATHIC  INGREDIENT  OF  THE  GABBRO  FROM  NEAR  THE  MOUTH 

OP  NIPIGON  RIVER. 

The  feldspar  of  specimen  1752,  separated  oiit  by  Thonlet's  method,  yielded  49.28 
per  cent,  of  silica.  The  optical  measurements  indicate  anorthite.  A  separation  of 
the  feldspars  into  two  parts  by  Thonlet's  method  was  tried,  but  without  success. 

N  OTE    lo. 

(Page  113.) 

SECONDARY  QUARTZ. 
The  secondary  quartz  of  the  orthoclase-gabbros  (p.  51),  augite-syenites,  and 
granitoid  porphyries  of  the  Keweenaw  Series  (p.  113)  is  chiefly  of  the  kind  called  by 
Fouqu6  and  L6vy  (Min^ralogie  Micrographique,  p.  193),  "quartz  de  corrosion."  Some . 
of  the  secondary  quartz  of  the  last-named  rocks  may  also  correspond  to  their  "quartz 
globulaire"  (op.  cit.,  p.  194),  but  I  find  nothing  in  their  descriptions  or  figures  which 
recalls  the  peculiar  arborescent  secondary  quartz  so  commonly  met  with  in  the 
matrices  of  the  Lake  Superior  felsitic  porphyries  (p.  92). 

NOTE    II. 

(Page  59.) 
ANORTHITE-ROCK. 
Since  this  account  of  the  rock  of  the  anorthite  bowlders  and  masses,  met  with 
inclosed  in  the  olivine-gabbro  of  the  Minnesota  coast,  was  put  in  type,  the  following 
analytical  determinations  have  been  made  upon  the  anorthite: 


729 

729  A 

729  B 

822 

822  A 

822  B 
(single 
crystal.) 

822  C 

Silica  fSiOs)        

47.50 

48.91 

48.19 

47.25 
31.56 
2.29 
15.39 
0.27 
0.37 
2.52 
0.40 

47.41 

46.56 

46.54 



r.iTriR  fOa  O) 

Sofia  I'Na?  O) 

100.  05 

No.  729  is  from  a  great  mass  of  anorthite  surrounded  by  black  olivine-gabbro,  near 
the  mouth  of  Split  Eock  Eiver,  on  the  Minnesota  coast  (see  p.  59).     The  sample  for 


NOTES. 


439 


analysis  was  broken  from  all  parts  of  the  specimen,  care  being  taken  to  select  parti- 
cles free,  so  far  as  could  be  detected,  from  any  included  substance.  Nos.  729  A  and 
Bf  are  from  portions  separated  by  Thoulet's  method  (iodide  of  mercury  solution).  At 
a  specific  gravity  of  2.713  a  very  few  dark-colored  particles  separated  out.  Ou  grad- 
ually diluting  (by  the  slow  addition  of  water,  and  mixing  after  each  addition)  no  more 
fell  until  the  specific  gravity  was  reduced  to  2.691,  by  which  time  two-thirds  of  .the 
powder  had  separated  out.  This  was  drawn  off  and  constitutes  729  A.  On  continuing 
the  dilution  the  deposition  continued,  until  at  2.663  specific  gravity  nearly  all  the 
powder  had  fallen.  This  was  then  drawn  off  and  constitutes  729  B.  Thus  the  depo- 
sition was  a  continuous  process  from  2.713  specific  gravity  to  2.663  specific  gravity, 
and  there  was  no  sharp  separation.  This,  taken  together  with  the  similarity  between 
the  silica  percentages  of  729  A  and  729  B,  seems  to  indicate  the  simplicity  of  the  feld- 
spar, whose  crystals  were  so  large  that  on  crushing  each  must  have  been  broken  into 
many  thousand  pieces,  so  that  the  deposited  material  could  hardly  have  been  compound 
because  of  an  interlocking  of  crystals.  Two  thin  sections  of  729,  in  addition  to  the 
ones  described  on  page  59,  were  made,  and  the  following  angular  measurements 
obtained  from  the  feldspar  individuals : 


Angles  on  opposite  sides  of 
cross-liairs. 

Whole  angle. 

33° 

38° 

71° 

31 

36 

67 

31 

28 

59 

33 

25 

58 

24 

29 

53 

39 

38 

77 

32 

34 

66 

No.  822  is  from  a  large,  bowlder-like  mass  included  in  the  black  olivine-gabbro  of 
the  Minnesota  coast  two  miles  below  Beaver  Bay  (p.  61).  The  complete  analysis  is 
made  upon  material  carefully  selected  from  all  parts  of  the  hand  specimen ;  the  silica 
determination  of  822  0  being  upon  less  carefully  selected  material;  822  A  is  the  mass 
of  the  powdered  rock  separated  out  by  Thoulet's  method  at  2.70  specific  gravity,  a  very 
small  portion  being  left  suspended ;  822  B  is  a  single  crystal  very  carefully  separated 
from  the  rock.  From  these  figures,  and  especially  from  the  close  correspondence  of 
the  silica  content  of  the  single  crystal  with  that  of  the  powdered  rock,  it  is  evident 
that  there  is  no  admixture  of  feldspars  in  this  case. 

The  following  additional  optical  measurements  were  made  from  new  sections  of 
822: 


Angles  on  opposite  sides  of 
cross-bair. 

Whole  angle. 

23° 

24° 

47° 

29 

23 

52 

23 

25 

48 

40 

41 

81 

28 

28 

56 

33 

38 

71 

37 

32 

69 

24 

25 

49 

33 

36 

69 

30 

29 

59 

32 

35 

67 

33 

20 

62 

440       COPPEE  BEAEING  EOCKS  OF  LAKE  SUPEEIOE, 

A]tliougii  these  investigations  plainly  indicate  that  there  is  only  one  feldspar  con- 
cerned in  this  rock,  yet  this  feldspar  does  not  correspond  in  composition  to  typical 
anorthite,  which  contains,  according  to  Dana,  silica,  43.1;  alumina,  36.9;  lime,  20; 
equals  100.  This  comjjosition  corresponds,  for  E  :  E:  Si,  to  the  quantivalent  ratio 
1:3:4;  whereas  the  analysis  above  given  gives  a  ratio  of  about  1  :  2.4  :  4.15.  Since 
a  single  crystal,  showing  no  sign  of  interijenetration  by  other  feldspar  individuals,  or 
of  admixed  impui'ity  of  any  kind,  gave  nearly  the  same  silica  content  as  shown  in  the 
complete  analysis,  it  appears  evident  that  the  latter  is  the  true  composition  of  the 
anorthite  comijosing  the  rock  now  under  consideration.  Dana  gives  a  number  of 
analyses  of  anorthite  which  are  closely  like  the  one  given  above.^  If  the  view  of 
Tschermak  as  to  the  nature  of  the  intermediate  triclinic  species  be  accepted,  then,  of 
course,  the  composition  expressed  by  this  analysis  requires  no  further  explanation. 


N  O  T  E    I  2. 

(Page  72.) 

EEEATUM. 

T.  51,  E.  42  W.,  should  read  T.  51,  E.  12  W. 

NOTE    13. 

(Page  82.) 
DIABASE-POEPHYRITE  OF  THE  PORCUPINE  MOUNTAINS. 
Specimen  1245,  diabase-porphyrite,  contains  59.75  per  cent,  of  silica. 

NOTE     14. 

(Page  84.) 

DIABASE-POEPHYRITE  OF  THE  GREAT  PALISAEES. 

Specimen  884,  from  the  compact  portion  of  one  of  the  flows  underlying  the  quartz- 
porphyry  of  the  Great  Palisades,  contains  only  47.90  per  cent,  of  silica,  and  is  hence 
one  of  the  most  basic  of  this  class  of  rocks. 

NOTE    15. 

(Page  85.) 
DIABASE-PORPHYRITE  FROM  TWO  MILES  BELOW  THE  MOUTH  OF  BAPTISM  RIVEE. 
Specimen  907,  diabase-porphyrite,  has  of  silica  52.56  per  cent. 

'  System  of  Miuoralogy,  p.  339. 


NOTES.  441 

N  O  T  E    I  6. 

(Page  109.) 

COMPOSITION  OF  QUARTZ-PORPHYRY  OF  THE  GREAT  PALISADES. 

Specimen  876,  representing  the  quartz-porphyry  of  the  Great  Palisades  of  the 
Minnesota  coast  of  Lake  Superior  contains  71.10  per  cent,  of  silica. 

NOTE    17. 
(Page  110.) 
QUARTZ-PORPHYRY  OF  BAPTISM  RIVER  POINT. 
Specimen  902  has  73.87  per  cent,  of  silica. 

NOTE     18. 

(Page  152.) 

ERUPTIVE  MATERIAL  IN  THE  UPPER  DIVISION  OF  THE  KEWEENAW  SERIES. 

A  slight  exception  to  the  general  absence  of  eruptive  material  from  the  Upper 
Division  of  the  Keweenaw  Series  is  found  in  the  olivine-diabase  dike  described  on  page 
223.  A  small  exposure  of  diabase  was  also  noted  among  the  sandstones  of  the  Upper 
Division  on  th«  Saint  Croix  Eiver  in  Sec.  35,  T.  44,  E.  13  W.,  Wisconsin,  by  the  late 
Moses  Strong  (see  Geol.  of  Wis.,  p.  424).  It  is  not  evident  whether  this  exposure  repre- 
sents a  thin  intercalated  seam  or  a  dike.  Both  these  occurrences  are  in  the  lower  part 
of  the  Upper  Division  of  the  Series. 

NOTE     19. 

(Page  224,  line  14  from  top  of  page.) 
ERRATUM. 


T.  40  should  read  T.  49. 


NOTE     20. 


(Pages  253-258.) 
THE  UNCONFORMABLE  CONTACT  OF  BLACK  RIVER,  DOUGLAS  C0U:NTY,  WISCONSIN. 

The  quotations  here  given  from  Mr.  Sweet's  descriptions  of  this  contact  are  per- 
liaps  not  extensive  enough  to  bring  out  all  the  important  facts.  As  the  occurrences  on 
Black  Eiver  and  on  the  other  streams  of  the  vicinity  have  a  very  considerable  impor- 


442 


COPPER  BEARING  ROCKS  OF  LAKE  SUPERIOR. 


tance,  I  add  three  cross-sections  of  the  gorge  (constructed  from  Mr.  Sweet's  descrip- 
tions), whose  southwest  wall  is  represented  in  Fig.  10,  with  the  design  of  bringing  out 
more  distinctly  the  relations  of  the  exposures  here  seen. 


Fig.  37.— Cross-sections  of  gorge  of  Black  Eiver,  Douglas  County,  Wisconsin.    I,  at  about  4  of  Kg.  10;  II, 
at  5  of  rig.  10 ;  in,  at  7  of  Fig.  10.    Scale  natural,  200  feet  to  the  inch. 

These  cuts  will  serve  to  make  plainer  Mr.  Sweet's  reading  of  the  structure  at  this 
point.  If  the  reading  is  correct,  of  which  I  have  no  doubt,  it  is  evident  not  only  that 
we  have  to  do  here  with  an  unconformable  contact,  but  also  that  the  newer  sandstone 
is  here  deposited  within  the  sinuosities  of  the  old  coast-line. 

NOTE     21. 

(Page  316.) 
DIABASE-PORPHYEITE  OF  THE  GREAT  PALISADES. 

The  compact  diabase-porphyrite  of  the  layer  immediately  beneath  the  quartz- 
porphyry  of  the  Palisades  contains  47.9  per  cent,  of  silica. 


NOTE     22. 

(Page  350.) 

GEOLOGICAL  POSITION  OF  THE  COPPER-BEARING  ROCKS. 

The  question  of  the  equivalency  of  the  Copper-Bearing  Rocks  with  geological 
formations  of  other  regions  is  not  directly  touched  upon  in  the  discussions  of  Chapter 
VIII,  in  which  I  have  contented  myself  with  an  attempt  to  demonstrate  their  complete 
distinctness,  structu.rally,  from  any  of  the  immediately  associated  formations  and  their 
consequent  right  to  a  distinct  name,  of  at  least  local  significance.  I  have  shown  that 
they  are  not  Huronian,  and  that  at  the  same  time  they  are  separated  by  a  great  uncon- 
formity from  the  overlying  fossiliferous  Cambrian  sandstones,  with  which  they  come  in 
contact.  Heretofore  most  of  the  differences  of  opinion  in  this  connection  have  been  upon 
these  very  points.  A  number  of  writers,  and  especially  Messrs.  Foster  and  Whitney, 
maintaining  the  unity  of  the  Keweenaw  Series  and  the  Cambrian  sandstones  above  re- 
ferred to,  and  maintaining  at  the  same  time  the  equivalency  of  these  sandstones  with  the 
so-called  Potsdam  of  New  York,  have  been  led  to  include  the  Copper-Bearing  Rocks  also 
with  the  Potsdam  sandstone.  On  the  other  hand,  those  who  have  maintained  the  pre- 
Potsdam  age  of  the  Copper-Bearing  Rocks,  including  the  writer  of  this  volume,  accept- 
ing the  reference  of  the  overlying  sandstones  to  the  Potsdam  of  New  York,  have 


NOTES.  443 

thought  it  sufficient,  in  order  to  establish  their  point,  to  show  the  existence  of  a  great 
\incouformity  between  the  Copper -Bearing  Eocks  and  the  overlying  sandstones. 

Eeceutly,  however,  two  writers,  Messrs.  Selwyn'  and  N.  H.  Winchell,''  while  ad- 
mitting the  existence  of  this  unconformity,  and  the  consequent  distinctness  of  the 
Copper-Bearing  Series  from  the  overlying  sandstones,  have  yet  maintained  the  Cam- 
brian age  of  the  former  rocks.  These  two  writers,  however,  differ  somewhat  between 
themselves,  Selwyn  merely  maintaining  that  the  Copper-Bearing  Eocks,  along  with 
the  overlying  Cambrian  sandstones  and  the  underlying  Animikie  slates,  "occupy  the 
geological  interval  elsewhere  filled  by  those  divisions  of  the  great  Paleozoic  system 
which  underlie  the  Trenton  Group,"  without  more  definitely  parallelizing  them  with 
the  older  Paleozoic  formations  of  the  Eastern  States.  He  also  says  that  he  prefers  "to 
call  them  all  Lower  Cambrian,  which  includes  the  Potsdam  sandstone  and  the  Primor- 
dial Silurian." 

Winchell,  on  the  other  hand,  would  make  the  Copper-Bearing  Eocks  the  direct 
equivalent  of  the  New  York  Potsdam,  while  regarding  the  sandstones  which  uncon- 
formably  overlie  them,  i.  e.,  the  "  Eastern"  and  "  Western"  sandstones  of  this  volume, 
and  the  fossiliferous  Cambrian  sandstone  of  the  Mississippi  Valley  (his  Saint  Croix 
sandstone),  as  later  than  the  New  York  Potsdam.  Stated  in  his  own  words,  the  follow- 
ing are  Wiuchell's  conclusions: 

1.  "  The  Taconic  Group  was  correctly  established  by  Professor  Emmons,  though  its 
limits,  stratigraphically  and  geographically,  were  at  first  wrongly  defined  by  him. 

2.  "  The  Georgia  Group  of  Vermont,  and  the  Animikie  Group  of  Thunder  Bay,  and 
the  Acadian  of  New  Brunswick  are  the  equivalent  of  the  Taconic  of  Emmons. 

3.  "  The  Taconic  has  the  true  Primordial  fauna  of  Barrande. 

4.  "The  Potsdam,  which  lies  conformably  above  it  in  the  east,  is  represented  by  the 
rocks  of  the  Copper-Bearing  Series  in  the  west. 

5.  "No  fossils,  representing  the  true  Primordial  fauna,  have  yet  been  discovered 
in  the  west,  nor  have  any  been  found  in  the  western  representative  of  the  Potsdam. 

6.  "The  'second  fauna'  of  Barrande  is  found  in  the  Quebec  Group  of  Canada,  and 
in  the  Saint  Croix  sandstone  of  the  west,  lying  in  each  case  above  the  Potsdam  sand- 
stone."^ 

Elsewhere^  Winchell  suggests  the  probability  of  a  former  continuity,  in  the  regio^ 
north  of  Lake  Superior,  of  the  Animikie  slates  and  the  schists,  which  in  that  region 
have  been  called  Huronian,  a  position  which  I  have  regarded  in  the  preceding  pages 
as  much  more  than  probably  true.  If  it  is  so,  and  Winchell's  reference  of  the  Ani- 
mikie to  the  Taconic  of  Emmons  is  correct,  then  the  Huronian  and  Taconic  are  also 
the  same,  which  would  extend  Winchell's  use  of  the  term  Cambrian  over  the  Huronian 
as  well  as  over  the  Copper  Series. 

Into  a  discussion  of  the  question  as  to  how  far  downwards  the  term  Cambrian  should 
be  stretched,  I  have  no  desire  to  enter  at  length,  since  I  think  it  would  be  a  profitless 
one.  I  will  only  say  that,  in  using  the  word  Keweenawan,  I  have  never  designed  to 
give  to  this  term  a  scope  equivalent  to  that  of  the  terms  Cambrian,  Silurian,  &c.,  but 

1  Science,  Vol.  I,  pp.  11,221. 

2Tenth  Annual  Report  of  the  Geol.  and  Nat.  Hist.  Snrv.  of  Minnesota,  pp.  123-136,  also  Science, 
Vol.  1,  p.  334. 

=  Tenth  Annual  Report  of  the  Geol.  and  Nat.  Hist.  Surv.  of  Minnesota,  pp.  135,  136. 
«0p.  cit.  pp.  90,  94,  95 ;  also  Science,  Vol.  I,  p.  834. 


444       COPPER  BEAEIKG  EOGKS  OF  LAKE  SUPERIOR. 

Lave  merely  designed  to  indicate  by  it  the  entire  structural  distinctness  of  tlie  Copper- 
Bearing  Rocks  from  tlie  oldest  of  the  fossiliferous  Cambrian  sandstones  of  tbe  region, 
as  well  as  from  the  underlying  Huroniau.  I  may  also  add  that  it  appears  to  me  very 
unreasonable  to  stretch  the  term  Cambrian  over  such  an  unconformity  as  subsists 
between  the  last-named  sandstones  and  the  Keweenaw  Series,  and  yet  more  to  stretch 
it  over  the  unconformity  between  these  sandstones  and  the  Huroniau.  Everywhere 
throughout  the  Northwestern  States,  where  the  Cambriau  sandstones  come  iu  contact 
with  the  Huroniau,  there  is  evidence  of  an  enormous  time-gap  between  the  two  forma- 
tions. As  one  illustration  of  this  relation,  out  of  many  that  might  be  cited,  I  may 
mention  the  occurrences  in  the  Baraboo  region  of  Wisconsin,  where  a  great  series  of 
quartzites,  including  siliceous  schists  and  immense  beds  of  a  felsitic  porphyry,  are 
overlain  by  the  fossiliferous  Cambrian  sandstones  in  such  a  manner  as  to  prove  beyond 
all  question  that  the  time  which  elapsed  between  the  two  periods  at  which  these  forma- 
tions were  deposited  was  safaciently  great  to  cover,  (1)  the  folding  and  alteration  of 
the  older  series,  measuring  upwards  of  20,000  feet  in  thickness ;  (2)  the  denudation  of 
the  elevations  of  land  thus  produced  to  such  an  extent  that  ridges  approaching  in 
height  the  highest  existing  mountains  of  the  globe  were  entirely  removed,  and  depres- 
sions made  in  their  place  ;  and  (3)  the  depression  of  this  area  beneath  the  sea  and  the 
wearing  by  wave  action  of  the  older  rocks  to  supply  the  material  for  the  newer.  Now 
the  older  of  the  formations  in  this  case  I  take  to  belong,  beyond  question,  to  the  same 
horizon  as  that  to  which  belong  the  Animikie  slates  and  the  Huronian  rocks  of  the 
Lake  Superior  region  generally.  Certainly  no  one  ever  has  referred  or  ever  would  refer 
them  to  a  lower  horizon,  while  Winchell  even  regards  them  as  the  equivalent  of  the  New 
York  Potsdam  and  of  the  Copper-Bearing  Rocks  of  Lake  Superior.  Inasmuch  as 
neither  the  Huronian  nor  the  Copper-Bearing  Series  has  thus  far  afforded  any  fossils, 
it  does  not  seem  to  me  reasonable  to  extend  to  them,  in  spite  of  these  great  uncontorm- 
ities,  the  name  of  Cambrian,  even  though  the  fossiliferous  rocks  immediately  overlying 
them  be  not,  as  Winchell  has  argued  from  their  paleontology,  the  equivalents  of  the 
oldest  of  the  typical  Cambrian  divisions  of  Barrande. 

But,  however  this  may  be,  it  seems  sufficiently  evident  that  Winchell's  reference  of 
the  Copper  Series  directly  to  the  horizon  of  the  New  York  Potsdam  is  untenable.  If 
I  understand  him  correctly,  he  supports  this  reference  by  three  kinds  of  evidence, 
stratigraphical,  lithological,  and  paleontological.  The  stratigraphical  evidence  con- 
sists in  the  occurrence  in  the  east  of  the  following  succession,  in  ascending  order:  (a) 
A  series  of  slates,  sandstones,  &c.,  of  considerable  thickness,  and  carrying  Barrande's 
first  fauna,  conformably  succeeded  by  (b)  the  typical  Potsdam  sandstone,  of  very  incon- 
siderable thickness,  witli  only  a  very  few^  fossils,  grading  up  into  (c)  the  Calciferous 
Sandrock,  in  which,  and  in  whose  continuation  in  Canada,  is  found  a  large  fauna,  cor- 
responding to  the  second  fauna  of  Barrande.  The  members  of  this  succession  he 
parallelizes,  respectively,  with  (a)  the  Animikie  Group  (and  hence  with  the  Huronian 
of  the  Lake  Superior  country  generally),  (b)  the  Keweenaw  Series,  and  (c)  the  Saint 
Croix  sandstone,  including  the  Eastern  Sandstone  of  this  volume,  and  the  lowest 
fossiliferous  Cambrian  sandstone  of  the  Mississippi  Valley.  Now,  not  to  speak  of  the 
grave  doubts  which  still  hang  about  the  relations  of  the  older  rocks  in  the  Eastern 
States,  there  are  serious  objections  to  this  scheme  of  stratigraphical  equivalence- 
<1)  It  disregards  the  entire  absence,  so  far  as  known,  of  fossil  remains  from  the  Ani- 


NOTES.  445 

mikie  aud  Keweeuawaii  rocks,  whicli  are  often  full  as  favorable  in  nature  to  tbe  occur- 
rence of  such  remains  as  their  supposed  equivalents  in  the  east.  (2)  It  disregards  tbe 
unconformity  between  the  Animikie  (and  Hurouiaa  generally)  and  the  Keweenaw 
Series,  which  unconformity  finds  no  parallel  in  the  eastern  series,  as  given  by  Win- 
chell.  (3)  It  disregards  the  immense  and  far  more  striking  and  pronounced  uncon- 
formity met  with  in  the  western  succession  between  the  Keweenaw  Series  and  the 
overlying  sandstones,  which  break  not  only  finds  no  parallel  in  the  east,  but  is  to  be 
contrasted  with  the  gradation  of  the  New  York  Potsdam  into  the  overlying  Oalciferous 
Sandrock.  (4)  It  parallelizes  the  Keweenaw  Series,  which  approaches  a  thickness  of 
50,000  feet,  of  which  fully  15,000  are  of  purely  detrital  matter,  with  a  sandstone  only  a 
few  hundred  feet  thick. 

The  lithological  evidence  advanced  is  hardly  worth  discussion,  because  of  the  well- 
recognized  untrustworthiness  of  such  evidence  when  applied  to  the  comparison  of  rock 
formations  at  long  distances  apart.  Winchell's  assertions,  however,  of  a  lithological 
correspondence  between  the  New  York  Potsdam  and  the  Keweenaw  Series  will  not 
bear  examination.  In  his  own  words,  the  New  York  formation  "is  a  red  or  gray  loose 
sandstone,  often  tilted  or  faulted,  also  metamorphosed,  and  then  having  the  name  of 
quartzite."  We  look  in  vain  in  it  for  the  great  beds  of  porphyry-conglomerate,  the 
immense  thicknesses  of  basic  and  acid  eruptive  rocks,  and  the  black  shales  of  the  Ke- 
weenaw Series.  Eveu  the  sandstones  of  the  two  formations  do  not  approach  each 
other  in  character,  those  of  the  typical  Potsdam  being  described  as  distinctly  quartzose, 
whereas  those  of  the  Keweenaw  Series  are  only  very  subordii.ately  so,  being  composed 
almost  wholly  of  fragments  of  the  feldspars  or  felsitic  matrix  of  the  acid  eruptives  of 
the  same  series.  The  occurrence  in  the  Keweenaw  Series  of  beds  of  metamorphic 
origin,  including  "gneiss,  syenite,  and  hard,  red  quartzites,"  as  stated  by  Winchell,  I  do 
not  admit.  Gneiss  is  never  met  with.  Peculiar  red  rocks,  to  wliich  the  name  of  sye- 
nite may  be  applied,  are  met  with  in  the  series,  but  are  plainly  of  an  intrusive  nature. 
Eocks  to  which  the  name  quartzite  could  be  applied  I  have  never  seen;  certainly  they 
must  be  very  rare,  if  they  occur  at  all.  Portions  of  sandstone  beds  locally  indurated 
by  a  quartz  infiltration  I  have  occasionally  seen,  but  such  rare  and  unimportant  occur- 
rences would  hardly  warrant  the  mention  of  quartzite  as  a  characteristic  of  the  forma- 
tion. On  the  other  hand,  there  is  a  distinct  similarity  between  the  typical  Potsdam  as 
described  and  the  so-called  Potsdam  of  Central  Wisconsin,  where  a  quartzose  compo- 
sition, with  local  indurations  due  to  quartz  infiltration,  and  local  developments  of  red 
sandstone,  often  of  considerable  thickness,  are  prominent  features. 

The  paleontological  evidence  advanced  by  Winchell  consists  in  the  occurrence,  in 
the  Calciferous  Sandrock  of  New  York,  and  in  its  extension  into  Canada,  of  a  fauna 
nearly  allied  to  that  of  the  lowest  fossiliferous  sandstone  of  the  Mississippi  Valley. 
Accepting  the  statement  as  to  this  similarity  so  far  as  it  goes,  I  have  to  say;  (1)  that  the 
evidence  is  too  meager  to  establish  a  complete  equivalency  between  the  Calciferous 
Sandrock  and  the  Mississippi  Potsdam ;  (2)  that  even  if  it  were  not  so,  it  would  remain 
to  show  that  the  Potsdam  itself  is  not  merely  a  downward  continuation  of  the  Calcifer- 
ous, the  few  fossils  that  occur  in  it  being  insufficient  to  disprove  this  relation,  while  the 
grada'ion  of  the  Potsdam  into  the  overlying  Calciferous  is  a  distinct  indication  of  such 
a  relation. 

In  conclusion,  then,  I  have  to  say  that  it  seems  to  me  quite  plain  that  the  horizon 


446       COPPER  BEARING  EOCKS  OF  LAKE  SUPERIOR. 

of  the  New  York  Calciferous  and  Potsdam  together  is  represented  in  the  west  by  the 
following  succession,  given  in  descending  order :  (a)  The  Lower  Magnesian  Limestone, 
grading,  by  alternations  with  sandstone,  and  decrease  of  calcareous  matter  into  (b) 
the  basal  sandstone  of  the  Mississippi  Valley,  of  whose  total  thickness  of  about  1,000 
feet,  from  one-half  to  two-thirds,  is  quartzose  and  non-calcareoiis,  and  whose  lower- 
most portions  are  equivalent  to  (c)  the  Western  Sandstone  and  to  the  Eastern  Sand- 
stone of  the  Lake  Superior  region,  while  it  is  regarded  as  probable  that  the  lowest 
portions  of  the  last-named  sandstones  are  at  a  lower  horizon  than  any  met  with  in 
the  Mississippi  Valley.  Probably  the  New  Tork  Potsdam  finds  its  near  equivalent 
in  these  lowest  sandstones  and  in  the  lowest  portions  of  that  of  the  Mississippi  Val- 
ley, while  the  Calciferous  Sandrock  is  represented  in  the  West  by  the  upper  half  of 
the  last-named  sandstone,  which  alone  is  fossiliferous,  and  by  the  Lower  Magnesian 
Limestone.  The  question  then  arises  as  to  whether  the  Keweenaw  Series  is  the 
equivalent  of  any  of  those  fossiliferous  rocks  which  in  the  east  are  said  to  be  beneath  the 
typical  Potsdam.  A  discussion  of  this  question,  however,  would  hardly  be  profitable, 
until  the  stratigraphical  relation  of  these  eastern  formations  to  the  true  Potsdam  is 
more  satisfactorily  made  out.  When  any  such  discussion  is  undertaken,  however,  it 
will  be  necessary  to  keep  constantly  in  mind  the  great  unconformity  between  the 
Keweenaw  Series  and  the  western  representative  of  the  Potsdam  sandstone. 

NOTE  25. 

(Page  417.) 

ERUPTIVES  OP  THE  ANIMIKIE  GEOUP. 

The  cut  on  page  417  does  not  differentiate  the  eruptives  of  the  Huronian  from  the 
rest  of  the  series.  Some  of  these  eruptives  may  of  course  have  been  contemporane- 
ous, or  nearly  so,  with  those  of  the  Keweenawan,  having  been  formed  intrusively, 
while  at  the  same  time  those  of  the  Keweenawan  above  were  poured  out  at  the  sur- 
face. The  Huronian  eruptives  are  commonly  without  amygdaloids,  which  may,  of 
course,  be  because  of  their  intrusive  nature.  Still,  they  ordinarily  partake  of  the  folds 
of  the  folded  Hiu'onian,  and  must  therefore  have  preceded  the  Keweenawan  erupt- 
ives at  least  in  large  part. 


I  ]SrD  E  X 


Acid  original  rocks,  91-126, 144-15L 
^,  Age  of,  12,  13. 

— ,  Absence  of  copper  in,  426. 

,  Chronological  relation  of,  to  basic  emptives,  32, 432- 

436. 

,  Classification  of,  91. 

,  Distribution  of,  155. 

— ,  Frequency  of,  91. 

,  Huronian  and  Keweenawan  compared,  403. 

,  Kinds  of,  112-125. 

,  augite-syenite,  &c.,  112-124. 

,  felsite  and  quartz-porphyry,  95-112. 

~- ,  granite,  125. 

^.quartzless  porphyry,  91-95. 

,  Summary  of,  126. 

— ^1  Views  on  origin  of,  12, 150. 

Agate  Bay,  Amygdaloida  of,  137,  287-289. 

,  Olivine-diabaae  of  72,  76,  287. 

,  Section  at,  288,  289. 

Agate  Bay  Group,  284-294. 

at  Agate  Bay,  72,  288-290. 

,  Amygdaloids  of,  136,  286-289. 

,  Anorthite-rock  of,  292. 

,  Bedding  of,  288. 

,  Copper  possibly  present  in,  429. 

,  Bikes  of,  293,  294. 

at  Encampment  BlufF,  47,  285. 

at  Encampment  River,  284, 285, 291. 

,  Faults  in,  290, 291. 

on  French  Eiver,  284. 

on  Gooseberry  River,  286, 293. 

near  Lester  River,  292. 

,  Olivine-diabase  of,  76, 286. 

,  Relation  of  acid  and  basic  rocks  of,  435. 

,  Sandstone  of,  292,  293. 

on  Split-Rock  River,  291. 

— . on  Talmage  River,  286. 

,  Thickness  of  beds  of,  290. 

,  Thinning  of,  at  eastern  end  of  Minn,  coast,  294,  298. 

,  "Warping  of  beds  of,  290. 

Agate  Harbor,  Trends  and  dips  near,  178,  413. 
Agogebic  Lake,  Eastern  Sandstone  at,  360. 

,  Lower  Division  at,  158. 

Albany  and  Boston  mine,  Augite-syenite  pebbles  of,  115, 118, 

119,  190, 191. 

,  Conglomerate  of,  190, 191. 

^  Dip  of  strata  at,  187. 

,  Qnartzl ess-porphyry  of,  94, 191. 

Albite  in  olivine-diabase,  70. 

Algoma  mine.  Ferruginous  sandstone  of,  374. 

Allouez  mine,  Dip  at,  187. 


Alteration  of  augite  to  chlorite,  43,  84,  79. 

dialiage,  52, 

ferrite,  79, 93, 102, 114, 125. 

green  substance,  71,  89. 

hornblende,  56, 114, 125. 

magnetite,  41. 

uralite,  52. 

— ,  epidote-quartz  of  amygdaloid,  199. 

—  of  feldspars  to  chlorite,  64,  89. 

through  prelinite  to  chlorite,  64. 

to  prehnite,  89. 

glass-base  in  quartz-porphyry,  101. 

ferruginous  prehnite,  89. 

in  olivine-diabase,  70. 

—  —  olivine  to  biotite,  viridite,  and  talc^  39. 

greenish  substance,  38. 

magnetite  (?),39. 

orthoclase  to  chlorite,  51. 

plagioclase  to  chlorite,  51. 

green  substance,  71. 

prehnite,  46. 

prehnite  to  chloilte,  89 

chlorite,  calcite,  and  green  eartb,  89. 

— epidute  and  calcite,  89. 

orthoclase,  89. 

— ,  Pseud-amygdaloidal,  63, 64.  ^ 

—  of  uralite  to  chlorite,  52. 

Aminicon  River,  Orthoclase  gabbro  of,  52, 54. 

,  Rocks  of,  255,  256. 

Amygdaloids,  87-91, 134-139. 
— ,  Absence  in  Huronian,  403, 445. 
— ,  Alteration  of,  89,  90, 199. 
— ,  Amygdules  of,  135, 136. 

—  of  ashbed-diabases,  183. 

— ,  Characteristics  of,  135, 136. 
— ,  CoDStituents  of,  88, 89. 

,  Ferrite  of,  88. 

,  Plagioclase  of,  88. 

— ,  Copper,  deposition  in,  90, 421, 422. 

— ,  Distribution  of,  155. 

— ,  Fluidal  structure  in,  88. 

— ,  Matrix  of,  87,  88. 

— ,  Pumpelly  on,  89-91. 

— ,  Relations  of,  to  diabase-porphyrites,  87, 89. 

— ,  Stratiform,  137. 

Amygdules  of  amygdaloids,  89,  00. 

— ,  Elongation  of,  136. 

Analcite  of  amygdaloids,  90. 

Animikie  Group,  367-386. 

—  — ,  Bell  on  "crowning  overflow'"  of,  381,  382. 
,  Contact  of,  with  Eleweenaw  series,  157, 297, 405. 

(447) 


448 


INDEX. 


Animikie  Group,  Dikes  of,  144, 367, 368. 

,  Dike-rock  from,  372, 373. 

at  Grand  Portage  Bay,  297,  369. 

-=■  — ,  Interbedded  diabases  of,  373, 374, 445. 

at  Kaministiquia  Eiver,  380. 

,  Limits  of,  northern  and  western,  384. 

at  Liicille  Islands,  369. 

on  Mesabi  Eange,  382-334. 

at  Pigeon  Point,  369,  370,  379. 

on  Pigeon  Eiver,  370, 382. 

at  Pokegoma  Falls,  383. 

,  Portage  Bay  Island,  297. 

,  Eelation  of,  to  granites  of  Mesabi  Eange,  399. 

,  Huronian,  443, 444. 

Keweenaw  Series,  297, 385, 405, 443-445. 

Sonth  Shore  Huronian.  385, 386. 

ou  Saint  Louis  Eiver,  262, 384. 

at  Silver  Islet,  378. 

on  Thunder  Bay,  371-379. 

,  Yiews  on  age  of,  157, 385, 443. 

criticized,  385, 386, 

on  "Wauswaugoning  Bay,  368, 369. 

Anorthite-rock,  59-61. 

in  Agate  Bay  Group,  292. 

—  in  Beaver  Bay  gabbro,  310, 436,  440. 
— ,  Characteristics  of,  59. 

— ,  Constituents  of,  59, 438-440. 

—  differs  from  European  gabbros,  59. 
— ,  Localities  of,  59. 

— ,  Mode  of  occurrence  of,  59. 

—  at  Split  Eock  Eiver,  302, 438, 439. 

— ,  Tabulation  of  observations  on,  59-61. 

Anorthite  of  anorthite-rock,  59. 

— ,  Analyses  of,  438,440. 

,  Determination  of,  by  Thoulet's  method,  438-440. 

—  of  olivine-diabaae  or  melaphyr,  70. 

—  of  orthoclase-free  gabbro,  40. 

Anse  aus  Crepes,  Eeweenawan  rocks  of,  348. 
Apatite  of  diabaseporphyrite, 79. 

—  of  homblende-gabbro,  57. 

—  of  orthoclase-gabbro,  52. 

of  orthoclase-free  gabbro,  43. 

Apostle  Islands,  Sandstones  of,  163, 154, 305. 

Argillite  of  Nipigon  Lake  Basin,  340. 

Ash,  volcanic,  Absence  of,  in  Keweenaw  Series,  82, 138, 139, 

436,  437. 
Ashbed-diabase  of  Bohemian  Eange,  183. 
— ,  Distribution  of,  155. 

—  of  Eagle  Eiver  section,  171-173. 

—  of  Lester  Eiver  Group,  279, 280. 
— ,  Origin  of  name,  138, 173. 

— ,  See  Diabase-porphyrite. 

Ashbed  Group  of  Keweenaw  Point,  140, 171-178. 

,  Characteristics  of,  138. 

,  Section  of,  186. 

,  Thickness  of,  178. 

Atlantic  Mill,  130, 192. 
Augite  of  amygdaloids,  88. 

anorthite-rock,  59. 

augite-syenite,  114. 

basic  original  rocks,  37, 38, 

diabase-porphyrite,  78,  79. 

(porphyritic),  79. 

olivine-di  bi^e,  70. 

olivine-free  diabase,  63,  64. 

orthoclase-gabbro,  41, 42. 


Anglte  of  orthoclase-free  gabbro,  52. 

quartz-porphyry  (porphyritic),  102. 

quartzless  porphyrj-  (porphyritic),  93. 

Augite-syenite,  112-125. 

—  of  Beaver  Bay,  122,  306. 
Brick  Island,  369. 

— ,  Characteristics  of,  112. 
— ,  Constituents  of,  112-115. 

,  augite,  114. 

— .feldspars,  112, 113,  114. 

,  ferrite,  113. 

,  hornblende,  113. 

,  magnetite,  114. 

,  quartz,  112, 113. 

,  secondary,  113, 114, 438. 

— ,  Localities  for,  115. 

—  of  Mount  Bohemia,  184. 
— ,  Name  of,  115. 

— ,  relations  of,  115. 

—  of  Saint  Louis  Eiver  Group,  270, 271. 
— ,  Tabulation  of  observation  on, 116-124. 

—  of  Victoria  Islands,  372,  373. 
— ,  See  Porphyry,  Granitic. 

Azoic  Eocks,  Eeport  on.    See  Hunt,  T.  S. 
Bad  Eiver,  Augite-syenite  of,  115. 

,  Gabbros  of,  40-41, 144, 154, 155, 231, 377, 435, 436. 

,  Granite  of,  233, 435. 

,  Hornblende-gab bro,  region  of,  56. 

,  Lapham  on  sandstone  of,  411. 

,  Orthoclase-gabbro  of,  52,  54. 

,  Orthoclase-free  gabbro  of,  44,  45. 

,  Quartz-porphyry  of,  103, 150, 231, 433. 

,  Eelation  of  acid  and  basic  rocks  on,  435. 

,  Sandstone  of,  132,  231. 

,  Sources  of  information  on,  207. 

,  Thickness  of  Keweenaw  Series  on,  231. 

,  Lower  Division  on,  158. 

Baptism  Iviver,  Bay,  and  vicinity,  Ashbed-diabase  of,  318, 326. 

,  Augite-syenite  of,  116,  123. 

,  Beaver  Bay  Group  on,  298, 318,  319. 

,  Diabase-porphyrite  of,  85,  326,440. 

,  Felsitic-porphyry  of,  110,  318. 

,  Gabbro  of,  272. 

,  QuartB-porphyry  of,  102, 103, 147, 158,  314,  318, 319, 447. 

,  Eelation  of  acid  and  basic  rocks  of,  435. 

,  Temperance  Eiver  Group  on,  323,  326. 

Baraboo,  Eelations  of  Huronian  and  Potsdam  at,  444. 

Bare  Hills,  Eelsite  of,  149, 182. 

Bare  Point,  Ashbed-diabase  of,  375. 

Barrande,  J.,  "Western  equivalents  of  1st  and  2d  faunas  ofi  443. 

Basic  original  rocks,  35-91, 134-144. 

,  Age  of,  12,13. 

,  Amygdaloids  of,  134-139. 

,  Characteristics  of,  134-144. 

,  Classifications  of,  35,  37. 

,  Coarse-grained,  37-61 . 

,  anorthite-rock,  59-61. 

,  homblende-gabbro,  56-58. 

,  orthoclase-gabbro,  50-56. 

,  orthoclase-free  gabbro,  37-50. 

,  Dikes  in,  143. 

,  Dip  of,  141,142. 

,  Effect  on  topography,  141. 

,  Extent  of  single  beds,  140. 

,  Ij^ne-grained,  61-91. 

,  amygdaloids,  87-91. 


INDEX. 


449 


Basic  original  rocks,  Fine-grained,  aslibed-dlabase,  77-87. 

,  diabase-porpliyrite,  77-S7. 

,  olivinitlo  diabase  and  melapbyr,  68-77. 

,  olivine-free  diabaae,  61-68. 

— . in  Huronian  and  Keweenawan,  Similarity  of,  402,403. 

,  Material  for  study  of,  36. 

,  origin  of,  Views  on,  11. 

,  Pumpelly 'e  description  of,  35. 

,  Relation  of,  to  acid  roclis,  148, 432-436. 

,  Eosenbuach's  classification  of,  35. 

,  Thickness  of  beds  of,  134, 141. 

Batcbewannng  Bay,  Conrse  of  Keweenaw  Series  at,  349. 

,  Huronian  of,  400. 

,  Keweenawan  roclss  of,  348. 

Battle  Islands,  Course  of  Keweenaw  Series  at,  334. 

,  Sandstone  of,  336. 

Bayfield,  H.  "W.,  5. 

— ,  Paper  by,  14. 

Bead  Island,  Qaartz-porphyry  of,  103,  111,  346. 

,  Quartzless  porphyry  of,  95, 304 

Beaver  Bay  and  vicinity,  Anorthite-rocli  of,  61,  310, 439, 440. 

,  Ashbed-diabase  of,  84, 305, 308. 

,  Dite  of,  307, 436. 

,  Gabbros  of,  42, 43, 48, 49,  305, 306, 309. 

,  Granite  of,  304, 306. 

■ ,  Granitic  porphyry  of,  122, 306. 

—  — — ,  Magnetite  of,  51. 
,  Olivine-diabase  of,  308. 

.  Quartz-porphyry  of,  99, 100, 103, 107, 108, 306, 307. 

Beaver  Bay,  Islands  iu,  Anortbite-rock  of,  307. 
Beaver  Bay  Group,  298-323. 

,  Acid  and  basic  eruptives  of,  433, 436. 

,  Anorthite-rock  of,  61, 308,  310. 

at  Beaver  Bay,  107, 108, 30 1-309. 

Beaver  Eiver,  306-308. 

,  Characteristics  of,  207, 298, 299. 

,  Diabase-porphyrite  of,  83, 84, 308, 440, 442. 

,  Dikes  of,  320, 322, 323. 

,  Telsite  of,  108, 109, 110, 310, 311, 320-322. 

,  at  Grand  Marais,  319, 320. 

,  Granite  of,  304. 

,  at  Great  Palisades,  109, 314-318, 434, 435, 440. 

,  Limits  of,  298. 

,  Quartz-porphyry  of,  107, 108, 314-318, 440. 

at  Bed  Eock  Bay,  110,  322. 

Split  Eock  Eiver,  107, 299-303. 

,  Thickness  of,  299. 

at  T.  56,  E.  7  W.,  108,  310, 311. 

Beaver  Eiver,  Anorthite-rock  of,  60, 306. 

,  Biabase-porphyrite  of,  83,  303. 

,  Gabbro  of,  306, 308. 

,  Orthoclase-free  gabbro  of,  48, 49, 306. 

BeU,  E.,  on  Animikie  Group,  age  of,  385. 

,  crowning  overflow,  382. 

,  at  Gunflint  Lake,  382. 

,  stratigraphy  of,  380. 

—  on  Huronian  Eocks  of  Pic  Eiver,  401, 402. 
Kamluistiquia  slates,  374. 

Keweenaw  Series,  age  of,  12. 

Nipigon  Lake  Basin,  27, 408. 

— ,  Publications  by,  21, 22. 

—  on  Pigeon  Eiver  dikes,  370,  371. 
Eainy  Lake  schists,  397. 

— ,  Eeferred  to,  5, 6, 10, 400. 

—  ou  Saganaga  Lake  schists,  397. 

—  on  schists,  age  of,  395. 

-.    29  L  s 


Bell  on  Thunder  Bay  schists,  396. 

sandstones,  157, 332. 

EMe  Grise  Bay,  Augite-syenite  of,  117. 

,  Eastern  Sandstone  on,  183,  353, 362, 363. 

Olivine-diabase  of,  72, 74, 184. 

,  Median  Valley  rocks  on,  179. 

Big  Trout  Bay,  Dikes  of,  371. 

,  Dike-rock  from,  372. 

Bigsby,  J.  J.,  5. 

— ,  Publications  of,  14, 17. 

—  on  Eadny  Lake  schists,  397. 

Biotite,  Alteration-product  of  olivine,  38. 

—  of  granite,  125. 

orthoclase-free  gabbro,  43, 

orthoclase-gabbro,  52. 

Black  Bay,  Dikes  near,  333. 

,  Dip  of  strata  on,  333. 

,  Lake  Superior  syncliDal  on,  414. 

,  Lower  Division  on,  160. 

,  Porphyry  of,  337. 

,  Sandstone  of,  24, 128, 150,  336, 337. 

,  Eelation  of  sandstones  and  diabases  near,  334. 

Black  and  Nipigon  bays,  331-338. 

,  Amygdaloids  of,  334. 

,  Detrital  rocks  of,  336. 

— ,  Diabases  of,  334-336. 

,  Relation  of  rocks  of,  to  Keweenaw  Point  rocks,  336, 

337. 

.    See  Nipigon  Bay,  Black  Bay. 

Black  Point.    See  Caribou  Point. 

Black  Eiver  (Mich.),  Lower  Division  on,  208. 

,  Shales  of  Porcupine  Mountains  on,  225. 

,  (Wis.),  Eocks  of,  251-254, 441. 

Black  Sturgeon  Lake,  Sandstone,  &c.,  of,  340. 

Black  Sturgeon  Eiver,  Contact  of  sandstone  and  gabbro  on, 

333. 
Bladder  Lake,  Contact  of  gabbro  and  Huronian  on,  232. 

,  Olivine-diabase  of,  38. 

Blake,  W.  P.,  Paper  by,  18. 

Bohemian  Mountain.   See  Mt.  Bohemia  and  Bohemian  Eange. 

Bohemian  Eange,  179-186. 

,  Augite-syenite  of,  115, 116, 117, 145, 184. 

,  "Chlorites"  of  =;  diabase-amygdaloids, 285. 

,  conglomerates  of.  Source  of,  186. 

,  Diabases  of,  183. 

,  Diabase-porphyrite  of,  81. 

,  Felsit«  oft  104, 182, 183. 

,  Foster  and  Whitney  on,  180. 

,  Hunt  on,  180. 

,  Jackson  on,  180. 

,  Kinds  of  rock  of,  181. 

,  Localities  on,  181. 

,  Lac  la  Belle  to  Delaware  mine,  183, 184. 

,  Mt.  Bohemia,  181, 184, 185,435. 

,  Mt.  Houghton,  181, 182, 433. 

,  Orthoclase-gabbro  of,  53. 

,  Olivine-free  diabase  of,  67. 

,  Eelation  to  Keweenaw  Series,  181. 

,  Summary  on,  186, 187. 

,  Thickness  of  beds  of,  187. 

,  Whittlesey  on,  180. 

,  See  Bare  Hills,  Lao  la  Belle,  Mt.  Bohemia,  Mt  Hoagh- 

ton. 
Brick  Island,  369. 
Brooks,  T.  B.,  5. 

—  on  gneiss  and  schist,  interbedded,  400. 


450 


INDEX. 


—  on  Hnronian  greenstones,  394. 

—  on  relation  of  Keweena-ff  Series  to  Hnionian,  407. 
— ,  Publications  of,  20, 21, 22. 

Brulfi  Lake,  Chanvenet's  and  MoKinlay's  Tisit  to,  272. 

,  Diabase  of,  275. 

,  Duluth  gabbro  at,  294. 

,  Gabbro  of,  56,272. 

,  Porphyry  of,  124, 272. 

Erul6  Eange,  Eocks  of,  257. 

Brul6  Eiver  and  vicinity,  Beaver  Bay  Group  on,  321, 322. 

,  Orthoclase-freo  gabbro  of,  38, 44, 49, 144, 321. 

Bmnscbweiler  Eiver,  Flattening  of  dips  at,  233. 

,  Ortboclase-gabbro  of,  54. 

,  Qnartzless  porphyry  of,  93, 95. 

,  Gabbros  of,  232,  233. 

,  Upper  Division  on,  153. 

Burlington  Bay,  264. 

Burt,  W.  A.,  and  Hubbard,  B,,  Paper  by,  16. 

Cabmons  Neiding  Point  (Island  near),  Diabase-porphyrite 

of,  338. 
Calclferous  Sandrock,  351, 443, 444, 445. 
Calcite  of  diabase-porpbyrite,  79. 

—  of  sandstones,  128. 

—  veins,  423. 

Calumet  and  Hecla  mine,  188.    See  Conglomerate,  Calumet. 
Calumet  Conglomerate.    See  Conglomerate,  Calumet. 
Cambrian,  extent  of  term,  443-445. 
Cambrian  Sandstone  of  Mississippi  Valley,  234-250,  366. 

.Extent  of,  366. 

,  Relation  to  Keweenaw  series,  366, 443. 

,  See  Eastern  Sandstone,  Potsdam,  Wiscons.n,   Korth 

western. 
Campbell,  A.  C,  3. 

—  on  Beaver  Bay  Group,  309. 
ITortb  Shore,  262. 

Mamainse,  348. 

Porcupine  Mountains,  207. 

—  and  McKinlay  on  Baptism  Eiver  Eocks,  272. 
Canadian  Naturalist,  335,  378, 405, 432. 

Canada,  Geology  of,  3, 374, 377, 385, 386-390, 400, 401, 402. 
— ,  Geological  Survey  of,  Eeport  of  1848,  390. 

-,  1863-'66,  400, 436. 

,  1866-'69,  123, 157,  339,  340,  374,  380,  381, 385. 

,  1870-'71,  402. 

,  1872-'73,  382. 

Cape  Choyye,  Keweenawan  rocks  of,  347, 415. 
Cape  Gargantua,  Keweenawan  rocks  of,  347, 415. 
Caribou  Island,  Olivine-gabbro  near,  375. 

,  Upper  Division  at,  154. 

Caribou  Point,  Olivine-diabase  of,  72, 77, 328. 

,  Sandstone  of  Temperance  Eiver  Group  at,  328. 

Carlton's  Peak,  Anorthite  rock  of,  59,  329. 

.Origin of,  329. 

Carp  Lake,  Felsite  of,  210. 

,  Porphyry  and  diabase  of,  213. 

,  Sandstone  (inner)  of  Porcupine  Mountains  at,  213. 

Carp  Eiver,  Conglomerates  of  (T.  51,  E.  43  W.),  215-717, 220. 

,  Diabase-amygdaloids  exposed  on  (T,  51,  E.  42  TV.),  215. 

,  Diabases  of  (T.  51,  E.  43  W.),  215-217. 

,  Pelsites  of  (T.  43  E.  43  W.),  105. 

(T.  51  E.  43  "W".),  106,  210. 

,  Outer  sandstone  and  conglomerate  of  Porcupine  Mount- 
ains on  (T.  50,  E.  45  W.),  220. 

,  Sandstone  of  (T.  51,  R.  42  W.),  132. 

,  Stratigraphy  on  (T.  51,  E.  43  W.),  213-217. 

Carp  Eiver.    See  Porcupine  Mountains. 


Cascade  Eiver,  Diabases  of,  296. 

,  Diabase-porphyrite  of,  85, 295. 

,  Duluth  and  Lester  Eiver  Groups  on,  296. 

,  Gabbro  of,  56, 295. 

,  Quartz-porphyry  of,  295. 

visited  by  Chauvenet  and  McKinlay,  273, 295. 

Castle  Danger,  Agate  Bay  Group  at,  290. 

Cedar  Island,  Felsitio  porphyry  of,  108, 306. 

Chamberlin,  Prof  T.  C,  4,  5, 7, 10, 11, 410. 

— ,  on  Keweenaw  series  of  Northwestern  Wisconsin,  234. 

St.  Croix  Valley,  236-238, 411, 412. 

Snake  Eiver  diabases,  242,  243, 412. 

,  Publications  of,  23, 432. 

Chase's  Brook,  Melaphyr  of,  247. 
Chassells,  J".,  4, 197. 
Chauvenet,  W.  M.,  3, 262. 
— ,  on  Brul6  Lake  rocks,  272. 

Eagle  Mountain  rocks,  145, 273. 

Eastern  Sandstone,  359, 360. 

Hungarian  Eiver  (sandstone  of),  354. 

Porcupine  Mountains,  207. 

Torch  Lake  railway  (sandstone  of),  358. 

—  and  McKinlay  at  Cascade  Eiver,  295. 

Bml6  Lake,  272-274. 

Eagle  Mountain,  273. 

Chegwatona  Lake,  Diabases  of,  243. 
Chester,  Prof.  A.  H.,  Assistance  of,  4. 

—  on  the  Mesabi  Eange,  383, 384, 398. 

Vermillion  Lake  schists,  397-399. 

Chester  Creek  and  vicinity.  Diabases  of,  276, 277. 
,  Dikes  of,  278. 

,  Gabbro  of,  277. 

,  Lester  Eiver  Group  at,  281. 

,  Eocks  of,  275-278. 

.    See  Duluth. 

Chlorite  of  amygdaloids,  90. 

olivine-free  diabase,  64. 

orthoclase-freo  gabbro,  43. 

sandstones,  128. 

Chief's  Bay,  Nipigon  Lake,  Limestones  of,  340. 
Chippewa  Harbor,  Sandstone  of,  331. 
Chynoweth,B.  C.,4. 
Clam  Falls  District,  Diabase.porphyrite  of,  83 

,  Felsitic  porphyry  of,  103, 107. 

Cloquet  Eiver,  Gabbros  of,  44, 46, 268, 269, 272. 

,  Feldspar  of,  438. 

Conglomerates,  127, 151. 
— ,  Copper  in,  420. 
— ,  Extent  of,  151. 
— ,  Huronian,  386-388. 
— ,  Origin  of,  127. 

,  Foster  and  Whitney  on,  8. 

,  Houghton  on,  9. 

— ,  Pebbles  of,  127, 167, 168, 195, 196. 
— ,  Porphyry,  Distribution  of,  155. 
— ,  Source  of  materials  of,  31,  32, 186, 195, 196. 

,  Foster  and  Whitney  on,  31,  32. 

— ,  Variations  in  composition  and  structure  of,  151. 
Conglomerate,  Albany  and  Boston,  118, 119, 151, 190, 191. 
— ,  Calumet,  Alterations  in,  195. 

,  Copper  of,  420,  421. 

,  Pebbles  of,  105, 195-196. 

,  Sandstone  of,  130. 

— ,  The  Great,  at  Eagle  Eiver  Section,  167-169. 

,  Montreal  Eiver,  227-229. 

Ontonagon  Eiver,  199. 


INDEX. 


451 


Conglomerate,  Great,  on  Porcnpine  Mountains,  209, 217, 219. 

Portage  Lake,  191, 192. 

— ,  Thickness  of,  177, 182, 186. 
— ,  Inner,  Extent  of,  215-217. 

,  Kingston,  176. 
— ,  Outer,  151, 179, 186, 

on  Bad  Eiver,  231. 

on  Manitou  Island,  179. 

Montreal  Eiver,  226. 

Porcupine  Mountains,  220, 224. 

Potato  Eiver,  230. 

,  Thickness  of,  186. 

Contact  of  acid  and  basic  rocks,  148, 432-436. 

at  Great  Palisades,  147, 314-318. 

Animikie  slates  and  interbedded  diabase,  374. 

Eastern  Sandstone  and  Lower  Magnesian  limestone,  351. 

Huronian  Quartzites  and  Cambrian  Sandstone  at  Bara- 

boo,  444. 
Keweenawan  gabbros  and  Huronian,  at  Bad  Eiver,  155, 

136. 

near  Bladder  Lake,  232. 

on  Saint  Louis  Eiver,  263. 

of  Keweenawan  Series  with  Animikie  Group,  157, 405. 

at  Grand  Portage  Bay,  297, 367. 

on  Thunder  Bay,  376. 

Saint  Louis  Eiver,  263. 

at  Silver  Islet  Landing,  378, 379. 

Cambrian  (Potsdam)  sandstone,  366. 

on  Kettle  Eiver,  244. 

at  Taylor's  Falls,  237. 

Eastern  Sandstone,  185,204,353-361. 

onB6t6GriseBay,353,354. 

Douglas  Houghton  River,  355, 356. 

Gratiot  Lake,  354. 

_ Hungarian  Eiver,  354, 355. 

in  Michigan  (T.  50,  E.  39  W  ),  339,  360. 

on  Lac  La  Belle,  354. 

Ontonagon  Eiver,  203-205, 359, 360. 

Portage  Lake,  359. 

Huronian,  155, 156, 404-408. 

in  Bad  Kiver  Eegion,  155, 156, 231, 232. 

at  Bladder  Lake,  232. 

^N'ipigon  Lake,  408. 

of  North  Shore,  405, 406. 

South  Shore,  404, 405. 

Western  Sandstone  on  Douglas  County  Copper 

Eange,  250-259, 366. 

limestone  and  trap  of  Nipigon  Lake,  309. 

sandstone  and  gabbro  on  Nipigon  Bay,  333. 

Nipigon  Lake,  339. 

"Western  sandstone  and  Saint  Louis  Eiver  slates,  263. 

Copper  of  amygdaloids,  90, 419-425. 

,  ashbed-diabase,  122. 

,  sandstones,  128. 

—  in  conglomerates,  420. 

Deposition  of,  420-422, 425. 
— ,  Modes  of  occurrence  of,  251, 419. 
— ,  Occurrence  of,  on  Douglas  County  Copper  Eange,  251. 

. Keweenaw  Point,  423,' 424. 

Ontonagon  Eiver,  423. 

Portage  Lake,  421, 422. 

— ,  Origin  of,  425, 426. 

— ,  Possible  productive  area  for,  427-429. 

in  Isle  Eoyale,  420. 

Michigan,  427. 

— ,  Productive  area  for,  Minnesota,  428, 429. 


Copper,  Productive  area  for,  "Wisconsin,  427, 428. 

— ,  Pumpelly  on  origin  of,  425. 

— ,  Enles  for  exploration  for,  425, 426. 

—  in  sandstones,  128, 420. 

— ,  Sulphuretted,  in  gabbro,  426. 

—  in  transverse  veins,  423, 424. 

Copper  Creek,  Keweenawan  rocks  of,  254, 255. 
Copper  Ealls  mine,  Sandstone  of,  129, 130, 171. 
Copper  Harbor,  151, 186, 413, 414. 

,  Dip  of  strata  at,  178. 

Credner,  H.,  Publications  of,  20, 21, 22. 

—  on  relation  of  Eastern  Sandstone  and  Keweenaw  Series, 

363. 

Current  Eiver,  Diabase-porphyrite  near,  375. 

Dacotah  Mine,  189. 

Dana,  J.  D.,  on  composition  of  anorthite,  440. 

Dawson,  J.  W.,  Publication  of,  18. 

Delessite  of  amygdaloids,  90. 

Deroux,  H.,  Publication  of,  19. 

Des  Cloizeaux  on  determination  of  plagioclase  feldspars,  39. 

Desor,  E.,  Publication  of,  17. 

Detrital  Eocks,  127-133, 151. 

.    Sec  Conglomerate,  Limestone,  Sandstone. 

Devereaux,  J.  E.,  4. 

Devil's  Track  Lake,  visited  by  Chauvenet  and  McKin- 
lay,  273. 

Devil's  Track  Eiver  (coastnearmouthof),  Eelsiteof,  103,  321. 

,  Quartz-porphyry  of,  149. 

Diabase,  Olivine-,  (coarse-grained).  See  Gabbro,  Orthoclase- 
free. 

Diabase,  Olivine-,  (fine-grained),  andMelaphyr,  68-77. 

of  Agate  Bay,  76, 287. 

,  Alterations  in,  70,  71. 

of  B6te  Grise  Bay,  74, 185. 

Caribou  Point,  77,  328. 

,  Characteristics  of,  68, 71. 

,  Color  of,  69. 

,  Constituents  of,  69,  70. 

,  Age  of,  70. 

,  augite,  70. 

,  magnetite,  70. 

,  olivine,  70. 

,  plagioclase,  69. 

,  pyroxene,  70. 

,  of  Eagle  River  section,  73, 170. 

French  Eiver,  73,  76,  279,  280. 

Gooseberry  River,  76,  289,  290. 

,  Graduation  of,  into  coarse  oUvine-gabbro,  69. 

,  Grain  and  texture  of,  69. 

of  Greenstone  Group,  73, 174, 175. 

Knife  Eiver,  76,  279. 

Lac  La  Belle,  74,  183. 

Lester  River,  75,  280. 

Moose  Creek,  75,  247-249. 

Porcupine  Mountains,  74,214,215. 

Potato  Eiver,  74,  231. 

,  Pumpelly's  descriptioa  of,  69, 70. 

of  Silver  Creek,  76. 

,  Specific  gravity  of,  69. 

near  Split  Eock  River,  76. 

of  Saint  Croix  River,  75,  243. 

Totogatig  district,  75. 

Diabase,  Olivine-free,  (coarse-grained).  See  Gabbro,  Ortho- 
clase-. 

Diabase,  Olivine-free,  (fine-grained),  61-68. 

,  of  Bohemian  Eange,  67, 183. 


452 


INDEX. 


Diabase,  Olif  ine-free,  Cliaraoteristios  of,  61-66. 

,  Color  of,  62,63. 

,  Constituents  of,  63. 

,  augite,63. 

,  magnetite,  63. 

,  pla^oclase,  63. 

of  Copper  Creek  67,  254. 

Eagle  Eiver  Section,  66,  67, 170. 

Fond  du  Lac  mine,  67, 251. 

,  Frecitiency  of,  65, 66. 

,  Localities  for,  65. 

,  Modes  of  occurrence  of,  62. 

of  Montreal  Eiver,  67,  68, 227. 

,  Pseudamygdaloidal,  alteration  in,  63,  64. 

,  Pseudomorphism  in,  64, 65. 

,  Pumpelly's  description  of,  61-66. 

,  Tabulation  of  observations  of  66-68. 

.Texture  of,  63. 

,  Types  of,  61. 

of  Union  mine,  67, 219. 

Diabase    Ortboclase-free,    {coarse-gr-ained).      See    Gabbro, 

Ortboclase-free. 
Diabase-porphyrite  and  ashbed-diabase,  77-87. 

,  Amygdaloids  of,  79. 

near  Baptism  Eiver,  85,  326, 440. 

.Base  of,  78,79. 

of  Beaver  Bay,  84,  305. 

Eiver,  83,  308. 

Bohemian  Eange,  81, 183. 

. Cascade  Eiver,  85,293. 

,  Characteristics  of,  77. 

,  Color  of,  77, 78. 

,  Constituents  of  78-80. 

. ,  apatite,  79. 

— ,  calcite,  79. 

— . ,  chlorite,  79. 

— ,  epidote,  79. 

,  oligoclase,  78, 79. 

. ,  orthoclase,  78,79. 

,  silica  content.  79, 440, 442. 

,  Copper  in,  422. 

,  Distribution  of,  155. 

of  Dulutb,  80,  83,  273-277. 

Eagle  Eiver,  171-173. 

Encinipment  Bluff,  84, 285. 

French  Eiver,  83. 

near  Great  Palisades,  84, 316, 442. 

,  Grain  and  texture  of,  77. 

of  Enif 6  Eiver,  84, 279. 

Lester  Eiver,  279.  280. 

Little  Carp  Eiver,  82, 215, 217. 

,  Localities  for,  80. 

of  Michipicoten  Island,  85,  87,  344-346. 

Porcupine  Mountains,  82, 215-220, 440. 

Portage  Bay  Island,  85, 296-298. 

Potato  Eiver,  83, 231. 

Eed  Eock  Bay,  85,  323. 

.  Eolations  of,  to  ashbed-diabaso,  78. 

olivine-diabase,  78. 

. orthoclase-gabbro  and  qxiartzless  porphyry,  79. 

near  Split  Eock  Eiver,  84,  301,  303. 

of  Suffolk  mine,  81, 176, 177. 

,  Tabulation  of  observations  on,  83-87. 

Diallage.    See  Augite. 
Dieffenbach,  O.,  Publication  of,  19. 
Dikes.  143, 144. 


Dikes  of  Agate  Bay  Group,  287, 293, 294. 

—  of  Animikie  Group,  144, 367, 368, 372, 373. 
Basic  Eocks,  143. 

Beaver  Bay  Group,  299, 306, 307, 320-323. 

Big  Trout  Bay,  371, 372. 

Black  and  Nipigon  Bays,  334. 

Chester  Creek,  278. 

Dnluth  Group,  278. 

Grand  Portage  Bay,  367. 

Hat  Point,  368. 

— ,  Huronian,  389. 

—  of  Kaministiquia  Eiver,  374. 

Lester  Eiver  Gronp,  283. 

Mamainse,  143. 

Minnesota  Coast,  143,  278,  283,  294,  301,  306,307,320-323, 

329, 384. 

Pigeon  Bay,  372, 373. 

Pigeon  Point,  370. 

Pigeon  Eiver,  371. 

Porcupine  Mountains,  223. 

Eed  Eock  Bay.  322.  323. 

—  —  Saint  Louis  Eiver,  384. 
SUver  Islet,  378. 

Silver  Islet  Landing,  379. 

Split  Eock  Eiver,  301. 

Temperance  Eiver  Group,  329. 

Thunder  Bay,  370. 

Thunder  Cape,  376. 

Victoria  Islands,  372. 

Wauswaugoning  Bay,  368, 369. 

Diorites,  393, 400. 

Dog  Lake,  Schists  of,  396. 

Dor6  Eiver,  Huronian  Eocks  of,  401. 

Douglas,  J.,  Paper  by,  21. 

Douglas  County  Copper  Eange,  250-259. 

onAminicon  and  Middle  Elvers,  253-257. 

in  Bayfield  County,  257, 258. 

on  Black  Eiver,  251-255. 

Copper  Creek,  254, 255. 

,  Copper  of,  251,  428. 

^,  Lower  Division  on,  159. 

,Eelationof,  to  Keweenaw  Series,  251. 

,  sandstone  of.  Age  of,  259. 

,  "Western  sandstone  ou,  259, 366. 

Douglas  Houghton  Eiver,  Eastern  Sandstone  on,  185,  335, 356. 

,  Wadsworth  on  rocks  of,  355, 363. 

Duluth,  Diabase-porphyrite  of,  80, 83, 275-2T7. 
— ,  Granitic  porphyry  of,  116, 119, 263. 270-272. 435. 

—  Orthoclase-gabbro  of,  51, 53, 55, 269. 

—  Orthoclase-free  gahhro  of  44, 46, 277. 
^,  Quartzless  porphyry  of,  95. 

— .  "Western  Sandstone  absent  at,  412. 
Duluth  Gronp,  155, 275-279. 

. ,  Absence  of  copper  from,  426. 

,  Characteristics  of,  266. 

. on  Chester  Creek,  275. 

.Dikes  of,  278. 

at  Eastern  end  of  Minnesota  coast.  294. 

,  Origin  of  rocks  of,  277. 

.     Sec  Duluth. 

Dupee,  J.  A.,  Publication  of,  18. 
Dutton,  T.  E.,  Publication  of  15. 
Eagle  Harbor,  413. 

.Sandstone  of,  128. 

,  Topography  at,  17S. 

E.agle  Mountain,  Granitic  porphyry  of,  116, 124, 145, 273, 274 


ISDEX, 


453 


Eagle  Mountain,  Orthoclaae-gabbro  of,  53, 56. 
Eagle  Eiver  Section,  167-178. 

,  Amygaloids  of,  170-172. 

,  Ashbed  Group  of,  171-173. 

,  Diabasea  of,  170. 

,  Diabase-porphyrite  of  (beds  45, 65, 66),  80. 

,  Extent  of,  141. 

•,  Great  Conglomerate  of,  166-168. 

,  Pebbles  of,  93, 104, 115, 118. 

,  The  Greenstone  of,  174, 175, 178, 187. 

,  Kingston  Conglomerate  of,  177. 

,  Limits  of,  166. 

— . ,  Marvine's  Group  "A"  of,  173, 174. 

,  Melaphyr  of  (beds  22, 69, 87),  66, 67, 170. 

,  Olivine  diabase  of  (greenstone  beds),  73. 

,  Orthoclase-gabbro  of  (bed  94),  52, 53, 174, 175, 176. 

,  Ortboclase-free  gabbro  of  (beds  96, 107),  44, 45, 175. 

,  Phcenis  Mine  Group  of,  175, 176. 

,  Pumpelly  on,  62, 168, 175. 

,  Sandstone  of,  169, 171. 

,  Summary  of,  177, 178, . 

,  Thickness  of,  178. 

Eames,  H.  H.,  on  East  coast  of  Lake  Superior,  27, 347, 349. 

,  Publication  by,  20. 

Eastern  Sandstone,  351-365. 

,  Age  of,  12, 351, 352. 

,  at  B6t6  Grise  Bay,  185, 353, 354. 

,  Clay  holes  in,  185, 357. 

,  Conclusions  regarding,  365. 

—  ^  on  Douglas  Houghton  Eiver,  355, 356. 

excluded  from  Keweenaw  series,  24. 

,  Extent  of,  351. 

,  Foster  and  "Whitney  on,  361, 362. 

on  Hungarian  Kiver,  354. 

on  Lake  Agogebic,  360. 

Portage  Lake,  185, 359. 

,  Kelationof,  toMississippi  Valley  sandstone,  351, 442-445. 

Keweenaw  Series,  300-365, 442-445. 

Trenton  limestone,  352. 

Western  Sandstone,  366. 

on  South  Eange,  360,  361. 

,  Theories  as  to,  361-365, 442^45. 

on  Torch  Lake  Eailroad,  356-358. 

,  Wadsworth  on,  3,54, 355, 357-359. 

,  Winohell  on,  442. 

Edward  Island,  Sandstone  conglomerate  and  trap  of,  336. 

Egleston,  F.,  Publication  of,  22. 

Emerson,  L.  G.,  3,  202. 

Emmons,  E.,  Taconic  Group  of,  443. 

Encampment  Bluff,  Diabase-porphyrite  of,  84, 285, 433. 

,  Olivine-diabase  near,  76. 

,  Ortboclase-free  gabbro  of,  47, 286. 

Encampment  Kiver,  Agate  Bay  Group  on,  284, 285, 289. 

,  Flowage  strnctnre  in  rocks  of,  291. 

,  Lester  Eiver  Group  on,  279,  281-283. 

Encampment  Island,  Orthoclase-free  gabbro  of,  286. 
English  Lake,  Hornblende-gabbro  of,  58. 
Epidote  of  amygdaloids,  89, 90. 

diabase-porphyrites,  79. 

sandstones,  128. 

Ernptives,  Acid.    See  Acid  original  rocks. 
— ,  Basic.    See  Basic  original  rocks. 
— ,  Tertiary,  Order  of,  432. 

,  Eolation  of  to  Keweenawan,  436. 

Faults  in  Agate  Bay  Group,  290, 291. 
— — Beaver  Bay  Group,  312. 


Faults  at  contact  Eastern  Sandstone  and  Keweenaw  Seriea, 
365. 

—  of  Douglas  County  Copper  Eange,  258. 

—  at  Great  Palisades,  319. 

—  on  Hungarian  Eiver,  355. 

Keweenaw  Point,  158, 205, 363. 

,  Foster  and  Whitney  on,  361. 

,  End  of,  208. 

on  Montreal  Eiver,  230. 

,  Effect  of,  on  South  E:iDgo,  205. 

—  of  the  Keweenaw  Series,  416, 417. 

North  Shore,  142, 290, 291,  312,  319, 329. 

Porcupine  Mountains,  219. 

Temperance  Eiver,  329. 

Feldspars  of  amygdaloids,  88. 

augite-syenite,  112, 113, 114. 

diabase-porphyrite  (porphyritic),  79. 

granite,  125. 

olivine-diabase,  70. 

quartz-porphyry,  98, 101. 

(porphyritic),  97. 

quartzless  porphyry,  92. 

— ,  Methods  of  determining,  Des  Cloizeaux',  39. 

,  Thoulet's,  437, 438, 439. 

— ,  nature  of,  39, 437-440. 

— ,  Silica  content  of,  437-440. 

— ,  Tschermak's  theory  of,  437-439.    See  Plagioclaae. 

Felsites,  95-115. 

— ,  Banding  in,  97. 

—  of  Bare  Hills,  182. 

Beaver  Bay  Group,  311, 312. 

— ,  Characteristics  of,  95-102. 

— ,  Color  of,  96. 

— ,  Constituents  of,  98, 102. 

,  augite,  102. 

,  ferrite,  98,  lOL 

,  "greenish  substance,"  100. 

,  orthoclase,  99. 

,  quartz,  99. 

,  secondary,  99, 100. 

— ,  Flowage  in,  97, 9S. 
— ,  Glass-base  in,  99. 
— ,  Localities  for,  102. 
— ,  Matrix  of,  96-98. 

—  of  Little  Montreal  Eiver  Bay,  183. 

—  of  Michipicoten  Island,  343. 

—  of  Mount  Houghton,  182. 
— ,  Origin  of,  102. 

—  of  Porcupine  Mountains,  212. 

— ,  Porphyritic  ingredients  of,  97, 98, 101, 102. 
— ,  Eosenbnsch  on,  96. 

—  of  Stannard's  Eoot,  197, 198. 

— ,  Tabulation  of  observations  on,  104-115. 
— .    See  Porphyry,  Quartz-. 
Felsitio  porphyry.    See  Felsites. 
Ferrite  of  amygdaloids,  88. 

augite-syenite,  113. 

felsites,  99, 101,  X02. 

ortboclase-free  gabbro,  4L 

quartz-porphyry,  98, 100, 102. 

quartzless  porphyry,  92. 

Fire  Steel  Eiver,  200. 

Flambeau  Trail,  Gabbros  of,  236. 

Flint  Steel  Eiver,  200. 

Flowage-stmcture  in  felsitic  porphyry,  139. 

—  in  amygdaloids,  88, 139. 


454 


INDEX. 


Flowage-stmcture  in  basic  rocks,  81, 139. 

—  of  Beaver  Bay  Group,  149, 313. 

Great  Palisades,  317, 318. 

Red  Eock  Bay,  322. 

Fond  da  Lac,  26. 

— ,  Sandstone  of,  262. 

Fond  dn  Lac  Mine,  Olivine-free  diabase  of,  68, 251. 

Fortieth  Parallel,  Geological  exploration  of,  98, 109, 312. 

Foster,  J.  W.,  Publications  by,  18. 

Foster,  J.  "W.,  and  Whitney,  J.  D.,  2,  5,  6, 197. 

on  acid  eraptives,  12. 

Bohemian  Range,  180, 185. 

conglomerates,  origin  of,  30, 31. 

,  pebbles  of,  31. 

Eastern  Sandstone,  361, 362. 

Isle  Eoyale,  330,  331. 

Keweenaw  Series,  age  of,  12, 352, 442. 

Lake  Superior  synclinal,  410. 

Mount  Bohemia,  184. 

^ Houghton,  149, 183. 

pebbles  of  conglomerates,  31. 

Pigeon  Point,  370. 

Porcupine  Mountains,  207, 224. 

,  Publications  of,  16, 17, 18. 

on  Saint  Croix  Yalley,  240. 

sandstone  of  B6te  Grise  Bay,  185,  353, 354. 

South  Range,  204, 205. 

Stannard's  Rock,  197. 

traps,  origin  of,  9. 

Fouqu6,  F.,  and  L6vy,  A.  M.,  on  secondary  quartz,  438. 
French  River,  264. 

,  Agate  Bay  Group  on,  284. 

,  Diabase-porphyrite  of,  83. 

,  Lester  River  Group  on,  279-281. 

,  Olivine-diabase  of,  72, 76, 2S0. 

,  Olivine-gabbro  of,  281. 

Gabbro,  Hornblende-,  56-58. 

—  of  Bad  River  country,  232. 
— ,  Characteristics  of,  56. 

— ,  Constituents  of,  57. 

— ,  Localities  for,  56. 

— ,  Tabulation  of  observations  on,  57, 58. 

Gabbro,  Olivine-.    See  Gabbro,  Orthoolase-free. 

Gabbro,  Orthoclase-,  50-56. 

—  of  Aminicon  River. 

Bad  River,  54, 144, 155, 156, 233, 233. 

Branschweiler  River,  54, 230, 231. 

Cascade  River,  56. 

— ,  Constituents  of,  apatite,  52. 

,  augite,  52. 

,  biotite,  52. 

,  copper  sulphide,  52. 

,  chlorite,  52. 

,  iron  sulphide,  52. 

,  magnetite,  51. 

,  orthoclase,  51. 

,p]agioclase,  51. 

■  — ,  quartz,  secondary,  51, 438. 

,  titanic  acid,  51. 

— ,  Distribution  of,  154. 
— ,  of  Dnluth,  51,  53,  55,  269. 

Eagle  Mountain,  56,273,274. 

Eagle  River  Section,  53, 174, 175, 176. 

—  Frequency  of,  50. 
— ,  Grain  of,  50. 

Lester  River,  55,  ?81, 282. 

— ,  Mode  of  occurrence  of,  52. 


Gabbro,  Orthoclase-,  of  Mount  Bohemia,  53, 184. 

—  —  Pigeon  Eiver,  372. 
Silver  Islet,  378. 

— ,  Tabulation  of  observations  on,  53-56. 
—of  Thunder  Bay,  376. 
Gabbro,  Orthoclase-free,  37-50. 

—  of  Animikie  Group,  368,  379. 
Bad  River,  45, 232. 

Beaver  Bay  Group,  48, 49, 302, 308. 

Brunschweiler  River,  46, 230, 231. 

Brulfi  River,  38, 49, 144, 321. 

— ,  Characteristics  of,  37. 

— ,  Constituents  of,  apatite,  43. 

,  augite,  42, 43. 

,  biotite.  43. 

,  chlorite,  43. 

,  f errite,  40. 

,  iron  oxide,  41. 

,  olivine,  37, 38. 

,  plagioclase,  39, 40. 

,  prehnite,  43. 

,  titaniferous  magnetite,  41. 

,  viridite,  43. 

— ,  Distribution  of,  154. 

—  of  Duluth46,277. 

Eagle  River  section,  45, 175. 

Encampment  Bluff  and  Eiver,  47, 286. 

— ,  Localities  for,  43. 

— f  Luster-mottling  of,  42. 

— ,  Mode  of  occurrence  of,  43. 

—  of  Nipigon  Bay,  50, 333, 334. 

Pigeon  Point,  370. 

River,  371, 372. 

Potato  Eiver,  45, 230. 

Saint  Louis  River,  208, 270. 

— ,  Specific  gravity  of,  38. 

—  of  Split  Rock  River,  49, 302. 

—  Tabulation  of  observations  on,  45-50. 

Thunder  Bay,  371,  376, 377. 

Gaujot,  E.,  on  Bohemian  Range,  180. 

Geological  Report,  Copper  Lands,  &c.      See  Foster  and 

Whitney. 
Glass-base,  96. 

of  felsites,  99,  100. 

Gogogashugun  Eiver,  Diabase,  &c.,  of,  229. 

,  Quartzless-porphyry  of,  93. 

Good  Harbor  Bay,  Sandstones  of,  329. 
Gooseberry  River,  Agate  Bay  beds  at,  289,  290. 

,  Conglomerate  on,  293. 

,Felsiteof,  231. 

,  Olivine-diabase  of,  7C- 

Goose  Point,  Olivine-gabbro  near,  375. 

Grand  Falls.    See  Kakabika  Falls. 

Grand  Marais  and  vicinity,  Beaver  Bay  rocks  at,  298, 319, 320. 

,  Felsite  of,  103, 110. 

. ,  Eeef  at  harbor  at,  327. 

— . Eolation  of  acid  and  basic  rocks  of,  436. 

,  Temperance  River  Group  at,  324, 327. 

. ,  Titanic  acid  in  rocks  of,  52.  • 

,  Titaniferous  magnetite  of,  51. 

,  Topography  near,  141, 142. 

Grand  Portage,  330. 

Bay,  Animikie  Group  at,  297,  367, 405. 

,  Gabbros  of,  297. 

Erosion  of  Huronian  beds  at,  405. 

Granit6,125. 

—  of  Bad  River,  144,  233, 435. 


INDEX. 


455 


— ,  Characteristics  of,  125. 

Granite,  Constituents  of,  augite,  125. 

,  feldspar,  125. 

,  horuMende,  125. 

,  quartz,  125. 

—  of  Duluth,  270-272, 435. 

— ,  Eruptive  and  non-eniptive,  395. 
— ,  HuTonian,  389, 390. 
— ,  Inclusions  in,  125. 

—  of  Kakabika  Falls,  375. 

—  at  Lake  Shebandowan,  396. 
— ,  Localities  for,  125. 

—  in  Menominee  Region,  394. 
Mesabi  Eange,  383, 399. 

—  north  of  Thunder  Bay,  396. 

— ,  Relation  of  Animikle  beds  to,  399. 
— ,  Relations,  125. 

Granitell.    See  Augite-syenite  and  Porphyry,  Granitic. 
Granitic  porphyry.    See  Augite-syenite  and  Porphyry,  Gra- 
nitic. 
Gratiot  Lake,  164, 186. 
Gratiot  River,  165, 187. 

Great  Conglomerate.    See  Conglomerate,  Great. 
Great  Conglomerate  Ridge,  164. 
Great  Palisades,  146-148, 314-318. 

,  Diabases  of,  315, 316, 317. 

.  Fluidal  structure  of  felsites  of,  98. 

,  Diabase-porphyrite  of,  84, 316, 440, 442. 

,  Glass  inclusions  in  rocks  of,  102. 

,  Quartz-porphyry  of,  103, 109, 146, 147, 317, 313, 440. 

,  Relation  of  acid  and  basic  rocks  of,  435. 

Great  Palisades  (near),  Gabbro  of,  318. 

,  Diabase-porphyrite  of,  36, 80,  84, 318. 

Greenstones,  Huronian,  389,  390. 

Greenstone  Range,  164, 165. 

Greenstone,  The,  61,  71, 73, 87, 154, 164, 187. 

,  Alteration  of,  70. 

at  Eagle  River  section,  174, 175. 

,  Extent  of,  140. 

,  Pumpelly  on,  69, 70, 174. 

,  Thickness  of,  178, 187. 

,  Thinning  of,  187. 

,  Veins  in,  434. 

Gros  Cap,  Amygdaloid  of,  349. 

,  Felsite  of,  349. 

,  Huronian  rocks  of,  401. 

,  Lake  Superior  synclinal  at,  415. 

Gull  Island,  330. 

Ganfiint  Lake,  Schists  of,  399. 

Hall,  C.  W.,  Publications  of,  18, 23. 

Hancock,  Sandstones  and  stales  near,  193. 

Harbor  Island,  Trap  of,  401. 

Hat  Point,  Animikie  Group  at,  368. 

Hill,  E.  P.,  163. 

Hinkley,  Potsdam  sandstone  near,  244. 

Hodge,  J.  T.,  Publication  by,  16. 

Hornblende  of  hornblende-gabbro,  56, 57. 

augite-syenite,  114. 

granite,  125. 

— ,  Uralitic  nature  of,  114, 396. 
Hornblende-gabbro.    See  Gabbro,  Hornblende. 
Houghton,  D.,  5. 

—  on  conglomerates,  9. 

,  Eastern  Sandstone,  ago  of,  352. 

,  Keweena-w  Series,  age  of,  12. 

— ,  Publications  of,  14, 15. 

—  on  traps,  origin  of,  10. 


Hubbard,  B.,  5, 16. 

Hungarian  River,  Eastern  Sandstone  on,  354, 355. 

,  Faulting  on,  355. 

,  Wadsworth  on  rocks  of,  363. 

Hunt,  T.  S.,  5. 

—  on  Animikie  Group,  157, 384. 
,  age  of,  385. 

,  Bohemian  Eange,  180. 

Diabases,  origin  of,  10. 

,  Huronian  of  Thrmder  Bay,  377, 405. 

Keweenawan  of  Thunder  Bay,  332. 

Nipigon  Group,  157. 

— ,  Publications  of,  19, 20, 21, 22, 23. 

—  on  quartz-porphyry  of  Kipigon  Straits,  31. 
Volcanic  ash,  437. 

Huron  Bay,  414. 

Huronian,  Marquette,  and  Menominee.  392-395. 

,  Diorites  of,  393. 

,  Gneiss  of,  393. 

,  Granite  of,  394. 

,  Greenstones  of,  393. 

,  kinds  of  rock  of,  393,  394. 

,  Relations  of,  to  Penokee  Huronian,  and  Animikie 

Group,  394,  395. 

,  Schists  of,  393. 

Huronian,  Original,  386-391. 

,  Characteristics  of,  386-389. 

,  Dikes  of,  389. 

,  Eruptives  of,  390,  434, 445. 

,  Relation  of,  to  Animikie  Group,  385, 386, 390, 391,  399, 443. 

,  Keweenaw  Series,  390, 391,  404, 405, 406,  407,  443- 

445. 

,  Lake  Superior  synclinal,  416-418. 

Huroni.-in,  Penokee,  391,  392. 

,  Relation  of,  to  Animikie  Group,  392. 

Hyperite  of  Macfarlane,  377. 

Iron  River,  Nonesuch  shale  belt  on,  221,  222. 

,  Outer   sandstone   and    conglomerate    of   Porcupine 

Mountains  on,  220. 

,  Sandstones,  &c.,  of,  222. 

Ironton  Trail,  Diabaso  porphyrite  of,  83. 

,  Granitic  porphyry  of,  231. 

,  Orthoclase-gabbro  of,  53. 

Isle  Royale,  05, 164, 329-331, 414. 

,  Area  of,  27. 

,  Copper  of,  430. 

,  Height  of,  331. 

,  Sandstone  of,  331. 

,  Thickness  of  Lower  Division  on,  159. 

,  Topography  of,  330. 

,  Upper  Division  on,  154. 

vein,  189. 

Isle  St.  Ignace,  27, 334, 335, 413, 414. 
Jackson,  C.  T.,  5. 

—  on  Bohemian  Range,  180. 

diabases,  origin  of,  10. 

,  Keweenaw  Series,  age  of,  12. 

— ,  PubUcations  of,  15, 10, 17, 18, 19, 20. 
Julien,  A.  A.,  23, 35, 43. 

—  on  alteration-products  of  olivine,  39. 
orthoclase  in  gabbros,  41. 

— ,  Publication  of,  23. 
Kakabika  Falls,  374, 375. 

. ,  Animikie  slates  at,  380. 

Kaministiquia  River,  Animikie  Group  at,  371. 
,  Dikes  on,  374. 


456 


INDEX. 


Kaministiquia  Eiver,  Ferraginons  slates  and  sandstones  of; 

374, 3S0. 
Kettle  Eiver,  243-245. 

.    See  Snake  Eiver. 

Keweena-w  Bay,  163. 

,  Eastern  Sandstone  at,  351. 

Keweenaw  Fault.    See  Faults. 
Keweenaw  Point,  163-197. 

,  Area  of  Keweenaw  Series  on,  27. 

,  Ashbed  Group  on,  171-173. 

,  Authorities  on,  163. 

,  Bohemian  Eange  on,  179-187. 

,  Dimensions  of,  163. 

east  of  Eagle  Eiver,  166, 167. 

from  Eagle  to  Gratiot  rivers,  187. 

. at  Eagle  Eiver  Section,  167-178. 

,  Eastern  Sandstone  on,  185, 351-360. 

,  Elevations  on,  164,  165. 

.Faults  on,  1C6. 

,  East  of  Gratiot  Eiver,  164-165. 

at  Gratiot  Eiver,  187,  188. 

,  Great  Conglomerate  of,  177. 

,  Greenstone  of,  174,  175. 

,  Kingston  Conglomerate,  177. 

,  L.ake  Shore  Trap  of,  178, 179,  186. 

,  Lower  Division  on,  157, 138. 

,  Marvine  on  stratigraphy  of,  167. 

,  Median  Valley,  Eocks  of,  179,  187,  188. 

,  Outer  Conglomerate  of,  151,  179,  186. 

,  Phoenii  Mine  Group,  175,  176. 

,  Portage  Lake  to  Gratiot  Elver,  195,  196. 

,  Portage  Lake  Section,  188-195. 

,  Eolation  of  Keweenaw  Series  and  Huronian  on,  406, 407. 

,  Eidges  of,  164,  165. 

,  Section  of  eastern  part  of,  186. 

,  Transverse  copper  veins  of,  423-425. 

,  Thickness  of  Lower  Division  on,  157,  166. 

,  Topography  of  rocks  of,  164-166. 

,  Upper  Division  on,  133. 

Keweenaw  Series,  Acid  rocks  of.    See  Acid  original  rocks. 

,  Age  of,  12,  13, 431, 442-445. 

,  Area  of,  27. 

,  Ash,  volcanic,  absent  in,  32, 487. 

,  Authorities  on,  4,  5. 

,  Basic  Eocks  of.    See  Basic  original  rocks. 

— . ,  Chronological  relation  of  eruptives  of,  32, 432-436. 

,  Conglomerates  of.    See  Conglomerates. 

,  Copper  deposits  of,  409-429.    See  Copper. 

,  Contact  of,  with  other  formations.    See  Contact. 

on  East  Shore.    Chap.  VII.    See  East  Shore. 

,  Eruptives  of.    See  Acid  and  basic  original  rocks. 

,  Extent  and  general  nature  of.  Chap.  H. 

,  Faulting  of,  416, 417.    See  Faults. 

,  Foster  and  Whitney  on,  7-9. 

on  Keweenaw  Point.    See  Keweenaw  Point. 

,  Literature  of,  14-23, 431,  432. 

,  Lithology  of.  Chap.  in. 

,  Lower  Division  of,  154-160.    See  Lower  Division. 

in  Michigan,  Chap.  VI.    See  Michigan. 

Minnesota,  Chaps.  VI,  VII.    See  Minnesota. 

on  North  Shore,  Chap.  VII.    See  Uorth  Shore. 

,  Porphyry  conglomerates  of.  Chap.  II. 

,  Eelations  of,  to  newer  formations,  351-366, 442-445. 

. older  formations,  367-409, 442-445. 

on  South  Shore,  Chap.  VI.    See  South  Shore. 

,  Upper  Division  of,  152-154.    See  Upper  Division. 

in  "Wisconsin,  Chap.  VI.    See  Wisconsin. 


King's  Creek,  G.'ibbro  of,  46,  270. 

Klapetko,  F.,  4. 

Kloos,  J.  H.,  Publication  by,  431.    See  Strong. 

Knife  Falls,  Animikie  slates  of,  384. 

Knife  Eiver  and  vicinity,  Agate  Bay  beds  on,  284, 288, 

,  Diabase-porphyrite  of.  84. 

,  Lester  Eiver  Beds  on,  279. 

,  Olivine-diabase  of,  76. 

,  Olivine-gabbro  of,  286. 

Labradorite  of  augite-syenite,  113. 

olivine-diabase,  70. 

(porphyritic),  of  diabase-porphyrite,  79 

— .    See  Anorthite. 

Lao  La  BeUe,  164,  179,  181,  181, 185. 

,  Ashbed-diabase  of,  183. 

,  Eastern  Sandstone  at,  354. 

,  Olivine-diabase  of,  74, 184. 

,  Orthoclase-gabbro  of,  53. 

■ ,  Porphyry-conglomerates  of,  183. 

.    See  Mount  Bohemia. 

Lake  Agogebic.    See  Agogebic  Lake. 
Lake  Sheb.andowan,  Schists  of,  396. 
Lake  Shore  Trap,  178, 179, 186. 
Lake  Superior,  Dimensions  of,  26. 

,  East  Shore  of,  27,  347-349. 

,  Generalized  Section  of,  417, 418. 

,  K'orth  Shore  of.    See  North  Shore. 

,  South  Shore  of.    See  South  Shore. 

—  —  synclinal,  410-418. 

,  Wisconsin  Geol.  Survey  on,  410-412. 

Lapham,  I.  A.,  411. 

L'Anse,  163. 

Lester  Eiver,  Agate  Bay  Group  near,  292,293. 

,  Diabase-porphyrite  of,  84, 155, 279. 

,  Dikes  ne.ar,  283, 293, 

,  Lester  Eiver  Group  on,  279, 280. 

,  Olivine-diabase  of,  75, 280, 283. 

,  Olivine-gabbro  of,  282. 

,  Orthoclase-gabbro  of,  55,  56. 

,  Porphyries  of,  282,  283. 

,  Sandstone  near,  292. 

Lester  Eiver  Group,  279-284. 

,  Characteristics  of,  267. 

.Dikes  of,  283. 

in  Eastern  Minnesota,  294. 

,  Limits  of,  279. 

,  Localities  for,  279. 

Limestone,  24, 128. 

— ,  Huronian,  386-388. 

— ,  Keweenawan,  of  Nipigon  Lake,  340. 

Little  Carp  Eiver,  213. 

,  Diabase-porphyrite  of,  82. 

,  Diabases  of  Porcupine  Mountains  oh,  215, 217. 

,  Outer  Trap-belt  of  Porcupine  Mountains  on,  219. 

Sandstone-belt  of  Porcupine  Mountains  on,  220, 

,  Quartz-porphyry  of,  106, 211. 

,  Stratigraphy  on,  216, 217. 

Little  Iron  Eiver,  222. 

Little  Montreal  Eiver,  104. 

Little  Montreal  Eiver  Bay,  Felsito  of,  182. 

Locke,  J.,  Publications  of,  15,  18. 

Logan.  Sir  Willi.am,  5,  6. 

—  on  acid  eruptives.  12. 

Animikie  Group,  age  of,  388. 

,  relation  of,  to  Huronian.  390. 

,  stratigraphy  of,  379. 

Black  and  Nipigon  Bays,  334-336. 


INDEX. 


457 


Logan  on  "crowning  overflow,"  381, 382. 

East  Coast  Keweenawan,  347,348. 

Huronian,  401. 

greenstones,  Huronian,  389. 

Eaministiquia  Biver  slates,  374. 

Keweenaw  Series,  age  of,  13,  341,  344. 

Mlchipicoten  Island,  24,  343. 

Nipigon  Group,  157. 

Original  Huronian,  386-388. 

Pic  Eiver  Huronian,  401. 

— ,  PubUcations  of,  15,  16,  17,  20. 
— ,  on  Thunder  Bay,  330, 332. 

,  sandstone,  332. 

,  schists,  396,  397. 

,  schists  west  of,  374. 

Lone  Eock,  Nonesuch  shale  belt  near,  223. 
Lower  Division,  Keweenaw  Series,  152,  154-160. 

,  on  Bad  Kiver,  158. 

,  Gabbros  of,  1 55. 

Black  and  Nipigon  Bays,  160. 

Douglas  County  Copper  Range,  159. 

Isle  Koyale,  159. 

. Keweenaw  Point,  157, 158. 

Mamainse,  160. 

Michipicoten  Island,  160. 

Minnesota  Coast,  266. 

_ ,  Gabbros  of,  156. 

Montreal  River,  158. 

,  ;Nipigon  sandstone  of,  157. 

,  on  Ontonagon  Eiver,  158. 

,  in  Saint  Croix  TaUey,  159. 

,  Thickness  of,  157-160. 

Lucille  Islands.  330. 

,  Anlmikie  Group  at,  369. 

,  Keweenawan  diabase  on,  369. 

Macfarlane,  T.,  6. 

— ,  on  Animikie  Group,  385. 

"Crowning  Overiiow,"  382. 

East  Coast  Keweenawan,  347, 348. 

Mamainse,  160,  348. 

Michipicoten  Island  and  its  rocks,  85,  86,  87,  93, 112, 160, 

341-340,  436,  437. 

Origin  of  traps,  10. 

felsites,  12. 

— ,  Publications  of,  20,  23. 

—  on  Silver  Islet,  378. 

Thunder  Bay  rocks,  332,  377-379,  405. 

Magnetite  of  augite-syenite,  134. 

diabase-porphyrite,  78, 79. 

olivine-diabase,  70. 

olivine-free  diabase,  63. 

orthoclase-g-abbro,  51. 

orthoclase-free  gabbro,  41. 

Main  Trap  Range,  165, 187, 188, 198, 201, 208, 229, 414. 

Mamainse,  160, 348, 415. 

-,  Dikes  of,  143. 

Manitou  Island,  Outer  Conglomerate  on,  179. 

,  Kind  of  strata  on,  414. 

Manitou  Elver,  Temperance  River  Beds  on,  324, 324 

Marcou,  J.,  5, 11. 

— ,  Publications  of,  16, 17. 

Marquette,  414, 415. 

Marvine,  A.  R.,  5, 7, 10, 21,  52, 163, 418. 

—  on  amygdaloids,  origin  of,  11. 

Calumet  Conglomerate,  195. 

Eagle  River  Section,  167-178. 

rocks  of  Houghton  and  Keweenaw  Cos.,  188. 


Mather,  "W.  "W.  on  Stnnnard's  Eock,  197. 
Maw-ske-quaw-caw-maw  Eiver,  Augite-syenite  of,  124,321, 
322. 

,  Orthoclase-gabbro  of,  49, 322. 

Mclntyre,  380,  381, 
McKay's  Mountain,  374. 
McKinlay,  E.,3,262. 
—  on  Beaver  Bay  Group,  278. 

Duluth  Group,  278. 

Snake  and  Kettle  River  district,  234. 

South  Range,  360-366. 

— .    See  Campbell,  Chauvenet. 
Median  Valley,  Keweenaw  Point,  179, 187, 188. 
Mendota  Mine,  Diabase  of,  184. 

Melaphyr  of  Pumpelly.     See   Diabase,  Olivine  — ,    (flne- 
gi-ained). 

Macfarlane,  85,  87, 93. 

Menom  inee  Region.    See  Huronian,  Marquette,  and  Menomi- 
nee. 
Mesabi  Range,  Animikie  group  at,  382-384. 

,  A.  H.  Chester  on,  383. 

,  Granite  of,  383. 

,  Schists  of,  398, 399. 

Metasomatic  Development  of  Rocks  of  Lake  Superior.    Set 

Pumpelly. 
Michigan,  Keweenaw  Series  in,  Chap.  VI. 

on  Keweenaw  Point,  163-198. 

from  Portage  Ljike  to  Ontonagon  Eiver,  198-201. 

on  the  South  Range,  201-205. 

.    ,566  Bohemian  Range,  Eagle  Eiver  Section  Eastern 

Sandstone,  Keweenaw  Point,  Portage  Lake  Sec- 
tion, South  Range. 
Michigan,  Geological  Survey  of,  31, 44,  52, 65, 73, 80,  91, 188, 189, 

190, 191, 195, 197,  247,  351,  352,  359,  3B0,  406. 
Michipicoten  Bight,  347, 415. 
Michipicoten  Island  and  vicinity,  341-346, 415. 

,  Amygdaloids  of,  345. 

,  Area  of  Keweenaw  Series  on,  27. 

,  Diabases  of,  343. 

,  Diabase-porphyrite  of,  78, 80, 85, 87, 433. 

,  Dip  of  rocks  on,  343. 

,  Felsite  of,  103, 112,  343,  346. 

,  Lower  Division  on,  160. 

,  Macfarlane's  specimens  from,  described,  85,  86,  87,  93, 

112,  342-346. 

, Peculiar  "breccias "  of,  430, 437. 

,  Quartz-porphyry  of,  93, 112, 346. 

,  Relation  of  acid  and  basic  rocks  of,  433, 434. 

,  Stratification  on,  341. 

,  Thickness  of  rocks  of,  342. 

Michipicoten  River,  Huronian  rocks  of,  400. 

Microfelsite  of  Rosenbusch,  96. 

Middle  Eiver,  Rocks  of,  256. 

Mineral  River,  ITonesnch  Shale  Belt  on,  224. 

Minnesota,  Area  of  Keweenaw  Series  in,  27. 

— ,  Authorities  on,  262. 

— ,  Course  of  rock-belts  in,  265. 

—,  Dikes  of,  143,  264, 278, 283, 293, 294, 301, 306, 307, 320-323, 329, 

384. 
~,  Duluth  to  French  River,  264. 
— ,  French  River  to  SpUt  Rock  River,  264. 
— ,  General  structure  of,  140, 265. 
— ,  Lakeward  dip  of  rocks  of,  141. 
— ,  Lake  Superior  coast  of,  262-329. 
_,  Mesabi  Range,  382-384,  398, 399. 
— ,  Snake  and  Kettle  River  District,  241-246. 
— ,  Split  Rock  River  to  Grand  Portage  Bay,  264, 265. 


458 


INDEX. 


Minnesota,  Subordinate  groups  of,  266. 

— ,  Temperance  Eiver  to  Grand  Portage,  265. 

— ,  Thickness  of  Lower  Division  on,  159. 

— .    See  also  Agate  Bay  Group,  Beaver  Bay  Group,  Duluth 

Group,  Lest-er  Kiver  Group,  Saint  Louis  Biver  Gab 

bros.  Temperance  Eiver  Group. 

,  Eocks  of,  266. 

— ,  Geological  Survey  of,  7tb  Annual  Eeport,  138, 142. 

,  Sth  Annual  Eeport,  138, 145. 

,  9th  Annual  Eeport,  101,382,  398. 

,  10th  Annual  Eeport,  399, 442, 443. 

—  mine,  Copper  vein  of,  422, 423. 
Mission  Creek,  Animikie  slates  of,  262. 
Montreal  River,  Diabase,  &c.,  of,  229. 
,  Amygdaloids  of,  136. 

,  Lower  Division  on,  158. 

,  Olivine-free  diabase  of,  16, 67, 68, 229. 

,  Sandstones  of,  133,  225,  404. 

,  Sources  of  inform.ition  on,  207. 

,  Upper  Division  on,  153. 

Montreal  Eiver  Section,  226-230. 
Moose  Creek,  247, 249. 

,  Olivine-diabase  of,  75. 

free  diabase  of,  68. 

Mosler,  C,  Publication  by,  23. 
Mosquito  Lake,  163. 
Mount  Bohemia,  184,  185. 

,  Augite-syenite  of,  115, 116, 185, 435. 

,  Olivine-diabase  of,  184. 

,  Foster  and  "Whitney  on,  184. 

,  Orthoclase-gabbro  of,  52, 63, 184. 

Mount  Houghton,  181,  184. 

,  Felsito  of,  102,  104,  182,  183,  343,  433. 

,  Foster  and  Whitney  on,  31, 149, 183. 

,  Quartz-porphyi'y  ot,  149. 

Miiller,  A.,  Publications  of,  18. 
Murray,  A.,  400. 

,  Publication  of,  15. 

,  on  Thunder  Bay  schists,  396,  397. 

National  mine,  Copper  vein  of,  423. 

,  Quartzless  porphyry  of,  94. 

Ifeebing,  Animikie  Group  at,  380. 

Kfiw  Hampshire,  Geology  of,  40,  51. 

2<'icholson,  H.  A.,  Publications  of,  21. 

Nipigon  Bay,  Contact  of  sandstone  and  gabbro  on,  333. 

,  Gabbro  of,  44,  50,  333,  334, 438. 

,  Lower  Division  on,  160. 

,  Sandstone  of,  128, 333, 336. 

Nipigon  Group,  Hunt  on,  157. 
Kipigon  Lake  and  Basin.  338-341. 

,  Age  of  rocks  of,  341. 

,  Augite-syenite  of,  339. 

,  Limestone  of,  24, 128,  340. 

,  Eelation  of  Keweenaw  Series  and  Huronian  on,  403. 

,  Sandstone,  24,  339. 

Nipigou  Straits  and  Islands  in,  334,  338. 
If  oncsuch  mine,  221. 

,  Copper  of,  420. 

,  Sandstones  of,  131,  132,221. 

,  Basic  detritus  in,  128. 

—  Shale  Belt,  221-224. 

on  Iron  Eiver,  222. 

Little  Iron  Ei.ver,  222. 

Lone  Eock  at,  223. 

on  Mineral  Eiver,  221. 

— Presqu'  Isle  Eiver,  223. 


Nonesuch  Shale  Belt  in  T.  50,  E.  39  "W. ;  E.  40  W. ;  E.  41  W., 
224,  225. 

possibly  like  "breccias"  of  Michipicoten   Island, 

436. 
North  Brother,  103. 
North  Shore  of  Lake  Superior,  Agate  Bay  Group,  284-294. 

,  Beaver  Bay  Group,  298-323. 

,  Black  and  Nipigon  bays,  331-338. 

,  Char.-icteristics  of,  260,  261. 

,  Conglomerates  of,  30. 

,  Distribution  of  Keweenaw  Eocks  on,  261-266. 

,  Duluth  Group,  275-279. 

,  Isle  Eoyale,  330,  331. 

to  Nipigon  Bay,  329,  330 

,  Lester  Eiver  Group,  279-283. 

,  Minnesota  coast,  261-329. 

,  east  end,  294. 

,  Nipigon  Lake,  338-341. 

,  St.  Louis  Eiver  Gabbros,  268-275. 

,  Temperance  Eiver  Group,  323-329. 

,  Thunder  Bay  to  Nipigon  Bay,  331-338. 

Norwood,  J.,  5. 

—  on  diabases,  origin  of,  10. 
dikes,  143,  264. 

Douglas  County  Copper  Eange,  258. 

Great  Palisades,  316. 

Metamorphic  sandstone  or  shale,  138,  287,283. 

,  North  Shore  rocks,  folding  of,  140. 

Olivine,  Alteration  of  in  basic  rocks,  39. 

—  of  anorthite  rock,  59. 

—  -— basic  original  rocks,  38. 
melaphyr,  69-71. 

orthoclase-f ree  gabbro,  38. 

Olivine-diabase.    See  Diabase,  Olivine-. 
Olivine-free  diabase.    See  Diabase,  Olivine-free. 
Olivine-gabbro.    See  Gabbro,  Orthoclase-free. 
Ontonagon  Eiver,  155,198-201. 

,  Copper  of,  422. 

,  Eastern  Sandstone  on,  359,  360. 

—  — ,  Falls  of,  Exposures  at,  302. 

,  Keweenawau  rocks  of,  199. 

,  Lower  Division  on,  158. 

,  Outer  Conglomerate  on,  149. 

,  Quartz-porphyry  on,  150,  199, 433. 

,  Sandstones  of,  199,  200. 

,  Stratigraphy  on,  198,  199. 

,  Thickness  of  Keweenaw  Series  on,  201. 

—  — ,  Trend  of  strata  east  of,  208. 
,  "West  Branch  of,  203. 

Ontonagon  and  Montreal  Elvers,  Country  between.  Dip  of 
strata  of,  208. 

-^^ — ,  Eastern  Sandstone  of  the,  208. 

,  Stratigraphy  in,  207,  208. 

,  Union  of  South  and  Main  ranges  in.  208. 

.    Sec  Porcupine  Mountains. 

"  Ordinary  type  "  diabase  of  Pumpelly.  5ee  Diabase,  olivine- 
free. 

Ortlioclase  of  augite-syenite,  113. 

diabase-porpbyrite,  78,  79. 

felsites,  99. 

hornblende-gabbro,  57. 

olivine-diabase,  69. 

orthoclase-gabbro,  51. 

quartzless  porphyry,  92. 

—  {porphyritic}  of  quartzless  porphyry,  93. 
quartz-porphyry,  99. 


INDEX. 


459 


Orthoclase-free  dialjase  (coarse  grained).   See  Gabbro,  Ortlio- 

clase-free. 
Orthoclase-gabbro.    See  Gabbro  Ortboclase. 
Orthoclase-freo  gabbro.    tiee  Gabbro,  Ortboolase-free. 
Osceola,  Contact  of  Keweenaw  Series  and  Potsdam  Sandstone 

in,  236,  287. 
Otter  Head,  Huronian  of,  400. 
Owen,  D.  D.,  5,  37,  403. 

—  on  Eastern  Sandstone,  351. 

Keweenawan  rocks,  112. 

St.  Croix  Sandstone,  411. 

,  Publications  of,  17. 

— .    See  Norwood. 

Owen,  E.,  on  Pigeon  Eiver  dikes,  370,  ^71. 

Pebbles  in  angite-syenite,  112. 

—  of  Albany  and  Boston  Conglomerate,  94,190, 191. 

Calumet  and  Hecla  Conglomer.ate,  105, 193, 196. 

M'ational  mine,  94, 190, 191. 

Penin.sula  Bay.  Trap  of,  401. 
Penoliee  Hnronian,  391,  392. 
Percival  mine,  257. 
Petit  Marais,  204. 

,  Temperance  Eiver  Group  at,  323. 

Pboenix  Mine  Group,  175, 176,  178. 

Physical  Geology  of  Lake  Superior.    See  'Whittlesey. 

Pic  Island,  Trap  of,  401. 

Pic,  The,  415. 

Pic  Eiver,  Huronian  of,  401. 

Pie  Island,  331. 

Pigeon  Bay,  Animikie  Group  on,  372-374. 

,  Dike-rock  from,  372. 

,  Gabbros  of,  372. 

Pigeon  Point,  Animikie  Group  at,  360,  370. 
Pigeon  Eiver,  414. 

,  Acid  eruptives  of,  372. 

Animikie  Group  on,  370-372. 

,  Dikes  on,  371. 

,  Eocksof,  372. 

,  Gabbros  of,  371,  372. 

Pike  Lake,  Gabbros  of  274. 
Pike  Eiver,  Slates  of,  401. 
Pine  Eapids,  Sandstone  of,  249. 
Plagioclase  of  amygdaloids,  88. 

augite-syenite,  113. 

hornblende-gabbro,  57. 

olivine-diabase,  69. 

oli vine-free  diabase,  63. 

ortlioclase-free  gabbro,  39,  40. 

,  Angles  of  39, 40,  439.    See  also  Tabulations  of  obser- 
vations of  various  rocks. 
Point  Magnet,  334. 
Pointe  aux  Mines,  347,  349,  415. 
Pokegoma  Falls,  Animikie  Group  at,  383,  384. 
Pcplar  Eiver,  Eocks  of  head-waters  of,  145,  272. 

,  Temperance  Eiver  Group  on,  328. 

Porcupine  Mountains,  65, 206-224. 

,  Authorities  on,  207. 

Basic  Belt  of,  first,  214-218, 432, 435. 

,  second,  219,  220. 

belts,  continuation  of,  224-226. 

at  Carp  Eiver  and  Little  Carp  Eiver,  215-217. 

,  Diabase-porphjTite  of,  82, 215, 440. 

.Dike  of,  253. 

,  Fault  of,  219. 

.Felsites  and  quartz-porphyries  of,  99,  102,  103,  105,  106, 

343, 150,  209-214. 


Porcupine  Mountain,  Formation  of,  206. 

,  Height  of,  209. 

,  Lower  Division  on,  209. 

,  Melaphyr  of,  214. 

,  Nonesuch  shale  belt  in,  221-224. 

,  Olivine-diabase  of,  72, 74,  214. 

,  Olivine-free  diabase  of,  67. 

,  Porphyry  of,  150,  209-212, 432,  433. 

,  Central,  208, 209. 

,  Distribution  of,  210-212. 

,  Limits  of,  212-214. 

,  Sandstone  of,  131, 132. 

,  Inner,  217, 218. 

,  Outer,  220,  224. 

,  Stratigraphy  of  208,  209,  413. 

,  Townships  50  and  51,  E.  42  W.,  Conglomerates  of,  217. 

,  Upper  Division  on,  153, 209. 

Pork  Bay,  Temperance  Eiver  group  at,  324. 
Porphyrite,  Diabase.    See  Diabase-porphyrite. 
Porphyries.    See  Acid  original  rocks. 
Porphyry-conglomerates.    See  Conglomerate,  Porphyry. 
Porphyry,  Felsitic.    See  Felsites. 

Porphyry,  Granitic,  of  Albany  and  Boston  Conglomerate,  118, 
119, 190, 191. 

Baptism  Eiver,  123, 272. 

Beaver  Bay,  122,  306. 

Bete  Grise  Bay,  117. 

Dulnth,  119,  270,  271. 

Eagle  Mountain,  124,  273,  274. 

Eiver  conglomerate,  118, 168. 

Maw-ske-quaw-caw-maw  Eiver,  123. 

Minnesota,  T.  56,  E.  7  "W .,  122. 

Mount  Bohemia,  116, 184. 

near  Split  Eock  Eiver,  121. 

Tischer's  Creek,  120. 

.    See  Augite-syenite. 

— ,  Quartz,  near  Baptism  Eiver,  110,  318. 

of  Beaver  Bay,  106-108,  307. 

Bead  Island,  111,  346. 

the  Calumet  Conglomerate,  105, 195, 196. 

Cedar  Island,  108,  306,  307. 

Eagle  Eiver  Conglomerate,  104, 168. 

Grand  Marais,  110,  .391. 

Great  Palisades,  109,314-318. 

Little  Carp  Eiver,  106, 210. 

Michipicolen  Island,  112,346, 

Potato  Eiver,  106, 231. 

Porcupine  Mountains,  42,  51, 105,  210,  211. 

Eed  Eock  Bay,  110,  322,  323. 

Stanuard' 3  Eock,  197,  198. 

Torch  Lake  Eailroad,  104, 196. 

Tyler's  Fork,  106, 

See  Felsites. 

— ,  Quartzless,  91-95. 

of  Albany  and  Boston  Conglomerate,  94,  190,  191. 

,  Base  of,  92. 

of  Bead  Island,  95,  304. 

. —  Brunschweiler  Eiver,  95. 

,  Characteristics  of  91,  92- 

.Color  of,  92. 

,  Constituents  of,  92,  9B. 

,  augite,  93. 

,  feldspars,  92,  93. 

,  quartz,  92,  438. 

of  Duluth,  95. 

,  Flowage  structure  in,  93. 


^ 


460 


INDEX. 


Porphyry,  Qnartzless,  Localities  for,  93. 

of  National  mine,  94. 

.Phases  of,  91. 

,  Eelation  of,  to  angite-syenite,  92. 

,  Silica  content  of,  92. 

,  Tabulation  of  observations  on,  94,  95. 

Portage  Bay  Island  and  vicinity,  296-298. 

,  Animikie  Group  on,  297. 

,  Diabase-porphyiite  of,  78,  85, 297. 

Portage  Lake,  Amygdaloids  of,  189, 345. 

,  Amygdaloids  (cupriferous)  of,  421,  422. 

,  Conglomerates  of,  93, 190-192. 

,  Copper  deposits  of,  420-422. 

,  Dip  of  strata  on,  187. 

,  ISTonesucb  Shale  belt  on,  193. 

,  Sandstone  of,  130, 131, 185, 192, 193. 

,  Thinning  of  beds  ou,  187. 

,  Upper  DiTision  on,  153, 193. 

Portage  Lake  Section,  188-195. 

,  Conglomerates  of,  32, 190,  191. 

,  Exposures  of,  188. 

,  Great  Conglomerate  of,  191,  192. 

,  Marvine  on,  188. 

■— ,  Pumpelly  on,  188-190,  194. 

,  Sandstone  of,  192,  193. 

,  Shales  of,  192,  193. 

,  Stratigraphy  of,  189. 

,  Summary  of,  194,  195. 

,  Thickness  of,  188. 

,  Upper  Division  on,  192,  193. 

Potato  Eiver  Section,  230,  231. 

,  Diabases  of,  74,  83, 231. 

,  ^elsitic  porphyry  of,  103,  106,  231. 

,  Gabbros  of,  45,  231. 

Potsdam  Sandstone  of  New  York,  Kelation  of,  to  Keweenaw 
Series,  443-445. 

,  "Western  equivalents  of,  442,  445. 

.    See  Cambrian,  Sandstone. 

Powell,  J.  W.,  Publication  of,  431. 
— ,  on  Tertiary  and  Keweenawan  emptives,  436. 
PraysviUe,  DIabase-porphyrite  of,  81, 176, 177. 
Presq'  Isle  Eiver,  Diabase  of,  212. 

,  Limits  of  Porphyry  on,  212. 

,  Lower  Division  on,  208. 

,  Nonesuch  shale  belt  on,  223. 

,  Outer  sandstone  exposed  on,  220. 

,  Outer  trap  exposed  on,  219. 

,  Upper  Division  on,  220. 

Prehnite  in  amygdaloids,  89, 90. 

olivine-diabase,  71. 

olivine-free  diabase,  65. 

(amygdaloidal),  64. 

pseudamygdaloids,  62-66. 

Pyroxene.    See  Angite. 
PnmpeUy,  E.,  2, 5, 7, 10, 163. 

—  on  amygdaloids,  88, 89, 135, 136, 416. 
,  copper  in,  421,  422. 

of  melaphyrs,  87. 

augite-diorite,  56. 

ashbed-diabases,  61. 

basic  original  rocks,  135. 

—  —  Calumet  Conglomerate,  1 95. 

conglomerates,  pebbles  of,  31. 

Des  Cloizeaux'  method,  39. 

diabases,  origin  of,  11. 

diabase-porphyrite,  77,  83, 235. 


PnmpeUy  on  Eastern  Sandstone,  relation  of  to  Keweenaw 
Series,  360,362,363. 

granitic  porphyry,  110. 

homblende-gabbro,  56. 

Keweenawan  sandstones,  nature  of,  30. 

Keweenaw  Series,  age  of,  13. 

,  relation  of,  to  Huronian,  106, 107. 

melaphyr,  68-70. 

olivine-diabase,  68-71, 73, 74. 

-~  —  oUvine-free  diabase,  60-66. 
"ordinary"  diabase,  61. 

—  —  orthoclase-free  gabbro,  45. 

—  —  orthoclase-gabbro,  53,  54. 

the  Porcupine  Mbuntains,  207. 

———Portage  Lake  Section,  188, 189. 

Pre-Cambrian  erosion  of  Keweenawan  rooks,  363. 

pseudamygdaloids,  60-66,135. 

— ,  Publications  of,  21, 22,  23. 

—  on  rocks  of  Saint  Croix  Valley,  239. 
Quartz  of  amygdaloids,  89,  90. 

augite-syenite,  112, 113, 114. 

diabase-porphyrite,  79. 

granitic  porphyry,  112, 113, 114. 

—  (secondary)  of  felsites,  99, 100, 211, 438. 
,  Fcuqu6  and  L6vy  on,  438. 

of  orthoclase-gabbros,  51. 

—  —  —  augite  syenite  and  granitell,  113, 114,438. 
qnartzless  porphyry,  92,  438. 

—  of  hornblende-gabbro,  57. 

—  (porphyritic)  of  quartz-porphyry,  93, 102. 
Quarzites,  Animikie,  367-386. 

— ,  Huronian,  386-388. 

Quartz-porphyry.    See  Felsites  and  Porphyry,  Quartz-. 

Quartzless  porphyry.    See  Porphyry,  Qnartzless. 

Kainy  Lake,  Schists  of,  397. 

Bed  Eiver  Eoad,  Animikie  Group  on,  380. 

Eed  Eock  Bay  and  vicinity,  Beaver  Bay  Group  at,  320. 

— . ,  Diabase-porphyrite  of,  85,  323. 

,  Dike  near,  322, 323, 436. 

,  Felsites  of,  322. 

,  Gabbro  of,  322.  ' 

,  Quartz-porphyry  of,  103,  110,  322. 

Eed  Eock  Post,  Gabbro  of,  333. 

Eeport  of  Geological  and  Mineralogical  Survey  of  Mineral 

Lands  of  United  States   in  Michigan.    See  Foster  and 

Whitney. 
Eice  Point  Quarry,  119.     See  Duluth. 
Kivot,  L.  E.,  5. 

—  on  origin  of  traps,  10. 
— ,  Publication  of,  18. 

Eoche  de  Bout,  Diabase  of  Island  near,  338. 
EocHand  (National  mine),  Quartzless  porphyry  of,  94. 
Eocky  Eun  Eiver,  Sandstone  of,  249. 
Sogers,  "W.  B.,  Publication  of,  15,  19. 
Eominger,  C,  5. 

—  on  Eastern  Sandstone,  age  of,  352. 

Keweenaw  Series,  age  of,  12. 

— ,  Publications  of,  21,  22. 
Eosenbusch,  H.,  on  augite-syenite,  115. 
basalt,  37. 

— ,  Classiflcation  of  plagioclase-augite  rocks  by,  35. 

— ,  Nomenclature  of  rocks  adopted,  28. 

— ,  Nomenclature  of  felsites,  96. 

Eutley  on  quartz-crystals  in  quartz-porphyry,  101. 

Saganaga  Lake,  Schists  of,  397. 

St.  Croix,  Dalles  of,  Diabase-porphyrite  of,  235,  236. 


INDEX. 


461 


St.  Croix  River,  Authorities  on  geology  of,  240. 

,  Contact  of  Potsdam  sandstone  and  Keweenaw  Series 

on,  236-238. 

,  Diabase  exposures  on,  343. 

,  Lower  Division  on,  159. 

,  Prospects  for  copper  on,  428. 

,  Eolation  of  roclis  of,  to  Keweenaw  Point  rocks,  239- 

241. 

,  Sandstone  of,  248,  411,443,444. 

. — ,  Termination  of  Lalio  Superior  synclinal  on,  411,  412, 

414. 

,  Upper  Division  on,  153. 

Saint  Louis  River  Gabbros,  156, 268-275. 

,  Augite-syenite  of,  270. 

atErnWLate,  274,  275,  294. 

,  Characteristics  of,  266. 

on  Clociuet  Eiver,  268, 269,  272. 

,  Dip  of,  269. 

atDuluth,  269-272. 

Eagle  Mountain,  273. 

—  ■ ,  Extent  of,  268. 

— . ,  Eelsitio  porphyry  of,  271,  272. 

Saint  Louis  River,  Anlmikie  Group  on,  262,  384,  412. 

.Dikes  of,  384. 

,  Gabbro  of,  263.    See  Saint  Louis  River  Gabbros. 

,  Sandstones  of,  262. 

Sand  Point,  178. 

Sandstone  of  Animikie  Group.    See  Animikie  Group. 

— ,  Cambrian.    See  Cambrian. 

— ,  Eastern.    See  Eastern  Sandstone. 

— ,  Keweenawan,  127-133,  420. 

of  Agate  Bay  Group,  292,  293. 

Albany  and  Boston  Conglomerate,  130. 

Atlantic  Mill,  130, 192. 

Bad  Eiver,  132, 227, 233. 

Batchewanung  Bay,  348. 

Black  Bay,  24, 128, 156,  333, 336, 337. 

Cape  Choyye,  347. 

Calumet  Conglomerate,  130. 

Caribou  Point,  328. 

Carp  Eiver,  132. 

Constituents  of,  127, 128. 

of  Copper  EaUs,  129, 130, 171. 

,  Copper  in,  420. 

of  Eagle  Harbor,  128. 

Eaglo  Eiver  Section,  169, 171. 

Good  Harbor  Bay,  329. 

Isle  Eoyale,  331. 

Lester  Eiver,  292. 

Montreal  River,  183,  225,404. 

■ NipigonBay,  333,  336. 

Lake,  24,  339, 340. 

Nonesuch  Mine,  181, 132, 22L 

,  Nonesuch  sandstone  belt,  221-224. 

of  Ontonagon  Eiver,  199,  200. 

Pointe  aux  Mines,  347. 

Porcupine  Mountains,  132,215-217,  220,  221. 

Portage  Entry,  198. 

,  West  of,  200. 

Lake,  192,  193. 

St.  Croix  Eiver,  246-250. 

Silver  Islet  Landing,  133, 378. 

Temperance  Eiver,  3C5. 

Thunder  Bay,  156, 157, 333,  376,  377, 411. 

Upper  Division,  152-164. 

,  Veins  of,  in  diabase,  139, 140, 292, 293. 

of  White  Eiver,  233. 


Sandstone,  Potsdam.    See  Potsdam. 

— ,  "  Mctamorphic."    See  Norwood. 

— ,  Nonesuch  Belt.    See  Nonesuch. 

— ,  Western.    See  Western  Sandstone. 

Sault  Ste.  Marie,  413. 

Sawteeth  Mountains,  142. 

Schists,  shales,  and  slates,  of  Batchewanung  Bay,  400. 

,  Grand  Portage  Bay,  247, 367, 405. 

,  Gunflint  Lake,  399. 

Hat  Point,  368. 

Kaministiquia  Eiver,  374, 375, 380. 

Lucille  Islands,  369. 

Mesabi  Eange,  382-384. 

—  —  Michipicoten  River,  400. 

Mission  Creek,  262. 

Nipigon  Lake,  408. 

Pigeon  Bay,  369-374.  , 

Pokegoma  EaUs,  383, 384. 

Rainy  Lake,  397. 

Eed  River  Road,  380. 

Saganaga  Lake,  397. 

St.  Louis  Eiver,  262, 384. 

Slate  Islands,  401. 

Thompson,  262. 

Thunder  Bay,  375-380,  396,  397. 

Wauswaugoning  Bay,  368. 

'  See  Animikie,  Huronian,  Nonesuch,  Porcupine  Mts. 

Science,  Vol.  I.,  441, 442, 443. 

Section  of  Animikie  Group  (BeJl),  380. 

—  on  Agate  Bay,  288-290. 

—  on  Bad  River,  232. 
Eagle  Eiver,  177, 178. 

—  of  Huronian  (Logan),  386-388. 

—  on  Keweenaw  Point,  186, 187. 
of  Lake  Superior  Basin,  417, 418. 

—  on  Minnesota  Coast,  266. 

Montreal  River,  227,  228. 

Ontonagon  Section,  198-201. 

— ,  Penokee  Huronian,  391,  392. 

—  on  Portage  Lake,  194, 195. 

Potato  River,  230, 231. 

Snake  Eiver,  242. 

Split  Rock  River,  301-803. 

Temperance  River  Group,  328. 

Thunder  Bay  (BeU),  332. 

T.60,E.2,  W.,328. 

Selwyn,  A.  R.  C,  on  Michipicoten  Island,  Specimens  from,  4, 

342. 
— ,  Publication  of,  23. 
Shales.    See  Sandstone  schists. 
Shumard,  B.  P.,  Publication  of,  17. 
Silver  Creek,  Diabase-porphyrite  of,  80, 84. 

,  Olivine-diabase  of,  76. 

.    See  Encampment  Bluft 

Silver  Islet,  Dikes  of,  878. 

,  Slates  of,  331. 

Silver  Islet  Landing,  Dikes  of,  877. 

,  Sandstone  of,  133, 378. 

Silver  Mountain,  202, 203. 

Simpson's  Island,  Sandstone  of,  836. 

Slate  Islands,  401. 

Slates.    See  Schists. 

"  Slide,  The,"  of  Eagle  River,  175. 

Snake  River,  Chamberlin  on  rocks  of,  242, 243. 

,  Conclusions  regarding  rocks  of,  245,  246, 412. 

,  Sandstone  of,  240. 

See  Kettle  Eiver. 


462 


INDEX. 


Soath  Eange,  201, 208. 

,  Dips  of  rocks  of,  202, 204, 205. 

,  Eastern  sandstone  on,  360. 

,  Foster  and  Whitney  on,  204. 

,  Limits  of,  201. 

on  Montreal  Eiver,  229. 

Ontonagon  River,  203. 

,  relation  of,  to  Keweenaw  Fault,  205. 

Main  Eange,  204, 205, 208. 

at  Silver  Mountain,  202, 203. 

South  Shore  Eocks,  Eastern  sandstone,  351-365. 

,  Huronian,  Marquette,  392-395. 

,  Penokee,  391, 393. 

,  Michigan,  161-229. 

,  Minnesota,  234-259. 

,  Wisconsin,  224-259. 

South  Side  Mine,  189. 

Spar  Island,  Dike  of,  273. 

Special  Eeport  on  Trap  Dikes,  &c.    See  Hunt,  T.  S. 

Spencer,  J.  W.,  Publication  of,  23. 

Split  Eock  Eiver  and  vicinity,  301-303. 

,  Anorthite-rock  of,  59,  60. 

,  Aiigite- syenite  of,  116, 121. 

,  Beaver  Bay  G-roup  on,  298. 

,  Diabase-porphyrite  on,  84. 

,  Felsites  of,  107,  291, 301. 

,  Granites  of,  304. 

,  Olivine-dl.ahase  of,  76,  291. 

,  Orthoclase-free  gahbro  of,  48, 302, 

Stannard's  Eock,  197, 198. 

,  Felsite  of,  93,  94, 197, 198. 

,  Foster  and  Whitney  on,  197. 

,  Mather  on,  197. 

Stevens,  W.  H.,  163.  ' 

Streng,  A.,  and  Kloos,  J.  H.,  on  diabase-poiphyrite,  235. 

on  Duluth  Group,  277. 

,  Saint  Louis  gabbro,  269. 

,  Publication  of,  22. 

Strong,  M.,  5,  7, 11,  35,  44, 410. 

—  on  Saint  Croix  Valley,  238, 411. 

Wisconsin,  Northwestern,  234. 

Stuntz,  G.  W.,  on  Mesabi  Eange,  399. 

Sucker  Bay,  Orthoclase-free  gabbro  of,  43, 47, 286. 
Sucker  Brook,  Orthoclase-gabbro  of,  373. 
Sucker  Eiver,  Olivine-diabase  near,  72,  76. 

,  Orthoclase-free  gabbro  of,  47. 

Suffolk  Mine,  81, 176, 177. 
Sweet,  E.  T.,  5,  7, 10, 11,  35,  44, 410. 

—  on  Black  Eiver  Eocks,  251-258, 441. 

—  on  Douglas  County  Copper  Eange,  250-258. 
— ,  Publications  of,  22, 23. 

—  on  Saint  Croix  Eiver,  238, 249, 411. 
Saint  Louis  Slates,  412. 

Wisconsin,  Northwestern,  234. 

Synclinal,  Lake  Superior.     See  Lake  Superior. 

Taconic  Group,  Winchell  on  Western  Equivalents  of,  443, 

Talc,  alteration  product  of  olivine,  39. 

Taylor's  Falls,  Contact  of  Potsdam  Sandstone  and  Keweenaw 

Series  at,  236-238. 
Temperance  Eiver  and  vicinity,  Anorthite-rock  of,  59. 

,  Beaver  Bay  Group  on,  298. 

,  Geological  descent  of  strata  at,  264, 265. 

,  Temperance  Eiver  Group,  323, 325, 326. 

Temperance  Eiver  Group,  323-329. 
at  Baptism  Eiver,  326. 


Temperance  Eiver  Group,  Beaver  Bay  Beds  of,  318, 319 

,  Caribou  Point,  328. 

,  Copper  possibly  in,  424. 

,  Characteristics  of,  265, 324. 

,  Detrital  rooks  of,  327-329. 

,  Dikes  of,  329. 

,  Dip  of  beds  of,  324. 

,  Faults  of,  329. 

at  Good  Harbor  Bay,  329. 

Grand  Marais,  327. 

Maniton  Eiver,  328. 

— Poplar  Eiver,  338. 

Temperance  Eiver,  325, 326. 

,  Thickness  of,  323. 

Thompson,  Saint  Louis  slates  at,  262. 
J  Thompson  Island,  Dike  of,  373. 
Thoulet,  J.,  method  of  determining  feldspars,  437-440. 
Thunder  Bay  and  vicinity,  Animikie  Group  on,  375, 380. 

,  Chert  of,  375. 

,  Dip  on,  333. 

,  Dolomitic  sandstone  of,  376. 

,  Erosion  on,  377, 405. 

,  Interbedded  diabases  of,  375. 

,  Keweenaw  Series,  331-333, 403. 

-I ,  Parallelism  of  Huronian  and  Keweenaw  Series  on, 

405. 

,  Sandstones  of,  156, 157, 332, 3B3. 

,  Schists  of,  331, 376, 396. 

,  Age  of,  397. 

.    See  Thunder  Cape. 

Thunder  Cape,  Dike  of,  376. 

,  Olivine-gabbro,  377. 

,  Slates  of,  331,  376,  378. 

Tischer's  Creek,  Augite-syenite  of,  120. 

,  Lester  Eiver  Group  near,  279. 

Titanic  acid  in  orthoclase-gabbro,  52. 
Titaniferous  magnetite.    See  Magnetite. 
Tobacco  Eiver,  Eastern  Sandstone  on,  354. 
Topography  affected  by  basic  rocks,  141-143. 
Torch  Lake,  Eastern  Sandstone  near,  185, 359. 
Torch  Lake  Eailroad,  Eastern  Sandstone  on,  356. 

,  Quartz-porphyry  of,  103, 104, 186, 196. 

.    ,See  Conglomerate,  Calumet. 

Totogatifi  District  (X.  44,  E.  9W.),  Olivine-diabtise  of,  75. 

(T.  42,  E.  U  W.),  Diabase-porphyrite  of,  83. 

Township,  Michigan,  46,  E.  39  W.,  203. 

,  E.  41  W.,  302, 203,  204,  360. 

,48,  E.  36  W.,  202. 

,  E.  43  W.,  105. 

,  E.  44  W.,  201,  202. 

,  49,  E.  36  W.,  202. 

,E.42  W.,103,214. 

,E.43  W.,214. 

,  E.  44  W.,  212,  213. 

,  E.  45  W.,  218, 319, 320, 223, 224. 

,E.46W.,225. 

,E.47W.,226. 

,E.48W.,236. 

,  60,  E.  39  W.,  94, 103, 199, 200, 214, 325,  359, 360. 

,  E.  40  W.,  199,  214, 235. 

,E.41W.,214, 332, 224. 

,  E.  42  W.,  317, 323, 224. 

,  E.  43  W.,  105, 131,  209,  210,  213,  324. 

,  E.44  W.,  82,  103,  106,  209,  211,  213,  215,  217,  218,  219, 

220. 


INDEX. 


463 


TowBBhip,  Michigan,  50,  E.  45  TV.,  219, 22''i  223, 224. 

,5],E,35"W.,352. 

,  E.  38  W.,  199,  200. 

,  B.  41  Tf.,  222,  224. 

,  E.  42  "W.,  67,  72,  74,  105,  132,  209,  211,  213,  215, 217, 218, 

219, 220,  222,  224. 
,  E.  43  W.,  82, 103, 106,  209, 210,  211, 212,  213, 215,  216, 218, 

219,  220,  221. 

,  E.  44  "VV.,  213,  219,  223. 

,  52,  E.  35  W.,  359. 

,54,E.33W.,359. 

,E.34'W.,359. 

,E.43  W.,132. 

,  55,  R.  33  W.,  94, 118, 119, 187. 

,  56,  E.  31  W.,  67, 176. 

,  E.  33  "W.,  103, 104, 105, 130, 183, 196. 

,  E.  34  \T.,  193. 

,  57,  E.  31  W.,  81, 176, 188. 

,E.32W.,187. 

,  58,  E.  27  W.,  183. 

,  E.  28  W.,  72,  74, 182. 

,  E.  29  TV.,  53,  74,  81, 104, 116, 117, 181, 183. 

,E.30W.,128. 

,  E.  31  W.,  53,  66,  67,  73, 104, 118, 129, 130. 

,E,38W.,181. 

— ,  Minnesota,  38,  E.  19  "W.,  241, 243. 

,  39,E.20"W.,241. 

,  E.21  W.,241. 

,40,E.  19W.,244. 

,  E.  20  W.,  244. 

,  41,  E.  20  W.  244. 

,  42,  E.  20  "W.,  244. 

,  43,  E.  20  W.,  245. 

,  45,  E.  20  W.,  245. 

,46,E.20W.,245. 

,  48,  E.  6  W.,  262. 

,E.15W.,262. 

,  E.  16  "W.,  384. 

,  49,  E.  15  W.,  46,  263,  270. 

,  E.  17  W.,  384. 

,  50,  E.  13  W.,  55,  75,  84,  279,  280. 

,  E.  14  W.,  46,  55,  83,  95, 119, 120, 279. 

,  51,  E.  11  W.,  76,  84. 

,  E.  12  W.,  47,  72, 75,  76,  83, 279, 280, 281,  286. 

,  E.  13  W.,  53,  55,  56,  84, 279, 284, 285,  292,  293,  296. 

,  52,  E.  11 W.,  7S,  278,  287. 

,E.12'W.,47. 

,  53,  E.  1  "W.,  283. 

,  E.  10  W.,  47,  48,  76,  80,  84,  279,  281,  282,  284,  285,  286,  291, 

292,  293,  297. 

,  E.  11  "W.,  278, 279, 281, 282. 

,E.  13W.,272. 

,  E.  14  "W.,  46,  263,  272. 

,  54,  E.  8  W.,  48,  59,  60,  84,  300,  302,  303. 

,  E.  9  W.,  76, 107,  284,  290,  291, 293. 

,  E.  13  W.,  272,  282. 

,  55,  E.  7  W.,  49,  61,  309. 

,  E.  8  W.,  48,  49,  60,  61,  83,  84,  107,  108,  121, 122,  303,  304, 

305,  309. 

,  E.  26  "W.,  384. 

,56,E.7  W.,  84,  85,  97,  100,  103,  108,  109,  110,  116,  122,146, 

310-318,  327,  329. 

,  E.  8  "W.,  49,  309. 

,E.  25  TV.,  384. 

,57,E.3E.,38. 

,E.  6  TV.,  324. 


Township,  Michigan,  57,  E.  7  TV.,  123, 324. 

,  58,  E.  5  TV.,  324,  325. 

,  59,  E.  4  TV.,  265,  329. 

,  60,  E.  2  TV.,  77,  328. 

,E.  3  TV.,  328. 

,E.  12  TV., 383. 

,  E.  13  TV.,  382,  383. 

,  61,  E.  1  E.,  110,  272, 320,  321. 

,  E.  ITV.,273. 

,  E.2E.,32I. 

,  62,  E.  1  TV.,  145. 

,  E.  2  TV.,  53,  56,  So,  145,  273,  295,  296. 

,E.  3  TV.,  145. 

,  E.3E.,49. 

,  E.  4  E.,  124,  320,  321. 

,63E.  ITV.,145. 

,  E.  2  TV.,  56, 124, 145,  273,  274, 294. 

,  R.3  TV.,  145,  291. 

,  E.  5  E.,  49,  85,  no,  111,  320,  322. 

— ,  TViscousin,  33,  E.  19  W.,  236. 

,  34,  E.  18  TV.,  238. 

,  E.  19  TV.,  235. 

,  36,  E.  16  TV.,  235. 

,  E.  17  TV.,  236. 

,  37,  E.  16  TV.,  103, 107,  203. 

,  E.  17  TV.,  83,  203. 

,  42,  E.  U  TV.,  83. 

,  E.  14  TV.,  246,  249. 

,  E.  15  TV.,  247,  248. 

,  E.  16  TV.,  249. 

,43,  E.  13  TV.,  246,  248,  249. 

,  E.  14  TV.,  75,  83,  241),  247, 248, 249. 

,  44,  E.  3  TV.,  45,  46,  56,  58. 

,  E.  4. TV.,  46,  56. 

,  E.  5  TV.,  46,  56,  58. 

,  E.  6  TV.,  54,  56. 

,E.9TV.,75. 

,  E.  13  W.,  68,  72,  75,  246,  247,  248,  249. 

,  E.  14  TV.,  248. 

,  45,  E.  1  E.,  45. 

,  E.  1  TV.,  45,  58,  231. 

,  E.  2  TV.,-45,  54, 103, 106. 

,  E.  3  TV.,  40,  56,  57,  58. 

,  E.  4  TV. ,  46,  54,  57,  58,  93,  95. 

,  E.  5  TV.,  40,  56. 

,  R.  6  TV.,  46,  S6. 

,  46  E.  IE.,  67,  229. 

,  E.  1  TV.,  63.  74,  83, 103, 106,  23L 

,  E.  2  E.,  68,  93,  229,  231. 

,E.  4  TV.,  233. 

,  47,  E.,  1 E.,  68, 133,  226, 227,  228, 229, 427. 

,E.  2E.,229. 

,  E.  3  TV.,  132,  233. 

,E.  11  TV.,  257. 

,  E.  12  TV.,  256. 

,  E.  13  TV.,  255. 

= ,  E.  14  TV.,  68,  252,  254. 

,  48,  E.  10  TV.,  267. 

,  E.  12  TV.,  53,  54,  256. 

,  50,  E.  6  TV.,  258. 

,E.7  W.,257. 

Trap  helt.  Porcupine  Mountains,  Inner  on,  214, 215, 

,  Oater  on,  218-220. 

Trenton  limestone,  352. 

Tschermak,  G-.,  Theory  of  feldspars,  437, 440. 

Two  Islands  Eiver,  Temperance  Eiver  Gronp  on.  325. 


464 


INDEX. 


Tylor*a  Fork,    See  Bad  River. 

,  Quartz-porphyry  of,  103, 106. 

Union  Mine,  Olivine-free  diabase  of,  67, 219. 

,  Outer  trap  belt  exposed  at,  219. 

,  Sandstone  of,  132, 220. 

Union  Biver,  Outer  sandstone  exposed  at,  220. 

,  Trends  of  strata  on,  200. 

United  States  Late  Survey,  330. 

Upper  Division,  Characteristics  of,  153-154. 

Eruptives  of,  152,440. 

on  Isle  Royale,  154. 

—  Keweenaw  Point,  153. 

— . Montreal  Kiver,  153. 

North  Shore,  154. 

Ontonagon  Kiver,  199. 

—  Porcupine  Mountains,  153, 208. 

Portage  Lake,  153, 193. 

in  Wisconsin. 

without  copper,  426. 

Vanbise,  C.  E.,  on  uralitic  hornblende,  396. 

— ,  Determination  of  feldspars  by,  437-440. 

Veins,  Calcitic,  in  Greenstone,  424. 

— ,  Nature  of,  424, 425. 

— ,  Transverse,  423-425. 

Vermillion  Lake,  Prof.  A.  H.,  Chester  on  rocka  of,  398. 

,  Schists  of,  397-399. 

^  Relation  of,  to  Huronian  Animikie  Group,  397. 

Victoria  Island.  Dike  rock  of,  372. 
Viridite  an  alteration  product  of  olivine,  39. 

—  of  orthoclaae-free  gabbro,  43. 

Von  Bnch  on  origin  of  conglomerates,  8. 
"Wadsworth,  M.  E.,  5. 

—  on  ashbed-diabase,  138, 422. 

bibliography  of  Lake  Superior  Rocks,  14, 431. 

diabase  orisin  of,  11. 

Douglas  Houghton  River,  sandstone  of,  356. 

Hungarian  River,  sandstone  of,  355. 

Keweenaw  Series,  age  of,  12, 13. 

— ,  Publication  of,  23. 

—  on  relation  of  diabases  of  Saint  Croix  Valley  and  Kewee- 

naw Point,  239. 
relation  of  Eastern  Sandstone  and  Keweenaw  Series, 

363-363. 

Torch  Lake,  sandstone  of,  357. 

"Wauaan,  Huronian  rocks  of,  400. 
"Wauswaugoning  Bay,  Animikie  Group  on,  368,  369. 
"Western  Sandstone,  153, 154, 365,  366. 

Characteristics  of,  365, 366. 

Extent  of,  365. 

■ ,  Relation  of,  to  Eastern  sandstone,  366. 

• Mississippi  Valley  sandstone,  366. 

.    See  Douglas  County  Copper  Range. 

White,  B.N.,  3.4. 

White  on  Porcupine  Monntains,  207,  224. 


White  on  South  Range,  202. 
White  River,  Sandstone  of,  233. 
White  Fish  River,  Sandstone  of,  351. 
Whitney,  J  W.,  Publications  of,  16-18. 
— .    See  Foster  and  Whitney. 
Whittlesey,  C,  5, 17, 19, 22, 44. 

—  on  Bad  River  Country,  207. 
Bohemian  Range,  180. 

Douglas  County  Copper  Range,  258. 

Montreal  River,  207, 

Porcupine  Mountains,  207. 

—.Publications  of,  17, 19,  22. 
Wichmann,  C,  Diorites  of,  393. 

—  on  Huronian  greenstones,  394. 

,  Marquette  and  Menominee,  394. 

Williams,  C.  P.,  163. 

Willis,  on  quartzites  of  Pokegoma  Falls,  384. 
Winchell,  N.  H.,  4, 5, 7, 101. 

—  on  acid  eruptives,  12. 

Animikie  Group,  382,  397,  399, 443. 

diabases,  origin  of,  10. 

Duluth  granite,  145. 

Group,  277. 

Eastern  Sandstone,  431. 

faults  of  North  Shore,  142. 

Great  Palisades,  316.  r 

Gunflint  Lake,  382, 399. 

Keweenaw  Series,  age  of,  12. 

Relation  of  to  other  formations,  443-445. 

— ,  "  Metamorphic  shales"  of,  138, 287. 

—  on  Pigeon  River  dikes,  370, 371. 
hills,  382. 

Poplar  River  granite,  145. 

— ,  Publications  of,  20, 23, 431. 

—  on  Saganaga  Lake,  399. 

Saint  Louis  River  gabbro,  269, 272. 

Thunder  Bay  schists,  extension  of,  397. 

traps,  11. 

Vermillion  Lake  schists,  397. 

Wisconsin,  Area  of  Keweenaw  Series  in,  27. 

— ,  Possible  copper  of,  427. 

— ,  Potato  and  Bad  River  districts,  230-234. 

— ,  Northwestern,  Diabase-porphyrite  of,  235,  236. 

,  Sources  of  information  on,  234. 

,  Stratigraphy  of,  234. 

,  Southern  belt  of,  234-241. 

.    See  Douglas  County  Copper  Range. 

.    See  Taylor's  Falls. 

— ,  Geological  Survey  of,  2,  7,  35,  39, 41, 44, 45, 46, 52, 54, 56, 63, 

69,74,77,89, 105, 125,"l56,  207,  227,  235,  251-258,  365,  383,391, 

392, 393, 394, 398, 404, 410, 411, 412. 
Zirkel,  F.,  on  rhyolites,  98, 109. 
trichites,  312. 


